[llvm] r373363 - [InstCombine] Preserve 'exact' in -(X >>u 31) -> (X >>s 31) fold

[Roman Lebedev via llvm-commits]

Author: lebedevri
Date: Tue Oct  1 10:50:09 2019
New Revision: 373363

URL: http://llvm.org/viewvc/llvm-project?rev=373363&view=rev
Log:
[InstCombine] Preserve 'exact' in -(X >>u 31) -> (X >>s 31) fold

https://rise4fun.com/Alive/yR4

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
    llvm/trunk/test/Transforms/InstCombine/high-bit-signmask.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp?rev=373363&r1=373362&r2=373363&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp Tue Oct  1 10:50:09 2019
@@ -1676,12 +1676,16 @@ Instruction *InstCombiner::visitSub(Bina
       if (match(Op1, m_LShr(m_Value(X), m_APInt(ShAmt))) &&
           *ShAmt == BitWidth - 1) {
         Value *ShAmtOp = cast<Instruction>(Op1)->getOperand(1);
-        return BinaryOperator::CreateAShr(X, ShAmtOp);
+        Instruction *NewShift = BinaryOperator::CreateAShr(X, ShAmtOp);
+        NewShift->copyIRFlags(Op1);
+        return NewShift;
       }
       if (match(Op1, m_AShr(m_Value(X), m_APInt(ShAmt))) &&
           *ShAmt == BitWidth - 1) {
         Value *ShAmtOp = cast<Instruction>(Op1)->getOperand(1);
-        return BinaryOperator::CreateLShr(X, ShAmtOp);
+        Instruction *NewShift = BinaryOperator::CreateLShr(X, ShAmtOp);
+        NewShift->copyIRFlags(Op1);
+        return NewShift;
       }
 
       if (Op1->hasOneUse()) {

Modified: llvm/trunk/test/Transforms/InstCombine/high-bit-signmask.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/high-bit-signmask.ll?rev=373363&r1=373362&r2=373363&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/high-bit-signmask.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/high-bit-signmask.ll Tue Oct  1 10:50:09 2019
@@ -12,7 +12,7 @@ define i64 @t0(i64 %x) {
 }
 define i64 @t0_exact(i64 %x) {
 ; CHECK-LABEL: @t0_exact(
-; CHECK-NEXT:    [[R:%.*]] = ashr i64 [[X:%.*]], 63
+; CHECK-NEXT:    [[R:%.*]] = ashr exact i64 [[X:%.*]], 63
 ; CHECK-NEXT:    ret i64 [[R]]
 ;
   %t0 = lshr exact i64 %x, 63
@@ -30,7 +30,7 @@ define i64 @t2(i64 %x) {
 }
 define i64 @t3_exact(i64 %x) {
 ; CHECK-LABEL: @t3_exact(
-; CHECK-NEXT:    [[R:%.*]] = lshr i64 [[X:%.*]], 63
+; CHECK-NEXT:    [[R:%.*]] = lshr exact i64 [[X:%.*]], 63
 ; CHECK-NEXT:    ret i64 [[R]]
 ;
   %t0 = ashr exact i64 %x, 63