[PATCH] D157308: [ValueTracking] Add support for `mul` in `isKnownToBeAPowerOfTwo`

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n created this revision.
goldstein.w.n added reviewers: nikic, Allen.
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pow2 * pow2 is a power of 2 or zero.

Proof: https://alive2.llvm.org/ce/z/FNiiXd


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D157308

Files:
  llvm/lib/Analysis/ValueTracking.cpp
  llvm/test/Analysis/ValueTracking/known-power-of-two.ll


Index: llvm/test/Analysis/ValueTracking/known-power-of-two.ll
===================================================================
--- llvm/test/Analysis/ValueTracking/known-power-of-two.ll
+++ llvm/test/Analysis/ValueTracking/known-power-of-two.ll
@@ -345,7 +345,8 @@
 ; CHECK-NEXT:    [[XP2:%.*]] = shl i16 4, [[X]]
 ; CHECK-NEXT:    [[ZP2:%.*]] = shl i16 2, [[Z]]
 ; CHECK-NEXT:    [[XX:%.*]] = mul i16 [[XP2]], [[ZP2]]
-; CHECK-NEXT:    [[R:%.*]] = urem i16 [[Y]], [[XX]]
+; CHECK-NEXT:    [[TMP1:%.*]] = add i16 [[XX]], -1
+; CHECK-NEXT:    [[R:%.*]] = and i16 [[TMP1]], [[Y]]
 ; CHECK-NEXT:    ret i16 [[R]]
 ;
   %xp2 = shl i16 4, %x
Index: llvm/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/lib/Analysis/ValueTracking.cpp
+++ llvm/lib/Analysis/ValueTracking.cpp
@@ -2057,6 +2057,10 @@
     if (Q.IIQ.isExact(cast<BinaryOperator>(I)))
       return isKnownToBeAPowerOfTwo(I->getOperand(0), OrZero, Depth, Q);
     return false;
+  case Instruction::Mul:
+    return OrZero &&
+           isKnownToBeAPowerOfTwo(I->getOperand(1), OrZero, Depth, Q) &&
+           isKnownToBeAPowerOfTwo(I->getOperand(0), OrZero, Depth, Q);
   case Instruction::And:
     if (OrZero) {
       // A power of two and'd with anything is a power of two or zero.


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