[llvm] [InstCombine] Preserve nuw/exact flags when transforming (C shift (A add nuw C1)) --> ((C shift C1) shift A). (PR #79490)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Craig Topper (topperc)

<details>
<summary>Changes</summary>

If we weren't shifting out any non-zero bits before the transform, we shouldn't be after.

Example alive2 proofs
https://alive2.llvm.org/ce/z/grqsED
https://alive2.llvm.org/ce/z/wja3g3
https://alive2.llvm.org/ce/z/sfiAUn

---
Full diff: https://github.com/llvm/llvm-project/pull/79490.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp (+6-1) 
- (modified) llvm/test/Transforms/InstCombine/shift-add.ll (+32) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 54490c46dfaefcd..03668f59dfdfb2c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -439,7 +439,12 @@ Instruction *InstCombinerImpl::commonShiftTransforms(BinaryOperator &I) {
   if (match(Op0, m_Constant(C)) &&
       match(Op1, m_NUWAdd(m_Value(A), m_Constant(C1)))) {
     Value *NewC = Builder.CreateBinOp(I.getOpcode(), C, C1);
-    return BinaryOperator::Create(I.getOpcode(), NewC, A);
+    BinaryOperator *NewShiftOp = BinaryOperator::Create(I.getOpcode(), NewC, A);
+    if (I.getOpcode() == Instruction::Shl)
+      NewShiftOp->setHasNoUnsignedWrap(I.hasNoUnsignedWrap());
+    else
+      NewShiftOp->setIsExact(I.isExact());
+    return NewShiftOp;
   }
 
   unsigned BitWidth = Ty->getScalarSizeInBits();
diff --git a/llvm/test/Transforms/InstCombine/shift-add.ll b/llvm/test/Transforms/InstCombine/shift-add.ll
index 6ea2718abb2bbc5..5b8f0c44bde8e56 100644
--- a/llvm/test/Transforms/InstCombine/shift-add.ll
+++ b/llvm/test/Transforms/InstCombine/shift-add.ll
@@ -159,6 +159,38 @@ define i32 @ashr_add_nuw(i32 %x, ptr %p) {
   ret i32 %r
 }
 
+; Preserve nuw and exact flags.
+
+define i32 @shl_nuw_add_nuw(i32 %x) {
+; CHECK-LABEL: @shl_nuw_add_nuw(
+; CHECK-NEXT:    [[R:%.*]] = shl nuw i32 2, [[X:%.*]]
+; CHECK-NEXT:    ret i32 [[R]]
+;
+  %a = add nuw i32 %x, 1
+  %r = shl nuw i32 1, %a
+  ret i32 %r
+}
+
+define i32 @lshr_exact_add_nuw(i32 %x) {
+; CHECK-LABEL: @lshr_exact_add_nuw(
+; CHECK-NEXT:    [[R:%.*]] = lshr exact i32 2, [[X:%.*]]
+; CHECK-NEXT:    ret i32 [[R]]
+;
+  %a = add nuw i32 %x, 1
+  %r = lshr exact i32 4, %a
+  ret i32 %r
+}
+
+define i32 @ashr_exact_add_nuw(i32 %x) {
+; CHECK-LABEL: @ashr_exact_add_nuw(
+; CHECK-NEXT:    [[R:%.*]] = ashr exact i32 -2, [[X:%.*]]
+; CHECK-NEXT:    ret i32 [[R]]
+;
+  %a = add nuw i32 %x, 1
+  %r = ashr exact i32 -4, %a
+  ret i32 %r
+}
+
 ; negative test - must have 'nuw'
 
 define i32 @shl_add_nsw(i32 %x) {

``````````

</details>


https://github.com/llvm/llvm-project/pull/79490