[PATCH] D118539: [InstCombine] try to fold one-demanded-bit-of-multiply

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri accepted this revision.
lebedev.ri added a comment.
This revision is now accepted and ready to land.

LGTM, thank you!



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Comment at: llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp:548
+  case Instruction::Mul: {
+    // The LSB of X*Y is set only if (X & 1) == 1 and (Y & 1) == 1.
+    // If we demand exactly one bit N and we have "X * (C' << N)" where C' is
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https://alive2.llvm.org/ce/z/f39Z6n


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Comment at: llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp:549-550
+    // The LSB of X*Y is set only if (X & 1) == 1 and (Y & 1) == 1.
+    // If we demand exactly one bit N and we have "X * (C' << N)" where C' is
+    // odd (has LSB set), then the left-shifted low bit of X is the answer.
+    if (DemandedMask.isPowerOf2()) {
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https://alive2.llvm.org/ce/z/RakJNM


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Comment at: llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp:551-560
+    if (DemandedMask.isPowerOf2()) {
+      unsigned CTZ = DemandedMask.countTrailingZeros();
+      const APInt *C;
+      if (match(I->getOperand(1), m_APInt(C)) &&
+          C->countTrailingZeros() == CTZ) {
+        Constant *ShiftC = ConstantInt::get(I->getType(), CTZ);
+        Instruction *Shl = BinaryOperator::CreateShl(I->getOperand(0), ShiftC);
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https://alive2.llvm.org/ce/z/jgKaGK


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https://reviews.llvm.org/D118539