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!

llvm-svn: 127146

llvm/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))) {

llvm/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);

llvm/test/Transforms/InstCombine/icmp.ll

@@ -475,3 +475,22 @@ entry:
   %cmp = icmp ult <2 x i32> %tmp11, <i32 4, i32 4>
   ret <2 x i1> %cmp  
 }
+
+; PR9343 #7
+; CHECK: @test50
+; CHECK: ret i1 true
+define i1 @test50(i16 %X, i32 %Y) {
+  %A = zext i16 %X to i32
+  %B = srem i32 %A, %Y
+  %C = icmp sgt i32 %B, -1
+  ret i1 %C
+}
+
+; CHECK: @test51
+; CHECK: ret i1 %C
+define i1 @test51(i16 %X, i32 %Y) {
+  %A = sext i16 %X to i32
+  %B = srem i32 %A, %Y
+  %C = icmp sgt i32 %B, -1
+  ret i1 %C
+}