Use SimplifyDemandedBits on div instructions.

This folds away silly stuff like (a&255)/1000 -> 0.

llvm-svn: 130614

llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp

@@ -320,6 +320,10 @@ Instruction *InstCombiner::commonIDivTransforms(BinaryOperator &I) {
     }
   }
 
+  // See if we can fold away this div instruction.
+  if (SimplifyDemandedInstructionBits(I))
+    return &I;
+
   // (X - (X rem Y)) / Y -> X / Y; usually originates as ((X / Y) * Y) / Y
   Value *X = 0, *Z = 0;
   if (match(Op0, m_Sub(m_Value(X), m_Value(Z)))) { // (X - Z) / Y; Y = Op1

llvm/test/Transforms/InstCombine/div.ll

@@ -111,3 +111,10 @@ define i32 @test13(i32 %x) nounwind  {
 ; CHECK-NEXT: ret i32 1
 }
 
+define i32 @test14(i8 %x) nounwind {
+	%zext = zext i8 %x to i32
+	%div = udiv i32 %zext, 257	; 0
+	ret i32 %div
+; CHECK: @test14
+; CHECK-NEXT: ret i32 0
+}