[Bug c/78037] New: Incorrect code generated at optimization level -O2 for tzcnt and binary and

[nlescoua at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78037

Bug ID: 78037
   Summary: Incorrect code generated at optimization level -O2 for
tzcnt and binary and
   Product: gcc
   Version: 6.2.0
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
  Assignee: unassigned at gcc dot gnu.org
  Reporter: nlescoua at gmail dot com
  Target Milestone: ---

System: Haswell (Linux 64bit)

The following program compiles incorrectly on gcc 6.2.0 (and also gcc 5.3.1).
---
#include 
#include 
#include 

int main(int argc, char ** argv){
  unsigned long l = atol(argv[1]);
  uint32_t t = _tzcnt_u64(l) & 63llu;
  printf("%u\n",t);
}

The tzcnt_u64 intrisic returns the number of trailing zero in the argument and
64 (the register width) if the argument is 0. Complete specification is
available here: http://www.felixcloutier.com/x86/TZCNT.html

Expected output:
~/g/d/gcc_bug ❯❯ gcc -mbmi -O0 toto.c && ./a.out 0
0

Erroneous output when using -O2 or -O3 (the binary and gets eliminated by the
optimizer).
~/g/d/gcc_bug ❯❯ gcc -mbmi -O2 toto.c && ./a.out 0
64

The result must be 0 because 64&63 = 0

When looking at the assembly generated, the "and" instruction is missing at
optimization level 2 or 3, as if the optimizer assumed that the values returned
by tzcnt are in the range [0:63] whereas the effective range is [0:64].

[Bug c/78037] Incorrect code generated at optimization level -O2 for tzcnt and binary and

[nlescoua at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78037

--- Comment #1 from Nicolas Le Scouarnec  ---
Created attachment 39836
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39836&action=edit
C file

[Bug c/78037] Incorrect code generated at optimization level -O2 for tzcnt and binary and

[nlescoua at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78037

--- Comment #2 from Nicolas Le Scouarnec  ---
Created attachment 39837
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39837&action=edit
i file

[Bug c/78037] Incorrect code generated at optimization level -O2 for tzcnt and binary and

[nlescoua at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78037

--- Comment #3 from Nicolas Le Scouarnec  ---
Created attachment 39838
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39838&action=edit
Correct Assembly at optimization level O1

[Bug c/78037] Incorrect code generated at optimization level -O2 for tzcnt and binary and

[nlescoua at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78037

--- Comment #4 from Nicolas Le Scouarnec  ---
Created attachment 39839
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39839&action=edit
Incorrect assembly at optimization level O2

[Bug target/78037] Incorrect code generated at optimization level -O2 for tzcnt and binary and

[nlescoua at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78037

--- Comment #8 from Nicolas Le Scouarnec  ---
The same problem may be present in lzcnt which is also expanded as
__builtin_clz which is also undefined if 0.
http://www.felixcloutier.com/x86/LZCNT.html