[InstCombine] Take in account the size in sext->lshr->trunc patterns.

Otherwise we end up miscompiling, transforming:

define i8 @tinky() {
  %sext = sext i1 1 to i16
  %hibit = lshr i16 %sext, 15
  %tr = trunc i16 %hibit to i8
  ret i8 %tr
}

into:

  %sext = sext i1 1 to i8
  ret i8 %sext

and the first get folded to ret i8 1, while the second gets folded
to ret i8 -1.

Eventually we should get rid of this transform entirely, but for now,
this at least fixes a know correctness bug.

Differential Revision:  https://reviews.llvm.org/D33338

llvm-svn: 303513

2 files changed, 16 insertions, 8 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 001a4bcf16f..f4bf5221f6a 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -585,6 +585,7 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
     return CastInst::CreateIntegerCast(Shift, DestTy, false);
   }
 
+  // FIXME: We should canonicalize to zext/trunc and remove this transform.
   // Transform trunc(lshr (sext A), Cst) to ashr A, Cst to eliminate type
   // conversion.
   // It works because bits coming from sign extension have the same value as
@@ -595,18 +596,24 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
     Value *SExt = cast<Instruction>(Src)->getOperand(0);
     const unsigned SExtSize = SExt->getType()->getPrimitiveSizeInBits();
     const unsigned ASize = A->getType()->getPrimitiveSizeInBits();
+    const unsigned CISize = CI.getType()->getPrimitiveSizeInBits();
+    const unsigned MaxAmt = SExtSize - std::max(CISize, ASize);
     unsigned ShiftAmt = Cst->getZExtValue();
+
     // This optimization can be only performed when zero bits generated by
     // the original lshr aren't pulled into the value after truncation, so we
     // can only shift by values no larger than the number of extension bits.
     // FIXME: Instead of bailing when the shift is too large, use and to clear
     // the extra bits.
-    if (SExt->hasOneUse() && ShiftAmt <= SExtSize - ASize) {
-      // If shifting by the size of the original value in bits or more, it is
-      // being filled with the sign bit, so shift by ASize-1 to avoid ub.
-      Value *Shift = Builder->CreateAShr(A, std::min(ShiftAmt, ASize-1));
-      Shift->takeName(Src);
-      return CastInst::CreateIntegerCast(Shift, CI.getType(), true);
+    if (ShiftAmt <= MaxAmt) {
+      if (CISize == ASize)
+        return BinaryOperator::CreateAShr(A, ConstantInt::get(CI.getType(),
+                                          std::min(ShiftAmt, ASize - 1)));
+      if (SExt->hasOneUse()) {
+        Value *Shift = Builder->CreateAShr(A, std::min(ShiftAmt, ASize-1));
+        Shift->takeName(Src);
+        return CastInst::CreateIntegerCast(Shift, CI.getType(), true);
+      }
     }
   }
 
diff --git a/llvm/test/Transforms/InstCombine/cast.ll b/llvm/test/Transforms/InstCombine/cast.ll
index 2f15617e065..486a617097e 100644
--- a/llvm/test/Transforms/InstCombine/cast.ll
+++ b/llvm/test/Transforms/InstCombine/cast.ll
@@ -1513,8 +1513,9 @@ define i4 @pr33078_3(i8 %A) {
 define i8 @pr33078_4(i3 %x) {
 ; Don't turn this in an `ashr`. This was getting miscompiled
 ; CHECK-LABEL: @pr33078_4(
-; CHECK-NEXT:    [[C:%.*]] = ashr i3 %x, 2
-; CHECK-NEXT:    [[B:%.*]] = sext i3 [[C]] to i8
+; CHECK-NEXT:    [[B:%.*]] = sext i3 %x to i16
+; CHECK-NEXT:    [[C:%.*]] = lshr i16 [[B]], 13
+; CHECK-NEXT:    [[D:%.*]] = trunc i16 [[C]] to i8
 ; CHECK-NEXT:    ret i8 [[D]]
   %B = sext i3 %x to i16
   %C = lshr i16 %B, 13