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Alignment is a counter, not a property of the prologue

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Every stack movement now goes through pushv/popv and increments a depth
word on the function context. A call pads to 16 from wherever the expression
evaluator has left rsp, and asserts the parity before it emits the call.
The nested probe passes.

The stack-argument path is folded into the same counter rather than keeping
its own, because two independent notions of parity is how the bug comes back.
This commit is contained in:
Joseph Ferano 2026-09-13 09:14:44 +07:00
parent faba8a49f8
commit ec5062a0ed
1 changed files with 46 additions and 30 deletions
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74
spike/backend/x86.ml
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@ -122,6 +122,17 @@ let patch b ~at ~target =
type fnctx = { type fnctx = {
b : buf; b : buf;
nslots : int; nslots : int;
(* How many 8-byte words this expression's evaluation has pushed since the
prologue. rsp is 16-aligned at the end of the prologue, so [depth] even
means rsp is aligned and [depth] odd means it is 8 out.
This counter is the answer to the one bug the ABI probe found. Alignment
is not a property of the prologue: the evaluator spills the left operand
across the right one's evaluation, so a call written in the right operand
runs with one word outstanding. Deriving it from a count kept here is the
only way that stays correct as the evaluator grows cases, and it is what
clang's [sub rsp, 8] before a call is doing. *)
mutable depth : int;
(* A symbol the code calls, resolved to an absolute address by the driver (* A symbol the code calls, resolved to an absolute address by the driver
before emission. movabs + call r is what a JIT does anyway: a rel32 call before emission. movabs + call r is what a JIT does anyway: a rel32 call
cannot reach an arbitrary mmap, and the 2-byte indirect call is cheaper cannot reach an arbitrary mmap, and the 2-byte indirect call is cheaper
@ -131,6 +142,11 @@ type fnctx = {
let slot_disp i = -8 * (i + 1) let slot_disp i = -8 * (i + 1)
(* Every stack movement goes through these two, so that nothing can move rsp
without the counter noticing. *)
let pushv f r = push f.b r; f.depth <- f.depth + 1
let popv f r = pop f.b r; f.depth <- f.depth - 1
(* Every type this spike handles is one 8-byte integer register. Everything (* Every type this spike handles is one 8-byte integer register. Everything
else is the real backend's problem and is enumerated in the verdict rather else is the real backend's problem and is enumerated in the verdict rather
than guessed at here. *) than guessed at here. *)
@ -234,10 +250,10 @@ and prim f e (p : Flan.Tast.prim) args =
and binop f x y = and binop f x y =
let b = f.b in let b = f.b in
value f x; value f x;
push b rax; pushv f rax;
value f y; value f y;
mov_rr b ~dst:rcx ~src:rax; mov_rr b ~dst:rcx ~src:rax;
pop b rax popv f rax
and emit_if f c t e = and emit_if f c t e =
let b = f.b in let b = f.b in
@ -267,41 +283,41 @@ and emit_if f c t e =
and call f (addr : int64) args ~xfer = and call f (addr : int64) args ~xfer =
let b = f.b in let b = f.b in
let n = List.length args + (if xfer then 1 else 0) in let n = List.length args + (if xfer then 1 else 0) in
if n > 6 then begin (* Bring rsp to 16 first, so everything below can count in pairs. *)
(* The stack half. Evaluate every stacked argument first, right to left, let pad = f.depth land 1 = 1 in
leaving them pushed, then fill the registers -- otherwise evaluating a if pad then (sub_imm32 b ~dst:rsp 8; f.depth <- f.depth + 1);
stacked argument would clobber a register already loaded. *)
let stacked = List.filteri (fun i _ -> i >= 6) args in let stacked = List.filteri (fun i _ -> i >= 6) args in
let nstack = List.length stacked + (if xfer then 1 else 0) in let nstack = List.length stacked + (if xfer && n > 6 then 1 else 0) in
if nstack land 1 = 1 then sub_imm32 b ~dst:rsp 8; (* The stack half, evaluated right to left so that the seventh argument ends
if xfer then (movabs b ~dst:rax 0L; push b rax); up at [rsp] and the eighth above it. The transfer channel is the last
List.iter (fun a -> value f a; push b rax) (List.rev stacked) argument of all, so it is pushed first. *)
end; if nstack land 1 = 1 then (sub_imm32 b ~dst:rsp 8; f.depth <- f.depth + 1);
(* The register half, and it needs a spill: rdi..r9 are argument registers if xfer && n > 6 then (movabs b ~dst:rax 0L; pushv f rax);
List.iter (fun a -> value f a; pushv f rax) (List.rev stacked);
(* The register half needs a spill of its own: rdi..r9 are argument registers
and rax is where every value lands, so an earlier argument would be and rax is where every value lands, so an earlier argument would be
clobbered by a later one's evaluation. Push each, then pop them into clobbered by a later one's evaluation. Push each, then pop them into their
their registers in reverse. *) registers in reverse. *)
let inreg = List.filteri (fun i _ -> i < 6) args in let inreg = List.filteri (fun i _ -> i < 6) args in
List.iter (fun a -> value f a; push b rax) inreg; List.iter (fun a -> value f a; pushv f rax) inreg;
let nreg = List.length inreg in let nreg = List.length inreg in
List.iteri (fun i _ -> pop b arg_regs.(nreg - 1 - i)) inreg; List.iteri (fun i _ -> popv f arg_regs.(nreg - 1 - i)) inreg;
if xfer && n <= 6 then movabs b ~dst:arg_regs.(nreg) 0L; if xfer && n <= 6 then movabs b ~dst:arg_regs.(nreg) 0L;
(* al = number of vector registers used. Required only for a variadic (* al = the number of vector registers used. Required only for a variadic
callee, and set unconditionally because it is free and a wrong al on a callee and set unconditionally because it is two bytes: a wrong al on a
printf-shaped raylib entry point (TraceLog is one) is a crash that looks printf-shaped entry point -- raylib's TraceLog is one -- is a crash that
like anything else. It must be set *after* the argument registers, since looks like anything else. After the argument registers, since al is rax's
al is rax's low byte. *) low byte. *)
u8 b 0xb0; u8 b 0x00; (* mov al, 0 *) u8 b 0xb0; u8 b 0x00; (* mov al, 0 *)
(* r11 always, never r9: r11 is the scratch register SysV reserves and is the (* r11 always, never r9: r11 is the scratch register SysV reserves and is the
one 64-bit register guaranteed not to be carrying an argument. Picking the one register guaranteed not to be carrying an argument. Choosing the
target conditionally is how a call with six arguments gets quietly wrong. *) target conditionally is how a six-argument call gets quietly wrong. *)
u8 b 0x49; u8 b 0xbb; u64 b addr; (* movabs r11, addr *) u8 b 0x49; u8 b 0xbb; u64 b addr; (* movabs r11, addr *)
assert (f.depth land 1 = 0);
call_r b 11; call_r b 11;
if n > 6 then begin let back = 8 * (nstack + (nstack land 1)) in
let nstack = List.length args - 6 + (if xfer then 1 else 0) in if back > 0 then (add_imm32 b ~dst:rsp back; f.depth <- f.depth - (back / 8));
let pop_bytes = 8 * (nstack + (nstack land 1)) in if pad then (add_imm32 b ~dst:rsp 8; f.depth <- f.depth - 1)
add_imm32 b ~dst:rsp pop_bytes
end
and node_name (k : Flan.Tast.expr_kind) = and node_name (k : Flan.Tast.expr_kind) =
match k with match k with
@ -331,7 +347,7 @@ and node_name (k : Flan.Tast.expr_kind) =
let fn ~resolve (fd : Flan.Tast.fn) : string = let fn ~resolve (fd : Flan.Tast.fn) : string =
let b = create () in let b = create () in
let nslots = Array.length fd.Flan.Tast.slots in let nslots = Array.length fd.Flan.Tast.slots in
let f = { b; nslots; resolve } in let f = { b; nslots; resolve; depth = 0 } in
push b rbp; push b rbp;
mov_rr b ~dst:rbp ~src:rsp; mov_rr b ~dst:rbp ~src:rsp;
(* Round the frame to 16 so that rsp is aligned at every call site. One (* Round the frame to 16 so that rsp is aligned at every call site. One