[Bug tree-optimization/56175] Issue with combine phase on x86.

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56175



Richard Biener <rguenth at gcc dot gnu.org> changed:



           What    |Removed                     |Added

----------------------------------------------------------------------------

                 CC|                            |rguenth at gcc dot gnu.org



--- Comment #12 from Richard Biener <rguenth at gcc dot gnu.org> 2013-02-21 
13:44:09 UTC ---

For the real testcase I see



.L2:

        shrw    %ax

        movl    %edi, %edx

        subb    $1, %dl

        movl    %edx, %edi

        je      .L9

.L4:

        movl    %ecx, %esi

        movl    %eax, %ebx

        andl    $1, %esi

        andl    $1, %ebx

        shrb    %cl

        movl    %esi, %edx

        cmpb    %bl, %dl

        je      .L2



thus



        andl    $1, %esi

        andl    $1, %ebx

        cmpb    %bl, %dl



for



   t = (u8)((x & 1) ^ ((u8)y & 1));

   if (t == 1)



and with disabling the forwprop transformation:



.L2:

        shrw    %ax

        subb    $1, %dl

        je      .L9

.L4:

        movl    %ecx, %ebx

        shrb    %cl

        xorl    %eax, %ebx

        andl    $1, %ebx

        je      .L2



to confirm the issue again.  There is one less used register and the

zero-flag use by the conditional jump.



The following testcase is too simple to be not optimized anyway

at the RTL level but it may serve as a testcase for forwprop.



void bar (void);

unsigned short

foo (unsigned char x, unsigned short y)

{

  unsigned char t = (unsigned char)((x & 1) ^ ((unsigned char)y & 1));

  if (t == 1)

    bar ();

  return y;

}