[PATCH] D138814: [InstCombine] Combine a/lshr of add -> uadd.with.overflow

[Pierre van Houtryve via Phabricator via llvm-commits]

Pierre-vh added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp:880-891
+  // Make sure that `Op0` is only used by `I` and `HalfWidth`-truncates.
+  SmallVector<TruncInst *, 2> Truncs;
+  for (User *Usr : Op0->users()) {
+    if (Usr == &I)
+      continue;
+
+    TruncInst *Trunc = dyn_cast<TruncInst>(Usr);
----------------
lebedev.ri wrote:
> Pierre-vh wrote:
> > Pierre-vh wrote:
> > > lebedev.ri wrote:
> > > > This is rather unusual in InstCombine.
> > > > Would it not be sufficient to just zext the result?
> > > I think the original intent was to only to do the transform when it appears beneficial, but if we're fine with doing it all the time as a canonical form then we can indeed just zext + RAUW. What do you think?
> > Now that I think about it again, I don't think the transform would be valid if we don't look for the truncs?
> > We would be replacing the .add - which may have <32 leading zeroes (if overflow), with a zext of the uaddo, which will always have 32 leading zeroes or more
> Ah yes, you look at truncs of the wide add.
> It wouldn't be illegal to just leave it be.
> 
> I think what you want here, is:
> ```
> if(!Op0.hasOneUse()) {
>   for (User *Usr : Op0->users()) {
>     if (Usr == &I)
>       continue;
> 
>     TruncInst *Trunc = dyn_cast<TruncInst>(Usr);
>     if (!Trunc || Trunc->getType()->getScalarSizeInBits() > HalfWidth)
>       return nullptr;
>   }
> }
> ```
> 
> and then just
> ```
> if(!Op0.hasOneUse()) {
>    Value*WideOV = Builder.CreateZExt(Overflow);
>    replaceInstUsesWith(Op0, WideOV);
> }
> ```
Indeed that works too, but I think you meant `CreateZExt(UAdd)` instead of the Overflow bit?


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https://reviews.llvm.org/D138814