[Bug target/58928] Different results from gcc when -mlzcnt is used

[mikeb01 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58928

--- Comment #2 from Michael Barker <mikeb01 at gmail dot com> ---
> Hope helps

What hardware platform are you testing on?  I'm running on Ivy Bridge:

Intel(R) Xeon(R) CPU E5620 @2.40GHz

Could you put the assembler that you see on the ticket:

With -mlzcnt:

0000000000400580 <lzcnt>:
  400580:    55                       push   %rbp
  400581:    48 89 e5                 mov    %rsp,%rbp
  400584:    48 89 7d e8              mov    %rdi,-0x18(%rbp)
  400588:    48 8b 45 e8              mov    -0x18(%rbp),%rax
  40058c:    48 89 45 f8              mov    %rax,-0x8(%rbp)
  400590:    f3 48 0f bd 45 f8        lzcnt  -0x8(%rbp),%rax
  400596:    48 98                    cltq   
  400598:    5d                       pop    %rbp
  400599:    c3                       retq   

Without -mlzcnt:

0000000000400580 <lzcnt>:
  400580:    55                       push   %rbp
  400581:    48 89 e5                 mov    %rsp,%rbp
  400584:    48 89 7d e8              mov    %rdi,-0x18(%rbp)
  400588:    48 8b 45 e8              mov    -0x18(%rbp),%rax
  40058c:    48 89 45 f8              mov    %rax,-0x8(%rbp)
  400590:    48 0f bd 45 f8           bsr    -0x8(%rbp),%rax
  400595:    48 83 f0 3f              xor    $0x3f,%rax
  400599:    48 98                    cltq   
  40059b:    5d                       pop    %rbp
  40059c:    c3                       retq