Add more analysis of the sign bit of an srem instruction. If the LHS is negative

then the result could go either way. If it's provably positive then so is the
srem. Fixes PR9343 #7!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127146 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 25 insertions, 0 deletions

diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index 82f4f64fc6..231b95b618 100644
--- a/lib/Analysis/ValueTracking.cpp
+++ b/lib/Analysis/ValueTracking.cpp
@@ -460,6 +460,19 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
         assert((KnownZero & KnownOne) == 0 && "Bits known to be one AND zero?"); 
       }
     }
+
+    // The sign bit is the LHS's sign bit, except when the result of the
+    // remainder is zero.
+    if (Mask.isNegative() && KnownZero.isNonNegative()) {
+      APInt Mask2 = APInt::getSignBit(BitWidth);
+      APInt LHSKnownZero(BitWidth, 0), LHSKnownOne(BitWidth, 0);
+      ComputeMaskedBits(I->getOperand(0), Mask2, LHSKnownZero, LHSKnownOne, TD,
+                        Depth+1);
+      // If it's known zero, our sign bit is also zero.
+      if (LHSKnownZero.isNegative())
+        KnownZero |= LHSKnownZero;
+    }
+
     break;
   case Instruction::URem: {
     if (ConstantInt *Rem = dyn_cast<ConstantInt>(I->getOperand(1))) {
diff --git a/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index bda8cea4e4..b12d4c3687 100644
--- a/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -712,6 +712,18 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
         assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?"); 
       }
     }
+
+    // The sign bit is the LHS's sign bit, except when the result of the
+    // remainder is zero.
+    if (DemandedMask.isNegative() && KnownZero.isNonNegative()) {
+      APInt Mask2 = APInt::getSignBit(BitWidth);
+      APInt LHSKnownZero(BitWidth, 0), LHSKnownOne(BitWidth, 0);
+      ComputeMaskedBits(I->getOperand(0), Mask2, LHSKnownZero, LHSKnownOne,
+                        Depth+1);
+      // If it's known zero, our sign bit is also zero.
+      if (LHSKnownZero.isNegative())
+        KnownZero |= LHSKnownZero;
+    }
     break;
   case Instruction::URem: {
     APInt KnownZero2(BitWidth, 0), KnownOne2(BitWidth, 0);