[InstCombine] add folds for icmp with xor mask constant

These are the folds in Alive;
Name: xor_ult
Pre: isPowerOf2(-C1)
%xor = xor i8 %x, C1
%r = icmp ult i8 %xor, C1
=>
%r = icmp ugt i8 %x, ~C1

Name: xor_ugt
Pre: isPowerOf2(C1+1)
%xor = xor i8 %x, C1
%r = icmp ugt i8 %xor, C1
=>
%r = icmp ugt i8 %x, C1

https://rise4fun.com/Alive/Vty

The ugt case in its simplest form was already handled by DemandedBits,
but that's not ideal as shown in the multi-use test.

I'm not sure if these are all of the symmetrical folds, but I adjusted 
the existing code for one of the folds to try to show the similarities.

There's no obvious connection, but this is another preliminary step 
for PR14613...
https://bugs.llvm.org/show_bug.cgi?id=14613

llvm-svn: 341997

1 files changed, 19 insertions, 10 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 4b5891461bc..e8e9e86d08b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1499,16 +1499,25 @@ Instruction *InstCombiner::foldICmpXorConstant(ICmpInst &Cmp,
     }
   }
 
-  // (icmp ugt (xor X, C), ~C) -> (icmp ult X, C)
-  //   iff -C is a power of 2
-  if (Pred == ICmpInst::ICMP_UGT && *XorC == ~C && (C + 1).isPowerOf2())
-    return new ICmpInst(ICmpInst::ICMP_ULT, X, Y);
-
-  // (icmp ult (xor X, C), -C) -> (icmp uge X, C)
-  //   iff -C is a power of 2
-  if (Pred == ICmpInst::ICMP_ULT && *XorC == -C && C.isPowerOf2())
-    return new ICmpInst(ICmpInst::ICMP_UGE, X, Y);
-
+  // Mask constant magic can eliminate an 'xor' with unsigned compares.
+  if (Pred == ICmpInst::ICMP_UGT) {
+    // (xor X, ~C) >u C --> X <u ~C (when C+1 is a power of 2)
+    if (*XorC == ~C && (C + 1).isPowerOf2())
+      return new ICmpInst(ICmpInst::ICMP_ULT, X, Y);
+    // (xor X, C) >u C --> X >u C (when C+1 is a power of 2)
+    if (*XorC == C && (C + 1).isPowerOf2())
+      return new ICmpInst(ICmpInst::ICMP_UGT, X, Y);
+  }
+  if (Pred == ICmpInst::ICMP_ULT) {
+    // (xor X, -C) <u C --> X >u ~C (when C is a power of 2)
+    if (*XorC == -C && C.isPowerOf2())
+      return new ICmpInst(ICmpInst::ICMP_UGT, X,
+                          ConstantInt::get(X->getType(), ~C));
+    // (xor X, C) <u C --> X >u ~C (when -C is a power of 2)
+    if (*XorC == C && (-C).isPowerOf2())
+      return new ICmpInst(ICmpInst::ICMP_UGT, X,
+                          ConstantInt::get(X->getType(), ~C));
+  }
   return nullptr;
 }