[x86_64 PATCH] PR target/11877: Use xor to write zero to memory with -Os

Jakub Jelinek jakub@redhat.com
Mon Jun 21 09:58:54 GMT 2021


On Mon, Jun 21, 2021 at 11:19:12AM +0200, Richard Biener wrote:
> > --- gcc/config/i386/i386.c.jj   2021-06-21 09:39:21.622487840 +0200
> > +++ gcc/config/i386/i386.c      2021-06-21 10:21:12.389794740 +0200
> > @@ -15186,6 +15186,33 @@ ix86_lea_for_add_ok (rtx_insn *insn, rtx
> >    return ix86_lea_outperforms (insn, regno0, regno1, regno2, 0, false);
> >  }
> >
> > +/* Return true if insns before FIRST_INSN (which is of the form
> > +   (set (memory) (zero_operand)) are all also either in the
> > +   same form, or (set (zero_operand) (const_int 0)).  */
> > +
> > +bool
> > +ix86_zero_stores_peep2_p (rtx_insn *first_insn, rtx zero_operand)
> > +{
> > +  rtx_insn *insn = first_insn;
> > +  for (int count = 0; count < 512; count++)
> 
> Can't the peephole add a note (reg_equal?) that the
> SET_SRC of the previously matched store is zero?

I think REG_EQUAL is not valid, the documentation says that it should
be used on SET of a REG, which is not the case here - we have a MEM.

> That would avoid the need to walk here.

But we could do what I've done in
r11-7694-gd55ce33a34a8e33d17285228b32cf1e564241a70
- have int ix86_last_zero_store_uid;
set to INSN_UID of the last store emitted by the peephole2s and
then check that INSN_UID against the var.

	Jakub



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