nikic added inline comments.
================
Comment at:
llvm/test/Transforms/PhaseOrdering/unsigned-multiply-overflow-check.ll:20
+; FIXME: noundef should be attached to args
define i1 @will_not_overflow(i64 %arg, i64 %arg1) {
----------------
spatel wrote:
> aqjune wrote:
> > aqjune wrote:
> > > spatel wrote:
> > > > Any ideas about what it will take to get those argument attributes for
> > > > the C++ source example shown in the code comment?
> > > >
> > > > SDAG is still going to convert the `select` to `and`, so we can
> > > > probably avoid regressions by replicating InstSimplify's
> > > > omitCheckForZeroBeforeMulWithOverflow() as a DAG combine. Let me know
> > > > if I should do that.
> > > I promised to do the patch at D82317, but these days I'm occupied with
> > > other things, so it might not be a recent future (not in a month at
> > > least)...
> > >
> > > I think it is a good chance to use freeze here: We can add
> > > ```
> > > %cond = icmp ne %x, 0
> > > %v = call @llvm.umul.with.overflow(%x, %y)
> > > %ov = extractvalue %v, 1
> > > %res = select i1 %cond, %ov, false
> > > =>
> > > %y.fr = freeze %y
> > > %v = call @llvm.umul.with.overflow(%x, %y.fr)
> > > %ov = extractvalue %v, 1
> > > %res = %ov
> > > ```
> > > into CodeGenPrepare.
> > > What do you think? CodeGenPrepare is already using freeze for a few
> > > transformations.
> > Alive2 proof: https://alive2.llvm.org/ce/z/zgPUGT
> I don't think we want to add code to CGP if we can avoid it. CGP is supposed
> to be a temporary hack that is not needed with GlobalISel. I do acknowledge
> that "temporary" in the code may outlive the people working on it today (!).
>
> If we don't care about undef correctness in codegen (in other words, the
> select->and transform will live on in codegen forever), then we might as well
> add a DAG combine. Alternatively, are we comfortable creating freeze in
> instcombine for rare code like umul.with.ov?
I think this is one of the rare cases where inserting Freeze in InstCombine
makes sense. There's not much (any?) further optimization opportunities for a
umul.with.overflow with non-constant operands.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D101191/new/
https://reviews.llvm.org/D101191
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