Hello,
> >>> (parallel [
> >>> (set (pc)
> >>> (if_then_else (lt:SI (reg:SI 45) <---
> >>> (reg:SI 26))
> >>> (label_ref:HI 129)
> >>> (pc)))
> >>> (clobber (reg:SI 45)) <-- must be the
> >>> same
> >>> (clobber (scratch:BI))
> >>> ])
>
> >actually, I probably misunderstood what you meant. The problem is that
> >invariant motion rewrites just part of the considered insn, changing
> >reg:45 to another register in if_then_else, but leaving it the same in
> >the clobber?
>
> Correct. I originally had a patch like this
>
> Index: gcc/loop-invariant.c
> ===================================================================
> --- gcc/loop-invariant.c (revision 125308)
> +++ gcc/loop-invariant.c (working copy)
> @@ -1212,7 +1212,14 @@ move_invariant_reg (struct loop *loop, u
> if (inv->def)
> {
> for (use = inv->def->uses; use; use = use->next)
> - *use->pos = reg;
> + {
> + rtx orig = *use->pos;
> +
> + /* Use replace_rtx() in case there are other references to
> + ORIG inside the insn which are not recorded in the USE
> + list. eg references inside CLOBBERs. */
> + replace_rtx (use->insn, orig, reg);
> + }
> }
>
> return true;
>
> which I thought would solve the problem, but I found that it introduced
> regressions into the gcc testsuite for an x86_64-linux-gnulibc
> toolchain, so I abandoned it. (I did not investigate why it caused
> regressions, I just assumed that my approach was wrong).
It will break in situations like
loop {
insn 1: (set reg:1 something_invariant)
insn 2: (set reg:1 (plus reg:1 something_else))
}
When we move the insn 1 out of the loop, we want this to be rewritten as
insn 1: (set reg:3 something_invariant)
loop {
insn 2: (set reg:1 (plus reg:3 something_else))
}
While your patch would rewrite insn 2 to set reg:3. What could work
would be going through the it might be more efficient to go through defs
recorded by df, and rewriting those with DF_REF_MUST_CLOBBER flag.
Zdenek
