[PATCH] D130433: [InstCombine] Add fold for redundant sign bits count comparison

[Sanjay Patel via Phabricator via llvm-commits]

spatel added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp:1683
+  const APInt *ShiftC;
+  if (!match(Xor, m_c_Xor(m_Value(X), m_AShr(m_Deferred(X), m_APInt(ShiftC)))))
+    return nullptr;
----------------
What happens if the xor or ashr have other uses? We should bail out if this transform would increase the number of instructions.

We need 2 more tests to verify that. Try variations like this:

```
declare void @use(i8)
define i1 @ugt_extra_use(i8 %x) {
  %y = ashr i8 %x, 5
  %z = xor i8 %y, %x
  call void @use(i8 %z)
  %c = icmp ugt i8 %z, 1
  ret i1 %c
}

```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D130433/new/

https://reviews.llvm.org/D130433