Hi!
Before the recent PR111267 r14-8319 fwprop changes, fwprop would never try
to propagate what was not considered PROFITABLE, where the profitable part
actually was partly about profitability, partly about very good reasons
not to actually propagate and partly for cases where propagation is
completely incorrect.
In particular, classify_result has:
/* Allow (subreg (mem)) -> (mem) simplifications with the following
exceptions:
1) Propagating (mem)s into multiple uses is not profitable.
2) Propagating (mem)s across EBBs may not be profitable if the source EBB
runs less frequently.
3) Propagating (mem)s into paradoxical (subreg)s is not profitable.
4) Creating new (mem/v)s is not correct, since DCE will not remove the old
ones. */
if (single_use_p
&& single_ebb_p
&& SUBREG_P (old_rtx)
&& !paradoxical_subreg_p (old_rtx)
&& MEM_P (new_rtx)
&& !MEM_VOLATILE_P (new_rtx))
return PROFITABLE;
and didn't mark any other MEM_P (new_rtx) or rtxes which contain
a MEM in its subrtxes as PROFITABLE. Now, since r14-8319 profitable_p
method has been renamed to likely_profitable_p and has just a minor role.
Now, rule 4) above is something that isn't about profitability, but about
correct behavior, if you propagate mem/v, the code is miscompiled.
This particular case has been fixed elsewhere by Haochen in r14-9379.
But I think even the 1) and 2) and maybe 3) are a strong don't do it,
don't rely solely on rtx costs, increasing the number of loads of the same
memory, even when cached, is undesirable, canceling load hoisting can
be undesirable as well.
So, the following patch restores previous behavior of src contains any MEMs,
in that case likely_profitable_p () is taken as the old profitable_p ()
as a requirement rather than just a hint. For propagation of something
which doesn't load from memory this keeps the r14-8319 behavior.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2024-03-09 Jakub Jelinek <ja...@redhat.com>
PR target/114284
* fwprop.cc (try_fwprop_subst_pattern): Don't propagate
src containing MEMs unless prop.likely_profitable_p ().
--- gcc/fwprop.cc.jj 2024-03-08 09:07:29.371626376 +0100
+++ gcc/fwprop.cc 2024-03-08 16:18:16.125853619 +0100
@@ -451,6 +451,7 @@ try_fwprop_subst_pattern (obstack_waterm
if (!prop.likely_profitable_p ()
&& (prop.changed_mem_p ()
+ || contains_mem_rtx_p (src)
|| use_insn->is_asm ()
|| !single_set (use_rtl)))
{
Jakub
