Joseph Ferano
9ce51ba94e
as-slice was a warning, not an operation. The input type already decides which of the two things happens — a Vec can only be borrowed, an array or a string can only be viewed, and no call site picks between them — so the second name expressed no choice a reader could make. And it warned at the moment the view is taken, which is the one moment nothing is wrong; the danger arrives later, at the push. slice now takes a Vec at all three arities and as-slice is gone. (slice v lo) was free, and is the arity the Vec never had: the runtime already reads a hi of -1 as "to the end", so the tail form passes the caller's lo and the same -1 — no slot, no length read, no second evaluation. The merge is entirely in the checker; the Vec path builds the flan_vec_as_slice call it always built and neither backend has a line about any of it. A Vec a call returned is refused at every arity, and not for the array's reason. (slice (mk)) over an array dangles. (slice (make-vec)) does not — the storage outlives the expression — but the header is a temporary, so nothing can ever free the block. The refusal says that and names the let. The name's own refusal sits in ordinary_call after every table, so a program that defines an as-slice still reaches its own. It reads for somebody who has never heard of the old name and writes the call back out, spelling each argument that is a name or a number. The warning moved to where it bites: BUILT.md gains a section beside the Vec table and the push row points at it, spec-memory.md's Borrowing says the same. Investigated and deliberately not built — a diagnostic for a live view at the push. (reserve v 100) then a slice, a push and a read is correct code under the contract the spec chose, so any flag on it is a false positive by the language's own semantics rather than by an approximation. FIX.org has the finding and the syntactic sketch that does not work.
48 lines
2.3 KiB
Plaintext
48 lines
2.3 KiB
Plaintext
;;;; M2 queue item 5: typed = and != grow strings. Bytewise, with a
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;;;; length-mismatch fast path and a same-pointer fast path ahead of the byte
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;;;; loop (runtime/flan_rt.c, flan_str_eq). Ordering stays refused on a
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;;;; string -- that half is tested in test_flan.ml, because a program that
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;;;; wrote (< "a" "b") would not compile and so cannot be a row here.
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(defn main [] i32
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;; Same pointer: one local read twice is the same two words, ptr and len
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;; both, and the fast path answers before a single byte is looked at.
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(let [s "same"]
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(println (= s s)) ; true
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(println (!= s s))) ; false
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;; Differing lengths: the length check alone settles it, and never reaches
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;; the byte loop -- a common prefix would be no evidence otherwise.
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(println (= "abc" "ab")) ; false
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(println (!= "abc" "ab")) ; true
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;; Equal contents, distinct pointers. "abc" the literal lives in the
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;; read-only data section; to-lower of "ABC" is a fresh heap allocation,
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;; so this pair shares no address and the same-pointer fast path cannot
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;; fire -- what answers here is the byte loop, or the length check first
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;; ruling nothing out since both are three bytes.
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(let [heap (to-lower (bytes-view "ABC"))]
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(let [h (string (slice heap))]
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(println (= "abc" h)) ; true
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(println (!= "abc" h)))
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(free heap))
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;; A one-byte difference at the end, so the length check cannot rule it
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;; out and the byte loop has to run to the last byte before it can answer.
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(println (= "abd" "abc")) ; false
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(println (!= "abd" "abc")) ; true
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;; Empty strings: the length check's zero case, which the runtime helper
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;; also uses to skip a memcmp that would otherwise read through a null
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;; pointer -- two empty string literals, and empty against non-empty.
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(println (= "" "")) ; true
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(println (= "" "a")) ; false
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(println (= "a" "")) ; false
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;; A slice and the prefix it was cut from: same base pointer, different
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;; lengths -- the one pair the same-pointer fast path would answer wrong on
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;; if it ran before the length check instead of after.
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(let [s "abcd"]
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(println (= s (string (slice (bytes-view s) 0 2))))) ; false
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0)
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