Bug 112113 - [14 Regression] wrong code at -O3 on x86_64-linux-gnu since r14-2852-gf5fb9ff2396fd4
Summary: [14 Regression] wrong code at -O3 on x86_64-linux-gnu since r14-2852-gf5fb9ff...
Status: RESOLVED FIXED
Alias: None
Product: gcc
Classification: Unclassified
Component: tree-optimization (show other bugs)
Version: 14.0
: P1 normal
Target Milestone: 14.0
Assignee: Richard Biener
URL:
Keywords: wrong-code
Depends on:
Blocks:
 
Reported: 2023-10-28 06:11 UTC by Zhendong Su
Modified: 2023-11-14 14:16 UTC (History)
4 users (show)

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Host:
Target:
Build:
Known to work:
Known to fail:
Last reconfirmed: 2023-10-28 00:00:00

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Description Zhendong Su 2023-10-28 06:11:16 UTC 
It appears to be a recent regression.

Compiler Explorer: https://godbolt.org/z/vaY9GEnrz


[549] % gcctk -v
Using built-in specs.
COLLECT_GCC=gcctk
COLLECT_LTO_WRAPPER=/local/suz-local/software/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/14.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk/configure --disable-bootstrap --enable-checking=yes --prefix=/local/suz-local/software/local/gcc-trunk --enable-sanitizers --enable-languages=c,c++ --disable-werror --enable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 14.0.0 20231028 (experimental) (GCC) 
[550] % 
[550] % gcctk -O2 small.c; ./a.out
[551] % 
[551] % gcctk -O3 small.c
small.c: In function ‘main’:
small.c:3:29: warning: iteration 1 invokes undefined behavior [-Waggressive-loop-optimizations]
    3 |   for (int b = 0; b < 33; b += 3) {
      |                           ~~^~~~
small.c:3:21: note: within this loop
    3 |   for (int b = 0; b < 33; b += 3) {
      |                   ~~^~~~
[552] % ./a.out
Aborted
[553] % 
[553] % cat small.c
volatile int a;
int main() {
  for (int b = 0; b < 33; b += 3) {
    if (b > 31)
      a++;
  }
  if (a != 0)
    __builtin_abort();
  return 0;
}
Comment 1 Andrew Pinski 2023-10-28 06:17:46 UTC 
Confirmed. -fno-split-loops  fixes it ...
Comment 2 Thomas Koenig 2023-10-28 22:06:12 UTC 
According to bisection, f5fb9ff2396fd41fdd2e6d35a412e936d2d42f75
is the first bad commit.
Comment 3 Thomas Koenig 2023-10-28 22:10:30 UTC 
(In reply to Thomas Koenig from comment #2)
> According to bisection, f5fb9ff2396fd41fdd2e6d35a412e936d2d42f75
> is the first bad commit.

Or, if anybody wants a link, https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=f5fb9ff2396fd41fdd2e6d35a412e936d2d42f75 .
Comment 4 Andrew Pinski 2023-10-29 22:48:17 UTC 
Looks almost the same code as what was in PR 111652 (maybe it is a dup after all).
Comment 5 Richard Biener 2023-11-14 13:19:16 UTC 
patch_loop_exit simply copies the original guard code rather than using the analyzed one.  Testing a patch.
Comment 6 GCC Commits 2023-11-14 14:13:57 UTC 
The master branch has been updated by Richard Biener <rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:2066c29bf8dea87c9810eb261e342d941a6b2059

commit r14-5446-g2066c29bf8dea87c9810eb261e342d941a6b2059
Author: Richard Biener <rguenther@suse.de>
Date:   Tue Nov 14 14:26:17 2023 +0100

    tree-optimization/111233 - loop splitting miscompile
    
    The change in r14-2852-gf5fb9ff2396fd4 failed to update patch_loop_exit
    to compensate for rewriting of a NE/EQ_EXPR to a new code.  Fixed
    with the following.
    
            PR tree-optimization/111233
            PR tree-optimization/111652
            PR tree-optimization/111727
            PR tree-optimization/111838
            PR tree-optimization/112113
            * tree-ssa-loop-split.cc (patch_loop_exit): Get the new
            guard code instead of the old guard stmt.
            (split_loop): Adjust.
    
            * gcc.dg/torture/pr111233.c: New testcase.
            * gcc.dg/torture/pr111652.c: Likewise.
            * gcc.dg/torture/pr111727.c: Likewise.
            * gcc.dg/torture/pr111838.c: Likewise.
            * gcc.dg/torture/pr112113.c: Likewise.
Comment 7 Richard Biener 2023-11-14 14:16:20 UTC 
Fixed.