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* [Bug target/65368] New: _bzhi_u32 intrinsic generates incorrect code when -O1 or above is specified and index is an immediate
@ 2015-03-09 23:00 jamrial at gmail dot com
  2015-03-10  8:46 ` [Bug target/65368] [4.8/4.9/5 Regression]_bzhi_u32 " jakub at gcc dot gnu.org
                   ` (13 more replies)
  0 siblings, 14 replies; 15+ messages in thread
From: jamrial at gmail dot com @ 2015-03-09 23:00 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65368

            Bug ID: 65368
           Summary: _bzhi_u32 intrinsic generates incorrect code when -O1
                    or above is specified and index is an immediate
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jamrial at gmail dot com

The code generated is a simple AND instruction that zeroes the high bits based
on the index value starting from the highest bit rather than the lowest.

Sample program:

------
#include <stdio.h>
#include <x86intrin.h>

int main(int argc, char **argv)
{
    unsigned int j = _bzhi_u32(atoi(argv[1]), 11);
    printf("%x\n", j);
    return 0;
}
------

compiled with "gcc -O1 -mbmi2 -std=c99 -o bzhi bzhi.c"

[jamrial@ArchVM~]$ ./bzhi 4294967295
1fffff

Disassemble is as follows

0000000000000000 <main>:
   0:   48 83 ec 08             sub    rsp,0x8
   4:   48 8b 7e 08             mov    rdi,QWORD PTR [rsi+0x8]
   8:   ba 0a 00 00 00          mov    edx,0xa
   d:   be 00 00 00 00          mov    esi,0x0
  12:   e8 00 00 00 00          call   17 <main+0x17>
  17:   89 c6                   mov    esi,eax
  19:   81 e6 ff ff 1f 00       and    esi,0x1fffff
  1f:   bf 00 00 00 00          mov    edi,0x0
  24:   b8 00 00 00 00          mov    eax,0x0
  29:   e8 00 00 00 00          call   2e <main+0x2e>
  2e:   b8 00 00 00 00          mov    eax,0x0
  33:   48 83 c4 08             add    rsp,0x8
  37:   c3                      ret



compiled with "gcc -mbmi2 -std=c99 -o bzhi bzhi.c"

[jamrial@ArchVM~]$ ./bzhi 4294967295
7ff

Disassemble is as follows

0000000000000000 <main>:
   0:   55                      push   rbp
   1:   48 89 e5                mov    rbp,rsp
   4:   48 83 ec 20             sub    rsp,0x20
   8:   89 7d ec                mov    DWORD PTR [rbp-0x14],edi
   b:   48 89 75 e0             mov    QWORD PTR [rbp-0x20],rsi
   f:   48 8b 45 e0             mov    rax,QWORD PTR [rbp-0x20]
  13:   48 83 c0 08             add    rax,0x8
  17:   48 8b 00                mov    rax,QWORD PTR [rax]
  1a:   48 89 c7                mov    rdi,rax
  1d:   e8 00 00 00 00          call   22 <main+0x22>
  22:   89 45 f8                mov    DWORD PTR [rbp-0x8],eax
  25:   c7 45 f4 0b 00 00 00    mov    DWORD PTR [rbp-0xc],0xb
  2c:   8b 45 f4                mov    eax,DWORD PTR [rbp-0xc]
  2f:   c4 e2 78 f5 45 f8       bzhi   eax,DWORD PTR [rbp-0x8],eax
  35:   89 45 fc                mov    DWORD PTR [rbp-0x4],eax
  38:   8b 45 fc                mov    eax,DWORD PTR [rbp-0x4]
  3b:   89 c6                   mov    esi,eax
  3d:   bf 00 00 00 00          mov    edi,0x0
  42:   b8 00 00 00 00          mov    eax,0x0
  47:   e8 00 00 00 00          call   4c <main+0x4c>
  4c:   b8 00 00 00 00          mov    eax,0x0
  51:   c9                      leave
  52:   c3                      ret

When the index is not an immediate the actual bzhi instruction is always used
(with or without -Ox flags), and the result is the expected one.


^ permalink raw reply	[flat|nested] 15+ messages in thread
end of thread, other threads:[~2015-06-03 21:42 UTC | newest]

Thread overview: 15+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2015-03-09 23:00 [Bug target/65368] New: _bzhi_u32 intrinsic generates incorrect code when -O1 or above is specified and index is an immediate jamrial at gmail dot com
2015-03-10  8:46 ` [Bug target/65368] [4.8/4.9/5 Regression]_bzhi_u32 " jakub at gcc dot gnu.org
2015-03-10  8:48 ` jakub at gcc dot gnu.org
2015-03-10  9:06 ` ubizjak at gmail dot com
2015-03-10  9:18 ` jakub at gcc dot gnu.org
2015-03-10  9:24 ` ubizjak at gmail dot com
2015-03-10  9:31 ` ubizjak at gmail dot com
2015-03-10  9:41 ` jakub at gcc dot gnu.org
2015-03-10  9:52 ` ubizjak at gmail dot com
2015-03-10 10:04 ` ubizjak at gmail dot com
2015-03-10 21:03 ` jakub at gcc dot gnu.org
2015-03-10 21:09 ` [Bug target/65368] [4.8/4.9 " jakub at gcc dot gnu.org
2015-06-03 15:26 ` jakub at gcc dot gnu.org
2015-06-03 21:35 ` jakub at gcc dot gnu.org
2015-06-03 21:42 ` jakub at gcc dot gnu.org

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