On Thu, Jul 9, 2026 at 8:23 AM Jeffrey Law <[email protected]> wrote:
>
>
>
> On 7/8/2026 2:19 AM, Andrew Pinski wrote:
> > Since match and simplify can add unused statements to the sequence
> > in some cases, we should remove them before call phiopt_early_allow.
> > This is needed for 2 future patches. One is to allowing of comparisons,
> > optionally with a cast and optional with a negative expression.
> > The other is about supporting a way to handling
> > cond_removal_in_builtin_zero_pattern in match and also adding a new match
> > pattern to fix PR 126035 (ctz split back into one).
> >
> > Bootstrapped and tested on x86_64-linux-gnu.
> >
> > gcc/ChangeLog:
> >
> > * tree-ssa-phiopt.cc (remove_unused_stmts): New function.
> > (gimple_simplify_phiopt): Call remove_unused_stmts before
> > phiopt_early_allow.
> I'm torn. Not particularly happy with what appears to be a mini DCE
> pass on the sequence. Presumably the motivation is to clean things up
> in a way that makes phiopt_early_allow more likely to return true by
> removing statements that are going to be zapped by DCE later and thus
> shouldn't participate in phiopt_early_allow?
Yes. While working on a different patch I noticed this. Let me find
the testcase again.
The testcase is:
```
bool f1(bool a, bool b)
{
bool c = !a;
bool d = c & b;
if (d) return 0;
bool e = c | b;
return e;
}
```
This will also be rejected by the current version of
phiopt_early_allow. But we (Kyrylo and I) are looking to change that.
With the above patch what we get back is:
```
phiopt match-simplify back:
_12 = ~b_11(D);
_7 = a_9(D) | _12;
_6 = a_9(D) ^ b_11(D);
_5 = a_9(D) == b_11(D);
result: _5
rejected because early
```
But this change we get:
```
phiopt match-simplify back:
_5 = a_9(D) == b_11(D);
result: _5
rejected because early
```
which should then be accepted with the other change we are working on.
This was just one testcase I noticed the issue but I have seen in others.
The other reason to remove the dead statements from the sequence is
dealing with my next patch (which is not related to early). Which
extends phiopt match connection to handle (semi) arbitrary middle bbs
which allows the removal of cond_removal_in_builtin_zero_pattern and
allow for some of the patches that Daniel has been working on and also
allow for matching things like the 2 halved version of ctz into one
ctz with just a match pattern rather than some manual matching code.
Basically removing the dead statements makes sure we are no longer
referencing the middle-bb in some/many cases.
Thanks,
Andrea
>
> jeff
