[Bug rtl-optimization/55583] New: Extended shift instruction on x86-64 is not used, producing unoptimal code

[mtkilpailut at torni dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55583



             Bug #: 55583

           Summary: Extended shift instruction on x86-64 is not used,

                    producing unoptimal code

    Classification: Unclassified

           Product: gcc

           Version: 4.8.0

            Status: UNCONFIRMED

          Severity: enhancement

          Priority: P3

         Component: rtl-optimization

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: mtkilpai...@torni.org





Created attachment 28866

  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28866

Source code demonstrating bad code generation



On x86-64, extended shift instruction is not generated for some reason.

Combined with other problems this creates very bad code.



Test functions included for signed and unsigned 16,32,64-bit types for both

left and right shifts and for constant n and function parameter n.



Code of this form:

  unsigned int a, b; const int n = 2;

  void test32l (void) { b = (b << n) | (a >> (32 - n)); }



expected code:

  mov     a(%rip),%eax

  shld    $0x2,%eax,b(%rip)

  ret



produced code:

  mov    b(%rip), %edx   ; Size of register used here depends on gcc version

  mov    a(%rip), %eax   ; Size of register used here depends on gcc version

  sal    $2, %edx        ; Size of register used here depends on gcc version

  shr    $25, %eax

  or     %edx, %eax

  mov    %eax, b(%rip)

  ret





Tested with:

COLLECT_GCC_OPTIONS='-v' '-c' '-save-temps' '-O2' '-Wall' '-W' '-o'

'gcc_shld_not_used' '-mtune=generic'



I tried gcc versions:

GNU C (Debian 4.7.2-4) version 4.7.2 (x86_64-linux-gnu)

GNU C (Debian 4.6.3-11) version 4.6.3 (x86_64-linux-gnu)

GNU C (Debian 4.5.3-9) version 4.5.3 (x86_64-linux-gnu)

GNU C (Debian 4.4.7-2) version 4.4.7 (x86_64-linux-gnu)

GNU C (GCC) version 4.8.0 20121203 (experimental) [trunk revision 194106]

(x86_64-unknown-linux-gnu)



All produce the same code modulo register size differences mentioned above. gcc

HEAD changes sal to leal (,%rcx,4),%eax