InstSimplify: Optimize using dividend in sdiv

Summary:
The dividend in an sdiv tells us the largest and smallest possible
results.  Use this fact to optimize comparisons against an sdiv with a
constant dividend.

Reviewers: nicholas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3795

llvm-svn: 208999

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -2020,6 +2020,10 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
       APInt NegOne = APInt::getAllOnesValue(Width);
       if (!CI2->isZero())
         Upper = NegOne.udiv(CI2->getValue()) + 1;
+    } else if (match(LHS, m_SDiv(m_ConstantInt(CI2), m_Value()))) {
+      // 'sdiv CI2, x' produces [-|CI2|, |CI2|].
+      Upper = CI2->getValue().abs() + 1;
+      Lower = (-Upper) + 1;
     } else if (match(LHS, m_SDiv(m_Value(), m_ConstantInt(CI2)))) {
       // 'sdiv x, CI2' produces [INT_MIN / CI2, INT_MAX / CI2].
       APInt IntMin = APInt::getSignedMinValue(Width);

llvm/test/Transforms/InstSimplify/compare.ll

@@ -817,3 +817,12 @@ define i1 @compare_always_false_ne(i16 %a) {
 ; CHECK-LABEL: @compare_always_false_ne
 ; CHECK-NEXT: ret i1 true
 }
+
+define i1 @compare_dividend(i32 %a) {
+  %div = sdiv i32 2, %a
+  %cmp = icmp eq i32 %div, 3
+  ret i1 %cmp
+
+; CHECK-LABEL: @compare_dividend
+; CHECK-NEXT: ret i1 false
+}