[PATCH] D140798: [InstCombine] Fold zero check followed by decrement to usub.sat

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp:815
+    if (match(FalseVal, m_Zero())) {
+      if (Pred == ICmpInst::ICMP_NE) {
+        Pred = ICmpInst::getSwappedPredicate(Pred);
----------------
This is not going to trigger, because everything here is inside `Pred == ICmpInst::ICMP_EQ`. You'd want to check for isEquality() first (which accepts eq and ne), then have a check for ne where you invert the predicate and swap true/false, and then do the check for zero on the true value.

I'd suggest adding this test case to make sure the ne pattern is matched:
```
define i8 @test(i8 %a) {
  %i = icmp ne i8 %a, 0
  call void @use.i1(i1 %i)
  %i1 = add i8 %a, -1
  %i2 = select i1 %i, i8 %i1, i8 0
  ret i8 %i2
}

declare void @use.i1(i1)
```


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp:825
+    const APInt *Added;
+    if (match(FalseVal, m_Add(m_Specific(A), m_Negative(Added)))) {
+      APInt Sub = -*Added;
----------------
You can match `-1` using `m_AllOnes()`.


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  https://reviews.llvm.org/D140798/new/

https://reviews.llvm.org/D140798