[Bug target/54904] New: Large mode constant live in a register not used to optimize smaller mode constants

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54904

             Bug #: 54904
           Summary: Large mode constant live in a register not used to
                    optimize smaller mode constants
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jakub@gcc.gnu.org
                CC: uros@gcc.gnu.org


void
foo (char *p)
{
  __builtin_memset (p, 0x11, 23);
}

results in terrible code on x86_64-linux at -O2:
        movabsq $1229782938247303441, %rax
        movl    $286331153, 16(%rdi)
        movb    $17, 22(%rdi)
        movq    %rax, (%rdi)
        movq    %rax, 8(%rdi)
        movl    $4369, %eax
        movw    %ax, 20(%rdi)
        ret
E.g.
        movabsq $1229782938247303441, %rax
        movl    %eax, 16(%rdi)
        movb    %al, 22(%rdi)
        movq    %rax, (%rdi)
        movq    %rax, 8(%rdi)
        movw    %ax, 20(%rdi)
        ret
would be 10 bytes shorter.  Guess we don't want to do this kind of
optimizations before RA, it might increase register preassure, but somewhere
close to final would be nice.  Use a register instead of immediate if it is
available, makes the insn smaller and not slower.