[InstCombine] Fold X * (2^N + 1) >> N -> X + X >> N, or directly to X if X >> N is 0

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -1479,6 +1479,29 @@ static Value *simplifyLShrInst(Value *Op0, Value *Op1, bool IsExact,
   if (Q.IIQ.UseInstrInfo && match(Op0, m_NUWShl(m_Value(X), m_Specific(Op1))))
     return X;
 
+  // Look for a "splat" mul pattern - it replicates bits across each half
+  // of a value, so a right shift is just a mask of the low bits:
+  const APInt *MulC;
+  const APInt *ShAmt;
+  if (Q.IIQ.UseInstrInfo && match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC))) &&
+      match(Op1, m_APInt(ShAmt))) {
+    unsigned ShAmtC = ShAmt->getZExtValue();
+    unsigned BitWidth = ShAmt->getBitWidth();
+    if (BitWidth > 2 && (*MulC - 1).isPowerOf2() &&
+        MulC->logBase2() == ShAmtC) {
+      // FIXME: This condition should be covered by the computeKnownBits, but
+      // for some reason it is not, so keep this in for now. This has no
+      // negative effects, but KnownBits should be able to infer a number of
+      // leading bits based on 2^N + 1 not wrapping, as that means 2^N must not
+      // wrap either, which means the top N bits of X must be 0.
+      if (ShAmtC * 2 == BitWidth)
+        return X;
+      const KnownBits XKnown = computeKnownBits(X, /* Depth */ 0, Q);
+      if (XKnown.countMaxActiveBits() <= ShAmtC)
+        return X;
+    }
+  }
+
   // ((X << A) | Y) >> A -> X  if effective width of Y is not larger than A.
   // We can return X as we do in the above case since OR alters no bits in X.
   // SimplifyDemandedBits in InstCombine can do more general optimization for
@@ -1523,6 +1546,22 @@ static Value *simplifyAShrInst(Value *Op0, Value *Op1, bool IsExact,
   if (Q.IIQ.UseInstrInfo && match(Op0, m_NSWShl(m_Value(X), m_Specific(Op1))))
     return X;
 
+  const APInt *MulC;
+  const APInt *ShAmt;
+  if (Q.IIQ.UseInstrInfo && match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC))) &&
+      match(Op1, m_APInt(ShAmt)) &&
+      cast<OverflowingBinaryOperator>(Op0)->hasNoSignedWrap()) {
+    unsigned ShAmtC = ShAmt->getZExtValue();
+    unsigned BitWidth = ShAmt->getBitWidth();
+    if (BitWidth > 2 && (*MulC - 1).isPowerOf2() &&
+        MulC->logBase2() == ShAmtC &&
+        ShAmtC < BitWidth - 1) /* Minus 1 for the sign bit */ {
+      KnownBits KnownX = computeKnownBits(X, /* Depth */ 0, Q);
+      if (KnownX.countMaxActiveBits() <= ShAmtC)
+        return X;
+    }
+  }
+
   // Arithmetic shifting an all-sign-bit value is a no-op.
   unsigned NumSignBits = ComputeNumSignBits(Op0, Q.DL, 0, Q.AC, Q.CxtI, Q.DT);
   if (NumSignBits == Op0->getType()->getScalarSizeInBits())

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -1456,30 +1456,42 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
     }
 
     const APInt *MulC;
-    if (match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC)))) {
-      // Look for a "splat" mul pattern - it replicates bits across each half of
-      // a value, so a right shift is just a mask of the low bits:
-      // lshr i[2N] (mul nuw X, (2^N)+1), N --> and iN X, (2^N)-1
-      // TODO: Generalize to allow more than just half-width shifts?
-      if (BitWidth > 2 && ShAmtC * 2 == BitWidth && (*MulC - 1).isPowerOf2() &&
-          MulC->logBase2() == ShAmtC)
-        return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, *MulC - 2));
+    if (match(Op0, m_OneUse(m_NUWMul(m_Value(X), m_APInt(MulC))))) {
+      if (BitWidth > 2 && (*MulC - 1).isPowerOf2() &&
+          MulC->logBase2() == ShAmtC) {
+
+        // lshr (mul nuw (X, 2^N + 1)), N -> add nuw (X, lshr(X, N))
+        auto *NewAdd = BinaryOperator::CreateNUWAdd(
+            X, Builder.CreateLShr(X, ConstantInt::get(Ty, ShAmtC), "",
+                                  I.isExact()));
+        NewAdd->setHasNoSignedWrap(
+            cast<OverflowingBinaryOperator>(Op0)->hasNoSignedWrap());
+        return NewAdd;
+      }
 
       // The one-use check is not strictly necessary, but codegen may not be
       // able to invert the transform and perf may suffer with an extra mul
       // instruction.
-      if (Op0->hasOneUse()) {
-        APInt NewMulC = MulC->lshr(ShAmtC);
-        // if c is divisible by (1 << ShAmtC):
-        // lshr (mul nuw x, MulC), ShAmtC -> mul nuw nsw x, (MulC >> ShAmtC)
-        if (MulC->eq(NewMulC.shl(ShAmtC))) {
-          auto *NewMul =
-              BinaryOperator::CreateNUWMul(X, ConstantInt::get(Ty, NewMulC));
-          assert(ShAmtC != 0 &&
-                 "lshr X, 0 should be handled by simplifyLShrInst.");
-          NewMul->setHasNoSignedWrap(true);
-          return NewMul;
-        }
+      APInt NewMulC = MulC->lshr(ShAmtC);
+      // if c is divisible by (1 << ShAmtC):
+      // lshr (mul nuw x, MulC), ShAmtC -> mul nuw nsw x, (MulC >> ShAmtC)
+      if (MulC->eq(NewMulC.shl(ShAmtC))) {
+        auto *NewMul =
+            BinaryOperator::CreateNUWMul(X, ConstantInt::get(Ty, NewMulC));
+        assert(ShAmtC != 0 &&
+               "lshr X, 0 should be handled by simplifyLShrInst.");
+        NewMul->setHasNoSignedWrap(true);
+        return NewMul;
+      }
+    }
+
+    // lshr (mul nsw (X, 2^N + 1)), N -> add nsw (X, lshr(X, N))
+    if (match(Op0, m_OneUse(m_NSWMul(m_Value(X), m_APInt(MulC))))) {
+      if (BitWidth > 2 && (*MulC - 1).isPowerOf2() &&
+          MulC->logBase2() == ShAmtC) {
+        return BinaryOperator::CreateNSWAdd(
+            X, Builder.CreateLShr(X, ConstantInt::get(Ty, ShAmtC), "",
+                                  I.isExact()));
       }
     }
 
@@ -1686,6 +1698,21 @@ Instruction *InstCombinerImpl::visitAShr(BinaryOperator &I) {
       if (match(Op0, m_OneUse(m_NSWSub(m_Value(X), m_Value(Y)))))
         return new SExtInst(Builder.CreateICmpSLT(X, Y), Ty);
     }
+
+    const APInt *MulC;
+    if (match(Op0, m_OneUse(m_NSWMul(m_Value(X), m_APInt(MulC)))) &&
+        (BitWidth > 2 && (*MulC - 1).isPowerOf2() &&
+         MulC->logBase2() == ShAmt &&
+         (ShAmt < BitWidth - 1))) /* Minus 1 for the sign bit */ {
+
+      // ashr (mul nsw (X, 2^N + 1)), N -> add nsw (X, ashr(X, N))
+      auto *NewAdd = BinaryOperator::CreateNSWAdd(
+          X,
+          Builder.CreateAShr(X, ConstantInt::get(Ty, ShAmt), "", I.isExact()));
+      NewAdd->setHasNoUnsignedWrap(
+          cast<OverflowingBinaryOperator>(Op0)->hasNoUnsignedWrap());
+      return NewAdd;
+    }
   }
 
   const SimplifyQuery Q = SQ.getWithInstruction(&I);