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https://bugs.gentoo.org/757792
From 0dbfa88edafbe913a7a9099246041e0190aa3948 Mon Sep 17 00:00:00 2001
From: Jakub Jelinek <jakub@redhat.com>
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 <jakub@redhat.com>
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.
(cherry picked from commit f982a6ec9b6d98f5f37114b1d7455c54ce5056b8)
---
gcc/testsuite/gcc.dg/strcmpopt_12.c | 17 +++++++++++++++++
gcc/tree-ssa-strlen.c | 10 +++++++++-
2 files changed, 26 insertions(+), 1 deletion(-)
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 93d095e1896..b0874da5d1e 100644
--- a/gcc/tree-ssa-strlen.c
+++ b/gcc/tree-ssa-strlen.c
@@ -4485,7 +4485,15 @@ 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;
--
2.29.2
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