On Wed, 2021-03-03 at 11:34 -0700, Jeff Law wrote:
>
>
> On 3/2/21 3:37 PM, Ilya Leoshkevich via Gcc-patches wrote:
> > Bootstrapped and regtested on x86_64-redhat-linux, ppc64le-redhat-
> > linux
> > and s390x-redhat-linux. Ok for master?
> >
> >
> >
> > Commit efb6bc55a93a ("fwprop: Allow (subreg (mem))
> > simplifications")
> > introduced a check that was supposed to look at the propagated
> > def's
> > number of uses. It uses insn_info::num_uses (), which in reality
> > returns the number of uses def's insn has. The whole change
> > therefore
> > works only by accident.
> >
> > Fix by looking at def_info's uses instead of insn_info's uses.
> > This
> > requires passing around def_info instead of insn_info.
> >
> > gcc/ChangeLog:
> >
> > 2021-03-02 Ilya Leoshkevich <i...@linux.ibm.com>
> >
> > * fwprop.c (def_has_single_use_p): New function.
> > (fwprop_propagation::fwprop_propagation): Look at
> > def_info's uses.
> > (try_fwprop_subst_note): Use def_info instead of insn_info.
> > (try_fwprop_subst_pattern): Likewise.
> > (try_fwprop_subst_notes): Likewise.
> > (try_fwprop_subst): Likewise.
> > (forward_propagate_subreg): Likewise.
> > (forward_propagate_and_simplify): Likewise.
> > (forward_propagate_into): Likewise.
> > * iterator-utils.h (single_element_p): New function.
> Given we're well into stage4, I'd recommend deferring to gcc-12
> unless
> this fixes a code correctness issue.
>
> Jeff
>
Fortunately the issue here is not a miscompilation, but it's still
a regression: on s390 small functions that use long doubles get
a number of useless load/stores as well as a stack frame, where none
was required before. Basically, the same issue efb6bc55a93a failed to
fully fix due to the num_uses() / nondebug_insn_uses() mixup.
