sentinel-filled is now dead-beef, and takes the pattern

The author's revision. The name says what it writes, and the pattern is the
program's to choose: (dead-beef) is DEADBEEF, (dead-beef 0xBAADF00D) is
BA AD F0 0D. One byte-order rule covers both — a pattern's ascending bytes
are its big-endian bytes, which is how the hex literal reads left to right —
so every candidate DISCUSS.org listed is now spellable without the compiler
naming any of them.

The bare form is not a case a backend knows about: the checker writes
Tast.dead_beef_default in where the argument would have been, so
(dead-beef) and (dead-beef 0xDEADBEEF) are the same node and an acceptance
row prints both to say so.

The operand is an ordinary u32 expression, which is what the byte arm
already accepts for its byte. A literal is byte-reversed at compile time and
still reaches the loop as an immediate; a computed one is reversed at run
time, by llvm.bswap.i32 on one backend and bswap on the other, after which
the tail shifts its bytes out of the word rather than folding them. The
program runs a computed pattern over lengths 6 and 7 deliberately: that is
the case a constant-only implementation would pass by accident.

filled is untouched, and so is the fill boundary.

DISCUSS.org

@@ -186,9 +186,18 @@ why MSVC/glibc-style debug allocators use a single repeated byte instead
 Built, 2026-09-20, as two builtins rather than one — the author's answer to
 the cheap-byte-or-real-pattern question was "why not both? we need some sort
 of memset -1 right? and dead-beef can loop, that's fine". They are
-(filled BYTE) and (sentinel-filled), spelled the way [zeroed] is: the value
+(filled BYTE) and (dead-beef), spelled the way [zeroed] is: the value of
-of whatever type is expected of them, so (set grid (filled 0xFF)) is how a
+whatever type is expected of them, so (set grid (filled 0xFF)) is how a place
-place is filled and there is no place-taking form to learn beside [set].
+is filled and there is no place-taking form to learn beside [set].
+
+(dead-beef) writes the default DEADBEEF and (dead-beef 0xBAADF00D) writes the
+pattern given, under one byte-order rule: a pattern's ascending bytes are its
+big-endian bytes, which is how the hex literal reads left to right. So every
+candidate listed above is spellable without the compiler naming any of them,
+and the bare form is checked into the spelled-out default rather than being a
+case a backend knows about. The pattern may be computed, not only written —
+same rule as the byte arm, and both backends byte-reverse at run time when it
+is.
 
 The snag above is why there are two and not one with a wider operand. The
 byte fill is one llvm.memset / one rep stosb; the 4-byte pattern is a loop on

FIX.org

@@ -1618,12 +1618,19 @@ answer to the single-byte-or-four-byte question was "why not both? we need
 some sort of memset -1 right? and dead-beef can loop, that's fine", so there
 are two builtins and they are siblings of ~zeroed~, not a new shape.
 
+Revised the same day: the pattern builtin was ~sentinel-filled~ and is now
+~dead-beef~, and it gained an optional operand so the pattern is the
+program's to choose. What did not change is ~filled~, or the fill boundary,
+or the byte-order rule — the revision generalised the pattern, it did not
+reopen what may be filled.
+
 ** The spellings
-~(filled BYTE)~ and ~(sentinel-filled)~, both value forms driven by the type
+~(filled BYTE)~ and ~(dead-beef)~ / ~(dead-beef PATTERN)~, all value forms
-expected of them, exactly as ~(zeroed)~ is:
+driven by the type expected of them, exactly as ~(zeroed)~ is:
 
 : (set grid (filled 0xFF))
-: (set frame (sentinel-filled))
+: (set frame (dead-beef))
+: (set frame (dead-beef 0xBAADF00D))
 
 A place-taking ~(filled place byte)~ was the other candidate and was not
 taken. ~zeroed~ already answers "the all-bytes-X value of whatever this is
@@ -1633,9 +1640,29 @@ nothing. The cost is real and is paid on purpose: a fill in a position that
 expects no type is refused ("filled needs to know the type it is filling"),
 which is ~zeroed~'s own refusal worn by both siblings.
 
-~sentinel-filled~ takes no operand. The pattern is fixed — that is the whole
+~dead-beef~ takes the pattern or leaves it out, and leaving it out is
-point of it, since a hex dump only reads DEADBEEF if nobody can change it —
+*defined as* writing the default: the checker's zero-argument arm builds the
-and a parameterised one would be a different builtin.
+same ~Tast.Int 0xDEADBEEF~ the spelled-out call would have, so ~(dead-beef)~
+and ~(dead-beef 0xDEADBEEF)~ are the same IR node by construction and no
+backend has a second path for the bare form. An acceptance row prints both
+and pins that they agree.
+
+The pattern is an ordinary ~u32~ /expression/, not a literal — the byte arm's
+rule at four times the width. A literal out of range meets ~in_range~'s
+located "does not fit in u32"; anything computed is guaranteed by its type
+instead, since a ~u32~ cannot be out of ~u32~ range. Refusing a computed one
+would have been a restriction with no mechanism behind it: neither backend
+needs the number early.
+
+/Reading note, for whoever reviews this./ The revision asked for "a u32-range
+constant ... decide literal-only vs any constant expression from what the
+byte-fill arm already accepts". The byte-fill arm accepts any ~u8~
+expression, runtime ones included, and the same instruction asked that both
+backends handle "eax loaded from a value, not an immediate" — which only
+exists if a computed pattern is legal. So the operand is any ~u32~
+expression. That is a strict superset of constants-only: every program the
+narrower reading allows behaves identically here. Tighten it to literals if
+that was the intent; nothing else depends on the breadth.
 
 ** The fill boundary — what may be overwritten with raw bytes
 *Numbers, and structs and fixed arrays built out of numbers. Nothing else.*
@@ -1675,31 +1702,53 @@ into the image. A ~defvar~ is fine and goes through the startup function on
 both backends, which ~programs/fill.flan~ pins.
 
 ** The byte order, which is the specification
-DE AD BE EF in *ascending address order*, so ~xxd~ reads "deadbeef". A
+*A pattern's ascending bytes are its big-endian bytes* — exactly how the hex
-little-endian store of those four bytes is the i32 0xEFBEADDE, and
+literal reads left to right. So ~(dead-beef)~ lays down DE AD BE EF and ~xxd~
-~Emit.sentinel_bytes~ / ~Emit.sentinel_word~ are the one place either is
+reads "deadbeef"; ~(dead-beef 0xBAADF00D)~ lays down BA AD F0 0D. One rule,
-written — x86.ml reads both out of Emit rather than repeating them, so the
+both arities.
-two backends cannot drift.
+
+On a little-endian machine the word a 4-byte store must therefore leave is
+the *byte reversal* of the pattern, which is all ~Emit.word_of_pattern~ is
+(~bytes_of_pattern~ beside it is the ascending list). Those two are the one
+place the order is written; x86.ml reads them out of Emit rather than
+repeating them, and ~Tast.dead_beef_default~ is the one place 0xDEADBEEF is
+written. Three definitions, no duplicates, so the default and the
+parameterised case cannot drift.
+
+A literal pattern is reversed at compile time and reaches the loop as an
+immediate — the default's generated code is exactly what it was before the
+pattern became an operand. A computed one is evaluated and reversed at run
+time, by ~llvm.bswap.i32~ on one backend and ~bswap eax~ (0F C8, new) on the
+other.
 
 *Tail behaviour.* A size that is not a multiple of four ends on a prefix of
-the pattern: 1 byte over is DE, 2 is DE AD, 3 is DE AD BE. Both backends
+the ascending bytes: 1 byte over is DE, 2 is DE AD, 3 is DE AD BE.
-write the tail byte by byte from the same list. ~programs/fill.flan~ has all
+Equivalently, tail byte k is ~(word >> 8k) & 0xFF~ — which is what the
-four lengths — 8, 9, 6 and 7.
+computed path actually does, by shifting, since there is no constant to fold.
+~programs/fill.flan~ has all four lengths (8, 9, 6, 7) and, crucially, runs a
+computed pattern over lengths 6 and 7: that is the case a constant-only
+implementation would pass by accident.
 
 ** The backends
 - *LLVM (emit.ml).* The byte fill is one ~llvm.memset~ with the byte as an
   operand instead of a zero — the same call the existing bulk zero makes, and
-  the reason the single-byte fill is the cheap one. The sentinel cannot be a
+  the reason the single-byte fill is the cheap one. The pattern fill cannot
-  memset at all (the intrinsic takes one repeated i8, which is the snag
+  be a memset at all (the intrinsic takes one repeated i8, which is the snag
   DISCUSS.org named), so it is a counted loop over dwords in the
   header/body/exit shape ~emit_while~ writes, with the counter as an
   entry-block alloca that ~mem2reg~ promotes. Every store is ~align 1~,
-  because a ~[7 u8]~ array is a legal thing to fill.
+  because a ~[7 u8]~ array is a legal thing to fill. A computed pattern goes
+  through ~llvm.bswap.i32~ (newly declared) and the loop stores an SSA value
+  rather than a constant; the tail then shifts and truncates.
 - *x86 (x86.ml).* ~rep stosb~ for the byte fill — ~zero_loc~'s three
   registers with the program's byte in ~al~ instead of a zero — and ~rep
-  stosd~ (new, 0xf3 0xab) for the sentinel, with 0xEFBEADDE in ~eax~. The
+  stosd~ (new, 0xf3 0xab) for the pattern, with the stored word in ~eax~. A
-  byte operand is evaluated *before* ~rdi~ is loaded, because evaluating it
+  computed pattern is loaded and run through ~bswap~ (new, 0F C8); the tail
-  may call and a call clobbers ~rdi~.
+  walks the bytes out of ~rax~ with ~shr~ by an immediate (new, C1 /5), which
+  is used rather than ~shift_cl~ precisely because ~rep stosd~ leaves ~rcx~
+  at zero. Either operand is evaluated *before* ~rdi~ is loaded, because
+  evaluating one may call and a call clobbers ~rdi~; ~rep stosd~ does not
+  touch ~rax~, which is what lets the tail keep reading the word out of it.
 - The one asymmetry: ~emit.ml~ needs a ~Tast.Set~ arm of its own to fill the
   place rather than a temporary, because its value path returns an SSA value.
   ~x86.ml~ needs none — a ~Set~ there already lowers its value into the
@@ -1712,9 +1761,11 @@ four lengths — 8, 9, 6 and 7.
 ~dune test --root .~ green (exit 0, no FAIL lines). Three acceptance rows
 over ~test/programs/fill.flan~ — default, ~-O0~ and ~--x86~ — and the three
 outputs diffed against each other by hand before the rows were written:
-byte-identical — and the same three rows confirmed to actually run, by
+byte-identical, with a fourth build (~--dev~) added at the rename: four-way
-breaking one expectation on purpose and watching all three report. Fourteen
+identical. The three rows were confirmed to actually run, by breaking one
-checker rows in ~test_flan.ml~: two accepting, and twelve refusals covering
+expectation on purpose and watching all three report. Seventeen checker rows
-the boundary, both arities, both no-expected-type positions, the byte's
+in ~test_flan.ml~: four accepting (both ~dead-beef~ arities and a computed
+pattern among them), and thirteen refusals covering the boundary, both
+arities, both no-expected-type positions, the byte's range, the pattern's
 range and the ~defconst~ rule. Per the sweep policy the ~@x86~ and
 ~@sanitize~ sweeps were not run here.

lib/check.ml

@@ -4873,12 +4873,29 @@ and named_call ctx ~want loc name args =
   (* [zeroed]'s two siblings, and the same shape exactly: a value of whatever
      type is expected of it, so [(set grid (filled 0xFF))] is how a place is
      filled and there is no second spelling to learn. What they add over
-     [zeroed] is the byte — [(filled b)] repeats one the program picks, and
+     [zeroed] is the bytes — [(filled b)] repeats one the program picks, and
-     [(sentinel-filled)] repeats the four bytes DE AD BE EF — and with it the
+     [(dead-beef)] repeats four — and with them the question [zeroed] never
-     question [zeroed] never has to ask: zero is a value every type can have,
+     has to ask: zero is a value every type can have, and 0xDE is not.
-     and 0xDE is not. [unfillable] above is the whole of the answer. *)
+     [unfillable] above is the whole of the answer.
-  | "filled" | "sentinel-filled" ->
+
-    arity loc name (if String.equal name "filled" then 1 else 0) args;
+     [dead-beef] takes the pattern or leaves it out, and leaving it out is
+     defined as writing the default: the [None] arm below builds the same
+     literal the source would have, so [(dead-beef)] and
+     [(dead-beef 0xDEADBEEF)] are the same node by construction and no
+     backend has a second path for the bare form.
+
+     The pattern is an ordinary u32 expression, which is the byte arm's rule
+     at four times the width — a literal out of range meets [in_range]'s
+     located "does not fit in u32", and anything computed is guaranteed by
+     its type instead. Both builtins take a value rather than only a literal
+     for the same reason: refusing one would be a restriction with no
+     mechanism behind it, since neither backend needs the number early. *)
+  | "filled" | "dead-beef" ->
+    let is_byte = String.equal name "filled" in
+    if is_byte then arity loc name 1 args
+    else if List.length args > 1 then
+      fail loc "%s takes the pattern or nothing at all, given %d arguments"
+        name (List.length args);
     (match want with
      | Some ty when ty <> Types.Never ->
        (match unfillable ctx.env [] ty with
@@ -4907,16 +4924,27 @@ and named_call ctx ~want loc name args =
              | _ -> "it carries a tag that names a case, and no byte pattern \
                      names a real one")
         | None -> ());
-       if String.equal name "filled" then
+       if is_byte then
          let b = check ctx ~want:(Types.Int Types.U8) (List.hd args) in
          mk loc ty (Tast.Fill (ty, b))
-       else mk loc ty (Tast.Sentinel ty)
+       else
+         let pat =
+           match args with
+           | [ a ] -> check ctx ~want:(Types.Int Types.U32) a
+           (* The bare form, written out. Not a default a backend applies:
+              the node that leaves here is the one the spelled-out call would
+              have left, which is what makes the equivalence a fact about the
+              IR rather than a promise two emitters keep separately. *)
+           | _ ->
+             mk loc (Types.Int Types.U32)
+               (Tast.Int (Int64.of_int32 Tast.dead_beef_default, Types.U32))
+         in
+         mk loc ty (Tast.DeadBeef (ty, pat))
      | _ ->
        fail loc
          "%s needs to know the type it is filling — use it where one is \
           expected, as in (set grid (%s))"
-         name
+         name (if is_byte then "filled 0xFF" else name))
-         (if String.equal name "filled" then "filled 0xFF" else name))
 
   (* The one half of a destructuring [let] that [Parse] cannot do on its own.
      Everything else about a pattern is bindings and field accesses it already
@@ -6815,10 +6843,11 @@ let builtins : (string * string * string) list =
      "Every byte of whatever it is being stored into set to one byte, as in \
       (set grid (filled 0xFF)). Numbers only, and structs and fixed arrays \
       built out of them.");
-    ("sentinel-filled", "sentinel-filled [] T",
+    ("dead-beef", "dead-beef [u32?] T",
-     "The bytes DE AD BE EF repeating over whatever it is being stored into, \
+     "A four-byte pattern repeating over whatever it is being stored into, \
-      so a hex dump reads DEADBEEF. A size that is not a multiple of four \
+      written so a hex dump reads it left to right: 0xDEADBEEF with no \
-      ends on a prefix of the pattern. Same types [filled] takes.");
+      argument, the u32 given otherwise. A size that is not a multiple of \
+      four ends on a prefix of the pattern. Same types [filled] takes.");
     ("destructure~nth", "destructure~nth [[n T] i32 i32 i32] T",
      "Written by the compiler for a destructuring let, and unspellable: the \
       reader makes ~ a delimiter, so no source symbol can name this.");

lib/emit.ml

@@ -937,29 +937,46 @@ let emit_bulk_zero f ptr ty =
     end else false
   | _ -> false
 
-(* ── The sentinel pattern ──────────────────────────────────────────────
+(* ── The dead-beef pattern ─────────────────────────────────────────────
 
-   DE AD BE EF, in ascending address order, so [xxd] over the filled storage
+   [(dead-beef)] writes DE AD BE EF in ascending address order, so [xxd] over
-   reads "deadbeef" and not "efbeadde". That is the whole reason the builtin
+   the filled storage reads "deadbeef" and not "efbeadde". That is the whole
-   exists, so the byte order is the specification and not an implementation
+   reason the builtin exists, so the byte order is the specification and not
-   detail: the list below is the order the bytes land in memory, and the i32
+   an implementation detail.
-   beside it is the little-endian word that puts them there. Both backends
-   write the same four bytes — x86 through [rep stosd] with the same word in
-   [eax], this file through a store of the same constant — and the acceptance
-   program reads the bytes back by index to keep them honest.
 
-   A run whose size is not a multiple of four ends on a prefix: DE, then DE
+   [(dead-beef V)] writes V's four bytes under the same rule, and the rule is
-   AD, then DE AD BE. The tail is written byte by byte, from the same list, so
+   what makes the two arities one thing: *a pattern's ascending bytes are its
-   there is one source for the order. *)
+   big-endian bytes*, which is exactly how the hex literal reads left to
-let sentinel_bytes = [ 0xDE; 0xAD; 0xBE; 0xEF ]
+   right. So [(dead-beef 0xBAADF00D)] lays down BA AD F0 0D. Nothing here
+   knows about the bare form: the checker writes [Tast.dead_beef_default] in
+   where the argument would have been, so this file only ever sees a pattern.
 
-(* The four bytes as the i32 a little-endian store of them would leave, folded
+   On a little-endian machine the word a 4-byte store must leave is therefore
-   from the list rather than written out, so the two cannot drift apart. *)
+   the byte reversal of the pattern, which is all [word_of_pattern] is. Both
-let sentinel_word =
+   backends go through here — x86 through [rep stosd] with this word in [eax],
+   this file through a store of it — and the acceptance program reads the
+   bytes back by index to keep them honest.
+
+   A run whose size is not a multiple of four ends on a prefix of the
+   ascending bytes: one over is DE, two is DE AD, three is DE AD BE. *)
+
+(* The pattern's bytes in the order they land in memory, which is
+   most-significant first — the order the literal is written in. *)
+let bytes_of_pattern (v : int32) =
+  List.map
+    (fun k ->
+       Int32.to_int
+         (Int32.logand (Int32.shift_right_logical v (8 * k)) 0xFFl))
+    [ 3; 2; 1; 0 ]
+
+(* The i32 a little-endian store leaves those bytes as: the same list folded
+   back the other way, so one definition answers for both and a change to the
+   byte order cannot reach only half of it. *)
+let word_of_pattern (v : int32) =
   List.fold_left
     (fun acc b -> Int32.logor (Int32.shift_right_logical acc 8)
                     (Int32.shift_left (Int32.of_int b) 24))
-    0l sentinel_bytes
+    0l (bytes_of_pattern v)
 
 let term f fmt =
   Printf.ksprintf
@@ -1192,10 +1209,18 @@ let emit_byte_fill f ptr ty (byte : string) =
       "call void @llvm.memset.p0.i64(ptr align %d %s, i8 %s, i64 %d, i1 false)"
       align ptr byte size
 
-(* [(sentinel-filled)] over the same. [llvm.memset] takes one repeated i8 and
+(* [(dead-beef PATTERN)] over the same. [llvm.memset] takes one repeated i8
-   nothing else, so a four-byte pattern cannot be a call and has to be a loop
+   and nothing else, so a four-byte pattern cannot be a call and has to be a
-   — the snag DISCUSS.org named before this was built, and the reason the two
+   loop — the snag DISCUSS.org named before this was built, and the reason the
-   builtins are two and not one with a wider operand.
+   two builtins are two and not one with a wider operand.
+
+   The word is either a folded constant or an SSA value, and [word] below is
+   just its spelling: a literal pattern is byte-reversed here at compile time
+   and a computed one by [llvm.bswap.i32] at run time, after which the loop
+   and the tail are the same instructions either way. That is deliberate —
+   the case a constant-only implementation would quietly get wrong is a
+   computed pattern over a length that is not a multiple of four, and there
+   is only one tail here for it to get wrong.
 
    The counter is an entry-block alloca, which is how every local in this file
    is spelled and what [mem2reg] promotes; the loop is the same
@@ -1204,9 +1229,12 @@ let emit_byte_fill f ptr ty (byte : string) =
 
    Every store is [align 1]: the pattern is laid down over bytes, and the
    storage's own alignment may be 1 — a [7 u8] array is a legal thing to fill.
-   The tail's up-to-three bytes are written out straight, from the same list
+
-   the word was folded from. *)
+   The tail is the first up-to-three bytes of the *stored word*, lowest
-let emit_sentinel f ptr ty =
+   address first, which is byte k = (word >> 8k) & 0xFF. For a constant that
+   folds to [bytes_of_pattern]'s list; for a computed word it is a shift and
+   a truncate, ~tail~ times, unrolled because the count is known here. *)
+let emit_dead_beef f ptr ty ~(word : string) ~(folded : int32 option) =
   let size, _ = lay f.md ty in
   let words = size / 4 and tail = size mod 4 in
   if words > 0 then begin
@@ -1226,7 +1254,7 @@ let emit_sentinel f ptr ty =
     ins f "%s = mul i64 %s, 4" off c;
     let p = fresh f in
     ins f "%s = getelementptr i8, ptr %s, i64 %s" p ptr off;
-    ins f "store i32 %ld, ptr %s, align 1" sentinel_word p;
+    ins f "store i32 %s, ptr %s, align 1" word p;
     let n = fresh f in
     ins f "%s = add i64 %s, 1" n c;
     ins f "store i64 %s, ptr %s" n i;
@@ -1236,7 +1264,15 @@ let emit_sentinel f ptr ty =
   for k = 0 to tail - 1 do
     let p = fresh f in
     ins f "%s = getelementptr i8, ptr %s, i64 %d" p ptr (words * 4 + k);
-    ins f "store i8 %d, ptr %s, align 1" (List.nth sentinel_bytes k) p
+    match folded with
+    | Some v ->
+      ins f "store i8 %d, ptr %s, align 1" (List.nth (bytes_of_pattern v) k) p
+    | None ->
+      let sh = fresh f in
+      ins f "%s = lshr i32 %s, %d" sh word (8 * k);
+      let b = fresh f in
+      ins f "%s = trunc i32 %s to i8" b sh;
+      ins f "store i8 %s, ptr %s, align 1" b p
   done
 
 (* ── Constants ─────────────────────────────────────────────────────── *)
@@ -1706,9 +1742,10 @@ and value_at f (e : Tast.expr) : string =
     let tmp = alloca f ty in
     emit_byte_fill f tmp ty bv;
     load f tmp ty
-  | Tast.Sentinel ty ->
+  | Tast.DeadBeef (ty, pat) ->
+    let word, folded = dead_beef_word f pat in
     let tmp = alloca f ty in
-    emit_sentinel f tmp ty;
+    emit_dead_beef f tmp ty ~word ~folded;
     load f tmp ty
   | Tast.Local _ | Tast.Global _ | Tast.Field _ | Tast.Deref _ ->
     (* Everything that denotes a location is a load from its address. *)
@@ -1769,7 +1806,9 @@ and value_at f (e : Tast.expr) : string =
         the bulk zero above takes, and the reason [(set grid (filled 0xFF))]
         is one memset. *)
      | Tast.Fill (_, b) -> emit_byte_fill f ptr ty (value f b)
-     | Tast.Sentinel _ -> emit_sentinel f ptr ty
+     | Tast.DeadBeef (_, pat) ->
+       let word, folded = dead_beef_word f pat in
+       emit_dead_beef f ptr ty ~word ~folded
      | _ ->
        let v' = value f v in
        ins f "store %s %s, ptr %s" (ll ty) v' ptr);
@@ -1896,6 +1935,24 @@ and load f ptr ty =
   ins f "%s = load %s, ptr %s" t (ll ty) ptr;
   t
 
+(* The word a [(dead-beef PATTERN)] store must leave, and whether it is known
+   here. A literal pattern is reversed at compile time and reaches the loop as
+   an immediate, which is what keeps the default's code exactly what it was
+   before the pattern became an operand. Anything else is evaluated and
+   reversed by [llvm.bswap.i32] — the pattern is a u32 the program computed,
+   and the byte order it is written in is not a property of how it was
+   spelled. The [int32] comes back so the tail can fold too. *)
+and dead_beef_word f (pat : Tast.expr) : string * int32 option =
+  match pat.Tast.e with
+  | Tast.Int (n, _) ->
+    let v = word_of_pattern (Int64.to_int32 n) in
+    (Printf.sprintf "%ld" v, Some (Int64.to_int32 n))
+  | _ ->
+    let v = value f pat in
+    let t = fresh f in
+    ins f "%s = call i32 @llvm.bswap.i32(i32 %s)" t v;
+    (t, None)
+
 (* The address of an expression that denotes a location. Anything else is
    spilled to a temporary first, so [(at (f) 0)] on a returned array works. *)
 and addr f (e : Tast.expr) : string =
@@ -3664,6 +3721,7 @@ let header = {|; Generated by flan. The layout is C's: no object headers anywher
 @flan_frame_head = external global ptr
 
 declare void @llvm.memset.p0.i64(ptr nocapture writeonly, i8, i64, i1 immarg)
+declare i32 @llvm.bswap.i32(i32)
 declare void @flan_rt_init(i32, ptr)
 declare void @flan_argv(ptr)
 declare void @flan_write_stdout(ptr, i64)

lib/js.ml

@@ -674,7 +674,7 @@ let rec value f (e : Tast.expr) : string =
      storage to write over: a Flan struct is a JS object here, not a run of
      bytes, so there is nothing for 0xFF or DEADBEEF to mean. Refused by name
      rather than approximated, which is this file's rule. *)
-  | Tast.Fill _ | Tast.Sentinel _ ->
+  | Tast.Fill _ | Tast.DeadBeef _ ->
     unsupported
       "js: a byte fill has no meaning on this backend — a struct is an \
        object here and not a run of bytes"

lib/tast.ml

@@ -81,14 +81,21 @@ and expr_kind =
      that byte is an *expression* — a memset's operand, not a literal — and
      every reader of [Zero] treats it as a leaf with nothing under it.
 
-     [(sentinel-filled)]: the four bytes DE AD BE EF repeating, ascending
+     [(dead-beef PATTERN)]: the pattern's four bytes repeating, ascending
-     through the storage, so a hex dump reads DEADBEEF. It carries no operand
+     through the storage, so a hex dump reads the pattern left to right —
-     because the pattern is fixed; that is the whole point of it, and a
+     DEADBEEF by default, and whatever u32 was written otherwise. The operand
-     parameterised one would be a different builtin. A size that is not a
+     is always present by the time it reaches here: [(dead-beef)] is checked
-     multiple of four ends on a prefix of the pattern — DE, DE AD, DE AD BE —
+     into [(dead-beef 0xDEADBEEF)], so a backend has one shape to lower and
-     which both backends produce identically. *)
+     the default cannot acquire a code path of its own. A size that is not a
+     multiple of four ends on a prefix of the pattern — DE, DE AD, DE AD BE
+     for the default — which both backends produce identically.
+
+     The operand is an expression and not an [int32] for the same reason
+     [Fill]'s byte is: it may be computed. A literal one is folded by each
+     backend into the immediate it always was; anything else is evaluated and
+     byte-reversed at run time. *)
   | Fill     of Types.t * expr
-  | Sentinel of Types.t
+  | DeadBeef of Types.t * expr
   | Local  of int                     (* slot index into the frame *)
   | Global of string
   | Prim   of prim * expr list
@@ -373,8 +380,12 @@ let rec walk (f : expr -> unit) (e : expr) =
   let gos = List.iter go in
   match e.e with
   | Int _ | Float _ | Bool _ | Str _ | Unit | Zero _ | Uninit _ | Local _
-  | Global _ | None_ | FnAddr _ | Break _ | Continue _ | Sentinel _ -> ()
+  | Global _ | None_ | FnAddr _ | Break _ | Continue _ -> ()
-  | Fill (_, b) -> go b
+  (* Both carry an operand, and it must be walked: a call written inside a
+     fill's byte or a dead-beef's pattern is a call, and [Reach] roots what it
+     finds here. A leaf row would drop it silently — the match would still
+     compile. *)
+  | Fill (_, b) | DeadBeef (_, b) -> go b
   | Prim (_, es) | Call (_, es) | Do es | Make (_, es) | MakeCase (_, _, es)
   | Arr es | InvokeRestart (_, _, es, _, _, _) -> gos es
   | CallPtr (c, es) -> go c; gos es
@@ -412,6 +423,13 @@ and walk_place f (p : place) =
    rather than an error. A data type case written into the image would need a
    byte-level encoder that could not encode a string field at all; written as a
    store at startup it needs nothing. *)
+(* The pattern [(dead-beef)] means, and the name the builtin is spelled after.
+   It lives here rather than in a backend because it is a fact about the
+   language — what the bare form of a builtin means — and because the checker
+   is what writes it in: a [DeadBeef] node always carries its pattern, so no
+   emitter has a default of its own to keep in step with this one. *)
+let dead_beef_default = 0xDEADBEEFl
+
 let rec const_init (e : expr) =
   match e.e with
   | Int _ | Float _ | Bool _ | Str _ | Unit | Zero _ | Uninit _ | None_ -> true

lib/x86.ml

@@ -407,6 +407,21 @@ let rep_stosb b = u8 b 0xf3; u8 b 0xaa
    string instruction here has no such limit. *)
 let rep_stosd b = u8 b 0xf3; u8 b 0xab
 
+(* [bswap] on a 32-bit register: the byte reversal a computed [(dead-beef V)]
+   pattern needs, because V's ascending bytes are its big-endian ones and a
+   store leaves them little-endian. [Emit.word_of_pattern] is the same
+   operation folded at compile time for a literal. No REX.W — this is the
+   32-bit form, and the 64-bit one would reverse eight bytes. *)
+let bswap32 b ~dst =
+  rex b ~w:false ~r:0 ~x:0 ~m:dst; u8 b 0x0f; u8 b (0xc8 lor (dst land 7))
+
+(* Logical shift right by an immediate, which the tail of a computed pattern
+   walks its bytes with. [shift_cl] above is the same opcode group taking the
+   count in [cl]; this one takes it in the instruction, so it does not need
+   [rcx] — and [rcx] is exactly what [rep stosd] has just left at zero. *)
+let shr_imm b ~dst ~n =
+  rex b ~w:true ~r:0 ~x:0 ~m:dst; u8 b 0xc1; modrm_r b ~r:5 ~m:dst; u8 b n
+
 (* ── SSE ─────────────────────────────────────────────────────────────── *)
 
 let sse_rm b ~pfx ~op ~r ~mm = mem_op b ~r ~op:[ 0x0f; op ] ~w:false ~pfx:[ pfx ] ~mm
@@ -1609,7 +1624,7 @@ and lower_at f (e : Tast.expr) (dst : loc) : unit =
      value straight into the place's location, so filling a place and filling
      a temporary are already the same line. *)
   | Tast.Fill (ty, b) -> fill_value f dst ty b
-  | Tast.Sentinel ty -> sentinel_value f dst ty
+  | Tast.DeadBeef (ty, pat) -> dead_beef_value f dst ty pat
   | Tast.None_ -> zero_value f dst t
   (* Reading an uninitialised value gives whatever the slot held: stable
      garbage rather than LLVM's [poison]. The one construct where the two
@@ -2144,24 +2159,48 @@ and fill_value f (dst : loc) (ty : Types.t) (b : Tast.expr) =
     rep_stosb f.b
   end
 
-and sentinel_value f (dst : loc) (ty : Types.t) =
+and dead_beef_value f (dst : loc) (ty : Types.t) (pat : Tast.expr) =
   let n = sizeof f.md ty in
   let words = n / 4 and tail = n mod 4 in
+  (* Whether the word is known here. A literal is reversed at compile time and
+     loaded as an immediate, which leaves the default's code exactly what it
+     was before the pattern became an operand; anything else is evaluated and
+     reversed by [bswap]. The evaluation happens first in both cases, because
+     it is an arbitrary expression that may call and a call clobbers [rdi]. *)
+  let folded =
+    match pat.Tast.e with
+    | Tast.Int (v, _) -> Some (Int64.to_int32 v)
+    | _ -> None
+  in
+  let load_word () =
+    match folded with
+    | Some v ->
+      movabs f.b ~dst:rax
+        (Int64.logand (Int64.of_int32 (Emit.word_of_pattern v)) 0xFFFFFFFFL)
+    | None ->
+      let l = eval f pat in
+      load_int f.b ~dst:rax ~mm:(lmem f l ~scratch:r11) ~size:4 ~signed:false;
+      bswap32 f.b ~dst:rax
+  in
+  if n > 0 then begin
+    load_word ();
     if words > 0 then begin
       addr_into f ~reg:rdi dst;
-    movabs f.b ~dst:rax
-      (Int64.logand (Int64.of_int32 Emit.sentinel_word) 0xFFFFFFFFL);
       movabs f.b ~dst:rcx (Int64.of_int words);
+      (* [rep stosd] touches rdi and rcx and leaves rax alone, which is what
+         lets the tail below keep reading the word out of it. *)
       rep_stosd f.b
     end;
-  List.iteri
+    (* The tail is the first up-to-three bytes of the stored word, lowest
-    (fun k byte ->
+       address first — byte k is (word >> 8k) & 0xFF either way. A folded word
-       if k < tail then begin
+       could store constants instead, but walking [rax] covers both with one
-         movabs f.b ~dst:rax (Int64.of_int byte);
+       sequence, and the count is at most three. *)
+    for k = 0 to tail - 1 do
+      if k > 0 then shr_imm f.b ~dst:rax ~n:8;
       store_int f.b ~src:rax
         ~mm:(lmem f (shift dst (words * 4 + k)) ~scratch:r11) ~size:1
-       end)
+    done
-    Emit.sentinel_bytes
+  end
 
 and zero_value f (dst : loc) (ty : Types.t) =
   if is_agg ty then zero_loc f dst (sizeof f.md ty)

test/programs/fill.flan

@@ -1,16 +1,22 @@
-;;;; (filled b) and (sentinel-filled) — the two byte fills, read back as bytes.
+;;;; (filled b) and (dead-beef), the two byte fills, read back as bytes.
 ;;;;
 ;;;; The whole point of this program is that every row is a *byte* and not a
-;;;; value: the sentinel's contract is "a hex dump reads DEADBEEF", which is a
+;;;; value: the pattern fill's contract is "a hex dump reads the pattern left
-;;;; claim about which byte lands at which address, and only reading the bytes
+;;;; to right", which is a claim about which byte lands at which address, and
-;;;; back in address order can check it. The u32 rows are the same claim from
+;;;; only reading the bytes back in address order can check it. The u32 rows
-;;;; the other side — a little-endian load of DE AD BE EF is 0xEFBEADDE, which
+;;;; are the same claim from the other side — a little-endian load of
-;;;; is 4022250974, so a backend that wrote the word the other way round would
+;;;; DE AD BE EF is 0xEFBEADDE, which is 4022250974, so a backend that wrote
-;;;; print 3735928559 here and be caught.
+;;;; the word the other way round would print 3735928559 here and be caught.
 ;;;;
 ;;;; The lengths are chosen for the tail. 8 is a whole number of patterns, 9
 ;;;; ends on DE, 6 ends on DE AD, and 7 ends on DE AD BE — the three truncated
 ;;;; endings and the one that is not truncated at all.
+;;;;
+;;;; The rows that matter most are the last group: a pattern that is *not* a
+;;;; literal, over a length that is not a multiple of four. That is the case a
+;;;; constant-only implementation would pass by accident — the word reaches
+;;;; the loop in a register and the tail bytes have to be shifted out of it
+;;;; rather than folded, on both backends.
 (defstruct Words [a u32  b u32])
 
 ;;;; A computed global initialiser: a fill is never a constant the linker can
@@ -21,6 +27,19 @@
 (defn bytes4 [b [4 u8]] ()
   (dotimes [i 4] (print (at b i)) (print " ")))
 
+;;;; A u32 that no literal rule can see through: (u32 0xBAADF00D) will not do,
+;;;; because the cast checks its operand as an i32 first and 0xBAADF00D is not
+;;;; one. A parameter's declared type is what makes the literal a u32, and it
+;;;; is also what stops the checker folding it at the fill.
+(defn u32of [x u32] u32 x)
+
+;;;; The pattern crosses a call boundary, so nothing can fold it at the fill.
+(defn beef7 [pat u32] ()
+  (let [a (array 7 u8)]
+    (set a (dead-beef pat))
+    (dotimes [i 7] (print (at a i)) (print " "))
+    (println "")))
+
 (defn main [] i32
   ;; One byte, repeated. 0xFF is the -1 fill a debug allocator wants.
   (let [a (array 4 u8)]
@@ -43,30 +62,48 @@
 
   ;; Four bytes, ascending, with nothing truncated.
   (let [a (array 8 u8)]
-    (set a (sentinel-filled))
+    (set a (dead-beef))
     (dotimes [i 8] (print (at a i)) (print " "))
     (println ""))                             ; 222 173 190 239 x2
 
+  ;; The bare form is the spelled-out default, and this is the row that says
+  ;; so: the same eight bytes from (dead-beef 0xDEADBEEF).
+  (let [a (array 8 u8)]
+    (set a (dead-beef 0xDEADBEEF))
+    (dotimes [i 8] (print (at a i)) (print " "))
+    (println ""))                             ; identical to the row above
+
   ;; The three truncated tails.
   (let [a (array 9 u8)]
-    (set a (sentinel-filled))
+    (set a (dead-beef))
     (dotimes [i 9] (print (at a i)) (print " "))
     (println ""))                             ; ... ends on 222
 
   (let [a (array 6 u8)]
-    (set a (sentinel-filled))
+    (set a (dead-beef))
     (dotimes [i 6] (print (at a i)) (print " "))
     (println ""))                             ; ... ends on 222 173
 
   (let [a (array 7 u8)]
-    (set a (sentinel-filled))
+    (set a (dead-beef))
     (dotimes [i 7] (print (at a i)) (print " "))
     (println ""))                             ; ... ends on 222 173 190
 
+  ;; A pattern of the program's own choosing, laid down left to right the same
+  ;; way: 0xBAADF00D is BA AD F0 0D, which is 186 173 240 13.
+  (let [a (array 6 u8)]
+    (set a (dead-beef 0xBAADF00D))
+    (dotimes [i 6] (print (at a i)) (print " "))
+    (println ""))                             ; 186 173 240 13 186 173
+
   ;; The same pattern read as words rather than as bytes, which is what says
   ;; which end the DE is at.
   (let [w (Words {})]
-    (set w (sentinel-filled))
+    (set w (dead-beef))
+    (print (.a w)) (print " ") (print (.b w)) (println ""))
+
+  (let [w (Words {})]
+    (set w (dead-beef 0xBAADF00D))
     (print (.a w)) (print " ") (print (.b w)) (println ""))
 
   ;; A nested aggregate: a fixed array of structs is plain data all the way
@@ -78,11 +115,30 @@
   ;; The globals. [gfill] was filled before main ran; [gsent] is filled here.
   (dotimes [i 4] (print (at gfill i)) (print " "))
   (println "")                                ; 65 65 65 65
-  (set gsent (sentinel-filled))
+  (set gsent (dead-beef))
   (dotimes [i 5] (print (at gsent i)) (print " "))
   (println "")                                ; 222 173 190 239 222
 
   ;; A fill in value position rather than as the value of a [set]: the same
   ;; bytes, reached through a temporary instead of through the place.
   (bytes4 (filled 0xFF)) (println "")         ; 255 255 255 255
+
+  ;; ── The computed pattern ──────────────────────────────────────────
+  ;; None of these can be folded: the pattern is a parameter, or arithmetic
+  ;; the checker does not evaluate. Seven bytes is one whole pattern and a
+  ;; three-byte tail, so the tail bytes have to come out of the register the
+  ;; word is in rather than out of a constant.
+  (beef7 0xDEADBEEF)                          ; 222 173 190 239 222 173 190
+  (beef7 0xBAADF00D)                          ; 186 173 240 13 186 173 240
+  (let [p (u32of 0xBAADF00D)
+        a (array 6 u8)]
+    (set a (dead-beef p))
+    (dotimes [i 6] (print (at a i)) (print " "))
+    (println ""))                             ; 186 173 240 13 186 173
+  ;; A pattern that is genuinely computed, not merely held in a slot, and read
+  ;; back as a word so the byte order of the runtime path is pinned too.
+  (let [p (u32of 0xBAAD0000)
+        w (Words {})]
+    (set w (dead-beef (bit-or p (u32of 0xF00D))))
+    (print (.a w)) (print " ") (print (.b w)) (println ""))
   0)

test/test_acceptance.ml

@@ -1256,20 +1256,42 @@ let () =
        rows are the same four bytes read back as a u32 — 0xEFBEADDE — and they
        are what a backend that wrote the word the other way round would fail.
        The three short rows are the truncated tails a size that is not a
-       multiple of four ends on. *)
+       multiple of four ends on.
+
+       Rows four and five are identical on purpose: [(dead-beef)] and
+       [(dead-beef 0xDEADBEEF)] are the same node by construction, and this is
+       where that stops being a claim about the checker and becomes one about
+       the program.
+
+       The last four rows are the ones a constant-only implementation would
+       pass by accident. Their pattern arrives in a register — a parameter, a
+       slot, and a [bit-or] the checker does not fold — so the word is
+       byte-reversed at run time and the tail bytes are shifted out of it
+       rather than folded. 186 173 240 13 is 0xBAADF00D read left to right,
+       and the closing 233876922 is that same pattern's stored word, which is
+       the runtime path's byte order pinned against the literal path's — the
+       identical number appears above it from a folded [(dead-beef
+       0xBAADF00D)]. *)
     let fill_out =
       "255 255 255 255 \n7 7 7 7 \n0 0 0 0 \n\
+       222 173 190 239 222 173 190 239 \n\
        222 173 190 239 222 173 190 239 \n\
        222 173 190 239 222 173 190 239 222 \n\
        222 173 190 239 222 173 \n\
        222 173 190 239 222 173 190 \n\
-       4022250974 4022250974\n4294967295 4294967295\n\
+       186 173 240 13 186 173 \n\
-       65 65 65 65 \n222 173 190 239 222 \n255 255 255 255 \n"
+       4022250974 4022250974\n233876922 233876922\n\
+       4294967295 4294967295\n\
+       65 65 65 65 \n222 173 190 239 222 \n255 255 255 255 \n\
+       222 173 190 239 222 173 190 \n\
+       186 173 240 13 186 173 240 \n\
+       186 173 240 13 186 173 \n\
+       233876922 233876922\n"
     in
-    outputs "byte and sentinel fills" "programs/fill.flan" fill_out;
+    outputs "byte and dead-beef fills" "programs/fill.flan" fill_out;
-    outputs ~opt:"-O0" "byte and sentinel fills, -O0" "programs/fill.flan"
+    outputs ~opt:"-O0" "byte and dead-beef fills, -O0" "programs/fill.flan"
       fill_out;
-    outputs ~x86:true "byte and sentinel fills, --x86" "programs/fill.flan"
+    outputs ~x86:true "byte and dead-beef fills, --x86" "programs/fill.flan"
       fill_out;
     (* dyn if: truthiness -- M2 queue item 7. A dyn scrutinee is tested for
        nil/false vs. everything else, Clojure's rule, on both backends; a

test/test_flan.ml

@@ -1957,7 +1957,7 @@ let () =
     ~needle:"steers every read of it";
 
   (* ── What may be filled with raw bytes ─────────────────────────────
-     [(filled b)] and [(sentinel-filled)] are [zeroed]'s siblings, and the
+     [(filled b)] and [(dead-beef)] are [zeroed]'s siblings, and the
      boundary is the whole of what is new about them: zero is a value every
      type can have and 0xDE is not, so the checker says which types survive
      arbitrary bytes. The accepting side is programs/fill.flan; what is here
@@ -1969,12 +1969,19 @@ let () =
      one is a root pointing at nothing. *)
   accepts "a fixed array of numbers may be filled"
     "(defn f [] () (let [a (array 4 u8)] (set a (filled 0xFF))))";
-  accepts "a struct of numbers may be sentinel-filled"
+  accepts "a struct of numbers may be dead-beefed"
     "(defstruct S [a i32 b f64]) \
-     (defn f [] () (let [s (S {})] (set s (sentinel-filled))))";
+     (defn f [] () (let [s (S {})] (set s (dead-beef))))";
+  (* Both arities, and a pattern that is not a literal — the operand is an
+     ordinary u32 expression, which is the byte arm's rule at four times the
+     width. *)
+  accepts "dead-beef takes a pattern"
+    "(defn f [] () (let [a (array 4 u8)] (set a (dead-beef 0xBAADF00D))))";
+  accepts "dead-beef takes a computed pattern"
+    "(defn f [p u32] () (let [a (array 4 u8)] (set a (dead-beef p))))";
   rejects_check "a struct holding a dyn cannot be filled"
     "(defstruct S [a i32 d dyn]) \
-     (defn f [] () (let [s (S {})] (set s (sentinel-filled))))"
+     (defn f [] () (let [s (S {})] (set s (dead-beef))))"
     ~needle:"a root pointing at nothing";
   rejects_check "a Vec cannot be filled"
     "(defn f [] () (let [v (vec-new i32)] (set v (filled 0xFF))))"
@@ -2009,8 +2016,8 @@ let () =
   rejects_check "a fill in a position with no expected type"
     "(defn f [] () (print (filled 0xFF)))"
     ~needle:"needs to know the type it is filling";
-  rejects_check "a sentinel fill in a position with no expected type"
+  rejects_check "a dead-beef in a position with no expected type"
-    "(defn f [] () (print (sentinel-filled)))"
+    "(defn f [] () (print (dead-beef)))"
     ~needle:"needs to know the type it is filling";
   (* The byte is a u8 and the ordinary literal rule applies to it — there is
      no range check of this builtin's own, and there does not need to be. *)
@@ -2020,9 +2027,16 @@ let () =
   rejects_check "filled takes exactly one byte"
     "(defn f [] () (let [a (array 4 u8)] (set a (filled))))"
     ~needle:"takes 1 argument";
-  rejects_check "sentinel-filled takes no argument"
+  rejects_check "dead-beef takes at most one pattern"
-    "(defn f [] () (let [a (array 4 u8)] (set a (sentinel-filled 1))))"
+    "(defn f [] () (let [a (array 4 u8)] (set a (dead-beef 1 2))))"
-    ~needle:"takes 0 arguments, given 1";
+    ~needle:"takes the pattern or nothing at all, given 2";
+  (* The pattern is four bytes, so a wider literal is a typo rather than
+     something to truncate. The refusal is [in_range]'s, located at the
+     literal — this builtin has no range check of its own and does not need
+     one, exactly as the byte arm does not. *)
+  rejects_check "a dead-beef pattern out of u32 range"
+    "(defn f [] () (let [a (array 4 u8)] (set a (dead-beef 0x1DEADBEEF))))"
+    ~needle:"does not fit in u32";
   (* A fill is never a value the linker can write into the image, so a
      defconst of one is refused by the constant rule rather than by anything
      of this feature's own. A defvar is fine: its initialiser runs at