46 lines
2.4 KiB
Plaintext
46 lines
2.4 KiB
Plaintext
;;;; Bounds checks, TODO.org, "An index out of range is a condition". One
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;;;; program, one case per argument, so a trap is observable: the checked build
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;;;; exits 134 with the source location on stderr.
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;;;;
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;;;; The unchecked build is not uniform, and the split is the point. An index
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;;;; past the end runs off the end there and is not asserted on — that is what
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;;;; --no-bounds-checks buys. Two of the cases below still trap: the reversed
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;;;; range at n = 2 and the negative promise at n = -2, because neither is a
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;;;; bounds check. Both are the claim that a slice's length word is a count,
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;;;; and a build that drops them does not produce an unchecked slice, it
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;;;; produces a value that is not one. See check_slice in lib/emit.ml.
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;;;;
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;;;; The selector is also the index wherever it can be, which is what keeps the
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;;;; index dynamic — a literal would let the checker reject it outright one day
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;;;; (that is a separate job) and lets LLVM fold the branch away here.
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(defonce arr [3 i32])
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(defn main [args [string]] i32
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(let [n (i32 (bytes->i64 (bytes-view (at args 1))))
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s (bytes-view "hello")] ; len 5
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(cond
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;; In bounds, including both edges: the last index, and a slice that
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;; ends exactly at len. Neither may trap.
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(= n 0) (do (print (at arr 2))
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(print (slice s 1 5))
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(print (slice s 5 5)) ; empty at len is legal
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(println ""))
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(= n 3) (print (at arr n)) ; past the end of a fixed array
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(= n -1) (print (at arr n)) ; negative index
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(= n 9) (print (at s n)) ; past the end of a slice
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;; The write path lowers through place/Pindex rather than through At, so
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;; it is checked separately even though the message is the same.
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(= n 7) (set (at arr n) 1) ; write past the end
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(= n 4) (print (slice s n 9)) ; hi past the end
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(= n 2) (print (slice s n 1)) ; reversed range
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;; (slice-from p n) has nothing to check n against — the caller's
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;; number is the only length there is — so what it checks is that the
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;; number is not absurd. Signed, deliberately: the comparisons the other
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;; two checks use are unsigned, and a negative i32 sign-extended to i64
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;; is a huge unsigned value that sails straight through them.
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(= n -2) (print (length (slice-from (addr (at arr 0)) n)))
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:else (println "?"))
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0))
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