[llvm] fbb7866 - [InstCombine] fix one-use condition for shift transform

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2021-09-06T11:13:39-04:00
New Revision: fbb78668f2ee4a7ce390d9a5749168c5be26874e

URL: https://github.com/llvm/llvm-project/commit/fbb78668f2ee4a7ce390d9a5749168c5be26874e
DIFF: https://github.com/llvm/llvm-project/commit/fbb78668f2ee4a7ce390d9a5749168c5be26874e.diff

LOG: [InstCombine] fix one-use condition for shift transform

This transform is written in a confusing style,
and I suspect it is at fault for a more serious
bug noted in PR51567.

But it's been around forever, so I'm making the
minimal change to fix another bug - it could
increase instructions because it was not checking
uses.

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
    llvm/test/Transforms/InstCombine/shift-amount-reassociation-with-truncation-shl.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 4fff7abf2e98..544115c57a19 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -690,6 +690,9 @@ Instruction *InstCombinerImpl::FoldShiftByConstant(Value *Op0, Constant *Op1,
   if (Instruction *FoldedShift = foldBinOpIntoSelectOrPhi(I))
     return FoldedShift;
 
+  if (!Op0->hasOneUse())
+    return nullptr;
+
   // Fold shift2(trunc(shift1(x,c1)), c2) -> trunc(shift2(shift1(x,c1),c2))
   if (auto *TI = dyn_cast<TruncInst>(Op0)) {
     // If 'shift2' is an ashr, we would have to get the sign bit into a funny
@@ -728,9 +731,6 @@ Instruction *InstCombinerImpl::FoldShiftByConstant(Value *Op0, Constant *Op1,
     }
   }
 
-  if (!Op0->hasOneUse())
-    return nullptr;
-
   if (auto *Op0BO = dyn_cast<BinaryOperator>(Op0)) {
     // Turn ((X >> C) + Y) << C  ->  (X + (Y << C)) & (~0 << C)
     Value *V1;

diff  --git a/llvm/test/Transforms/InstCombine/shift-amount-reassociation-with-truncation-shl.ll b/llvm/test/Transforms/InstCombine/shift-amount-reassociation-with-truncation-shl.ll
index 4b762075fbdf..6c1d29d0990e 100644
--- a/llvm/test/Transforms/InstCombine/shift-amount-reassociation-with-truncation-shl.ll
+++ b/llvm/test/Transforms/InstCombine/shift-amount-reassociation-with-truncation-shl.ll
@@ -217,8 +217,7 @@ define i16 @shl_tr_shl_constant_shift_amount_uses(i32 %x) {
 ; CHECK-NEXT:    call void @use32(i32 [[SHL]])
 ; CHECK-NEXT:    [[TR:%.*]] = trunc i32 [[SHL]] to i16
 ; CHECK-NEXT:    call void @use16(i16 [[TR]])
-; CHECK-NEXT:    [[X_TR:%.*]] = trunc i32 [[X]] to i16
-; CHECK-NEXT:    [[R:%.*]] = shl i16 [[X_TR]], 7
+; CHECK-NEXT:    [[R:%.*]] = shl i16 [[TR]], 4
 ; CHECK-NEXT:    ret i16 [[R]]
 ;
   %shl = shl i32 %x, 3