revert r314984: revert r314698 - [InstCombine] remove one-use restriction for icmp (shr exact X, C1), C2 --> icmp X, (C2<<C1)

Recommitting r314698. The bug exposed by this change should be fixed with:
https://reviews.llvm.org/rL315579 

llvm-svn: 315857

2 files changed, 7 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 67b5e5c130b..c3a3c4fbc4c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2060,15 +2060,15 @@ Instruction *InstCombiner::foldICmpShrConstant(ICmpInst &Cmp,
           (!IsAShr && C.shl(ShAmtVal).lshr(ShAmtVal) == C)) &&
          "Expected icmp+shr simplify did not occur.");
 
-  // Check if the bits shifted out are known to be zero. If so, we can compare
-  // against the unshifted value:
+  // If the bits shifted out are known zero, compare the unshifted value:
   //  (X & 4) >> 1 == 2  --> (X & 4) == 4.
   Constant *ShiftedCmpRHS = ConstantInt::get(ShrTy, C << ShAmtVal);
-  if (Shr->hasOneUse()) {
-    if (Shr->isExact())
-      return new ICmpInst(Pred, X, ShiftedCmpRHS);
+  if (Shr->isExact())
+    return new ICmpInst(Pred, X, ShiftedCmpRHS);
 
-    // Otherwise strength reduce the shift into an 'and'.
+  if (Shr->hasOneUse()) {
+    // Canonicalize the shift into an 'and':
+    // icmp eq/ne (shr X, ShAmt), C --> icmp eq/ne (and X, HiMask), (C << ShAmt)
     APInt Val(APInt::getHighBitsSet(TypeBits, TypeBits - ShAmtVal));
     Constant *Mask = ConstantInt::get(ShrTy, Val);
     Value *And = Builder.CreateAnd(X, Mask, Shr->getName() + ".mask");
diff --git a/llvm/test/Transforms/InstCombine/icmp-shr.ll b/llvm/test/Transforms/InstCombine/icmp-shr.ll
index 77eaf09eb96..214f315f317 100644
--- a/llvm/test/Transforms/InstCombine/icmp-shr.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-shr.ll
@@ -483,7 +483,7 @@ declare void @foo(i32)
 define i1 @exact_multiuse(i32 %x) {
 ; CHECK-LABEL: @exact_multiuse(
 ; CHECK-NEXT:    [[SH:%.*]] = lshr exact i32 %x, 7
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[SH]], 1024
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 %x, 131072
 ; CHECK-NEXT:    call void @foo(i32 [[SH]])
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;