InstCombine: Avoid introducing poison values when lowering llvm.amdgcn.[us]bfe

Summary:
When the 3rd argument to these intrinsics is zero, lowering them
to shift instructions produces poison values, since we end up with
shift amounts equal to the number of bits in the shifted value.  This
means we can only lower these intrinsics if we can prove that the
3rd argument is not zero.

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: bnieuwenhuizen, jvesely, wdng, nhaehnle, llvm-commits

Differential Revision: https://reviews.llvm.org/D53739

llvm-svn: 346422

1 files changed, 5 insertions, 13 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index bdd9a43be27..c1872a1c9ce 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -3479,22 +3479,14 @@ Instruction *InstCombiner::visitCallInst(CallInst &CI) {
 
     bool Signed = II->getIntrinsicID() == Intrinsic::amdgcn_sbfe;
 
-    // TODO: Also emit sub if only width is constant.
-    if (!CWidth && COffset && Offset == 0) {
-      Constant *KSize = ConstantInt::get(COffset->getType(), IntSize);
-      Value *ShiftVal = Builder.CreateSub(KSize, II->getArgOperand(2));
-      ShiftVal = Builder.CreateZExt(ShiftVal, II->getType());
-
-      Value *Shl = Builder.CreateShl(Src, ShiftVal);
-      Value *RightShift = Signed ? Builder.CreateAShr(Shl, ShiftVal)
-                                 : Builder.CreateLShr(Shl, ShiftVal);
-      RightShift->takeName(II);
-      return replaceInstUsesWith(*II, RightShift);
-    }
-
     if (!CWidth || !COffset)
       break;
 
+    // The case of Width == 0 is handled above, which makes this tranformation
+    // safe.  If Width == 0, then the ashr and lshr instructions become poison
+    // value since the shift amount would be equal to the bit size.
+    assert(Width != 0);
+
     // TODO: This allows folding to undef when the hardware has specific
     // behavior?
     if (Offset + Width < IntSize) {