i386: Use xor to write zero to memory with -Os even for more than 4 stores [PR11877]

i386: Use xor to write zero to memory with -Os even for more than 4 stores [PR11877]

> > 2021-06-20  Roger Sayle  <roger@nextmovesoftware.com>
> >
> > gcc/ChangeLog
> >         PR target/11877
> >         * config/i386/i386.md: New define_peephole2s to shrink writing
> >         1, 2 or 4 consecutive zeros to memory when optimizing for size.

It unfortunately doesn't extend well to larger memory clearing.
Consider e.g.
void
foo (int *p)
{
  p[0] = 0;
  p[7] = 0;
  p[23] = 0;
  p[41] = 0;
  p[48] = 0;
  p[59] = 0;
  p[69] = 0;
  p[78] = 0;
  p[83] = 0;
  p[89] = 0;
  p[98] = 0;
  p[121] = 0;
  p[132] = 0;
  p[143] = 0;
  p[154] = 0;
}
where with the patch we emit:
        xorl    %eax, %eax
        xorl    %edx, %edx
        xorl    %ecx, %ecx
        xorl    %esi, %esi
        xorl    %r8d, %r8d
        movl    %eax, (%rdi)
        movl    %eax, 28(%rdi)
        movl    %eax, 92(%rdi)
        movl    %eax, 164(%rdi)
        movl    %edx, 192(%rdi)
        movl    %edx, 236(%rdi)
        movl    %edx, 276(%rdi)
        movl    %edx, 312(%rdi)
        movl    %ecx, 332(%rdi)
        movl    %ecx, 356(%rdi)
        movl    %ecx, 392(%rdi)
        movl    %ecx, 484(%rdi)
        movl    %esi, 528(%rdi)
        movl    %esi, 572(%rdi)
        movl    %r8d, 616(%rdi)
Here is an incremental patch that emits:
        xorl    %eax, %eax
        movl    %eax, (%rdi)
        movl    %eax, 28(%rdi)
        movl    %eax, 92(%rdi)
        movl    %eax, 164(%rdi)
        movl    %eax, 192(%rdi)
        movl    %eax, 236(%rdi)
        movl    %eax, 276(%rdi)
        movl    %eax, 312(%rdi)
        movl    %eax, 332(%rdi)
        movl    %eax, 356(%rdi)
        movl    %eax, 392(%rdi)
        movl    %eax, 484(%rdi)
        movl    %eax, 528(%rdi)
        movl    %eax, 572(%rdi)
        movl    %eax, 616(%rdi)
instead.

2021-06-22  Jakub Jelinek  <jakub@redhat.com>

	PR target/11877
	* config/i386/i386-protos.h (ix86_last_zero_store_uid): Declare.
	* config/i386/i386-expand.c (ix86_last_zero_store_uid): New variable.
	* config/i386/i386.c (ix86_expand_prologue): Clear it.
	* config/i386/i386.md (peephole2s for 1/2/4 stores of const0_rtx):
	Remove "" from match_operand.  Emit new insns using emit_move_insn and
	set ix86_last_zero_store_uid to INSN_UID of the last store.
	Add peephole2s for 1/2/4 stores of const0_rtx following previous
	successful peep2s.

	* gcc.target/i386/pr11877-2.c: New test.