From 6e17c00a88e23d0cc4cb491bfd0830f3df7fff66 Mon Sep 17 00:00:00 2001
From: Sanjay Patel <spatel@rotateright.com>
Date: Mon, 2 Oct 2017 18:26:44 +0000
Subject: [PATCH] [InstCombine] remove one-use restriction for icmp (shr exact
 X, C1), C2 --> icmp X, (C2<<C1)

llvm-svn: 314698
---
 llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp | 12 ++++++------
 llvm/test/Transforms/InstCombine/icmp-shr.ll            |  2 +-
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 1b38266..c548445 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2058,15 +2058,15 @@ 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(Shr->getType(), *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(Shr->getType(), 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 eba9151..e002b20 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]]
 ;
-- 
2.7.4