From f982a6ec9b6d98f5f37114b1d7455c54ce5056b8 Mon Sep 17 00:00:00 2001 From: Jakub Jelinek Date: Tue, 25 Aug 2020 13:47:10 +0200 Subject: [PATCH] strlen: Fix handle_builtin_string_cmp [PR96758] The following testcase is miscompiled, because handle_builtin_string_cmp sees a strncmp call with constant last argument 4, where one of the strings has an upper bound of 5 bytes (due to it being an array of that size) and the other has a known string length of 1 and the result is used only in equality comparison. It is folded into __builtin_strncmp_eq (str1, str2, 4), which is incorrect, because that means reading 4 bytes from both strings and comparing that. When one of the strings has known strlen of 1, we want to compare just 2 bytes, not 4, as strncmp shouldn't compare any bytes beyond the null. So, the last argument to __builtin_strncmp_eq should be the minimum of the provided strncmp last argument and the known string length + 1 (assuming the other string has only a known upper bound due to array size). Besides that, I've noticed the code has been written with the intent to also support the case where we know exact string length of both strings (but not the string content, so we can't compute it at compile time). In that case, both cstlen1 and cstlen2 are non-negative and both arysiz1 and arysiz2 are negative. We wouldn't optimize that, cmpsiz would be either the strncmp last argument, or for strcmp the first string length, but varsiz would be -1 and thus cmpsiz would be never < varsiz. The patch fixes it by using the correct length, in that case using the minimum of the two and for strncmp also the last argument. 2020-08-25 Jakub Jelinek PR tree-optimization/96758 * tree-ssa-strlen.c (handle_builtin_string_cmp): If both cstlen1 and cstlen2 are set, set cmpsiz to their minimum, otherwise use the one that is set. If bound is used and smaller than cmpsiz, set cmpsiz to bound. If both cstlen1 and cstlen2 are set, perform the optimization. * gcc.dg/strcmpopt_12.c: New test. --- gcc/testsuite/gcc.dg/strcmpopt_12.c | 17 +++++++++++++++++ gcc/tree-ssa-strlen.c | 12 ++++++++++-- 2 files changed, 27 insertions(+), 2 deletions(-) create mode 100644 gcc/testsuite/gcc.dg/strcmpopt_12.c diff --git a/gcc/testsuite/gcc.dg/strcmpopt_12.c b/gcc/testsuite/gcc.dg/strcmpopt_12.c new file mode 100644 index 00000000000..d8077b62f7f --- /dev/null +++ b/gcc/testsuite/gcc.dg/strcmpopt_12.c @@ -0,0 +1,17 @@ +/* PR tree-optimization/96758 */ +/* { dg-do run } */ +/* { dg-options "-O2" } */ + +int v = 1; + +int +main () +{ + const char *s = v ? "a" : "b"; + char x[5]; + char y[5] = "a\0a"; + __builtin_memcpy (x, y, sizeof (y)); + if (__builtin_strncmp (x, s, 4) != 0) + __builtin_abort (); + return 0; +} diff --git a/gcc/tree-ssa-strlen.c b/gcc/tree-ssa-strlen.c index fbaee745f7d..96962286735 100644 --- a/gcc/tree-ssa-strlen.c +++ b/gcc/tree-ssa-strlen.c @@ -4485,11 +4485,19 @@ handle_builtin_string_cmp (gimple_stmt_iterator *gsi, const vr_values *rvals) ++cstlen2; /* The exact number of characters to compare. */ - HOST_WIDE_INT cmpsiz = bound < 0 ? cstlen1 < 0 ? cstlen2 : cstlen1 : bound; + HOST_WIDE_INT cmpsiz; + if (cstlen1 >= 0 && cstlen2 >= 0) + cmpsiz = MIN (cstlen1, cstlen2); + else if (cstlen1 >= 0) + cmpsiz = cstlen1; + else + cmpsiz = cstlen2; + if (bound >= 0) + cmpsiz = MIN (cmpsiz, bound); /* The size of the array in which the unknown string is stored. */ HOST_WIDE_INT varsiz = arysiz1 < 0 ? arysiz2 : arysiz1; - if (cmpsiz < varsiz && used_only_for_zero_equality (lhs)) + if ((varsiz < 0 || cmpsiz < varsiz) && used_only_for_zero_equality (lhs)) { /* If the known length is less than the size of the other array and the strcmp result is only used to test equality to zero, -- 2.30.2