[llvm-bugs] [Bug 50331] New: clang spend relative much time on a simple code at -O3 (affect clang-6.0 above versions)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50331

            Bug ID: 50331
           Summary: clang spend relative much time on a simple code at -O3
                    (affect clang-6.0 above versions)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: haoxintu at gmail.com
                CC: llvm-bugs at lists.llvm.org

Hi all.

As the title shows, clang-6.0 upwards versions spend much time (exceeded my
expectation) to compile the following simple C code. Is this a latent
performance bug existing for many years or it's just the intention of LLVM? If
it's the intention of LLVM, I apologize for the invalid report, and please
close it asap.

$cat small.c
#include <stdint.h>
int a;
void b() {
  int16_t *c;
  uint8_t d;
  int16_t e = 1;
  int64_t f;
  for (;; c++)
    for (d = 30; d <= 63; d++)
      for (f = -19; f <= 87; f++)
        *c += e ^= a;
}

I tested the following versions on my machine:

$time clang-trunk -w -O3 small.c
real    0m15.075s
user    0m14.764s
sys     0m0.310s

$time clang-12 -w -O3 small.c
real    0m20.437s
user    0m20.080s
sys     0m0.289s


$time clang-11 -w -O3 small.c
real    0m25.627s
user    0m21.636s
sys     0m0.425s


$time clang-10 -w -O3 small.c
real    0m28.128s
user    0m24.602s
sys     0m0.423s

$time clang-6.0 -w -3 small.c
real    0m14.500s
user    0m14.240s
sys     0m0.243s


And reproduced in Godbolt: https://godbolt.org/z/KjfEdax7b
(Output is "Killed - processing time exceeded" on clang-6.0 above versions)


Thanks,
Haoxin

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