[PATCH] Teach ComputeNumSignBits about signed divisions

[Nadav Rotem]

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D8028

Files:
  ../llvm/lib/Analysis/ValueTracking.cpp
  ../llvm/test/Transforms/InstCombine/intrinsics.ll

Index: ../llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- ../llvm/lib/Analysis/ValueTracking.cpp
+++ ../llvm/lib/Analysis/ValueTracking.cpp
@@ -1723,6 +1723,22 @@
     Tmp = TyBits - U->getOperand(0)->getType()->getScalarSizeInBits();
     return ComputeNumSignBits(U->getOperand(0), TD, Depth+1, Q) + Tmp;
 
+  case Instruction::SDiv:
+    const APInt *Denominator;
+    // sdiv X, C -> adds log(C) sign bits.
+    if (match(U->getOperand(1), m_APInt(Denominator))) {
+
+      // Ignore non-positive denominator.
+      if (!Denominator->isStrictlyPositive())
+        break;
+
+      unsigned NumBits = ComputeNumSignBits(U->getOperand(0), TD, Depth+1, Q);
+
+      // Add floor(log(c)) bits.
+      return NumBits + Denominator->logBase2();
+    }
+    break;
+
   case Instruction::AShr: {
     Tmp = ComputeNumSignBits(U->getOperand(0), TD, Depth+1, Q);
     // ashr X, C   -> adds C sign bits.  Vectors too.
Index: ../llvm/test/Transforms/InstCombine/intrinsics.ll
===================================================================
--- ../llvm/test/Transforms/InstCombine/intrinsics.ll
+++ ../llvm/test/Transforms/InstCombine/intrinsics.ll
@@ -363,3 +363,14 @@
 ; CHECK-NEXT: call i32 @llvm.cttz.i32(i32 %Value, i1 false)
 ; CHECK-NEXT: ret i32
 }
+
+; CHECK-LABEL: @overflow_div(
+; CHECK: ret i1 false
+define i1 @overflow_div(i32 %v1, i32 %v2) nounwind {
+entry:
+  %div = sdiv i32 %v1, 2
+  %t = call %ov.result.32 @llvm.sadd.with.overflow.i32(i32 %div, i32 1)
+  %obit = extractvalue %ov.result.32 %t, 1
+  ret i1 %obit
+}
+

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