[PATCH] D98299: [InstSimplify] Simplify smul.fix and smul.fix.sat

[Simonas Kazlauskas via Phabricator via llvm-commits]

nagisa added inline comments.


================
Comment at: llvm/lib/Analysis/InstructionSimplify.cpp:5765
+    if (Q.isUndefValue(Op1))
+      return Constant::getNullValue(ReturnType);
+
----------------
Wouldn't this fail to trigger if the intrinsic call starts as follows?

```
lang=llvm
%res = call i16 @llvm.smul.fix.i16(i16 42, i16 undef, i32 ...)
```

If I understand it correctly, the canonicalization step above would swap the two operands, making the `Op0 = undef` and `Op1 = 42`, thus preventing this simplification from happening.

(This does not appear to be an issue for `0`, it seems there's code somewhere else that const-evaluates this operation if both operands are constant)


Repository:
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  https://reviews.llvm.org/D98299/new/

https://reviews.llvm.org/D98299