[PATCH] D127469: [InstCombine] Optimize lshr+shl+and conversion pattern

[chenglin.bi via Phabricator via llvm-commits]

bcl5980 created this revision.
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if `C1` and `C3` are pow2 and `Log2(C3) >= C2`:

  ((C1 >> X) << C2) & C3 -> X == (Log2(C1)+C2-Log2(C3)) ? C3 : 0

https://alive2.llvm.org/ce/z/zvrkKF


https://reviews.llvm.org/D127469

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp


Index: llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -1908,25 +1908,40 @@
     Constant *C1, *C2;
     const APInt *C3 = C;
     Value *X;
-    if (C3->isPowerOf2() &&
-        match(Op0, m_OneUse(m_LShr(m_Shl(m_ImmConstant(C1), m_Value(X)),
-                                   m_ImmConstant(C2)))) &&
-        match(C1, m_Power2())) {
-      Constant *Log2C1 = ConstantExpr::getExactLogBase2(C1);
+    if (C3->isPowerOf2()) {
       Constant *Log2C3 = ConstantInt::get(Ty, C3->countTrailingZeros());
-      Constant *LshrC = ConstantExpr::getAdd(C2, Log2C3);
-      KnownBits KnownLShrc = computeKnownBits(LshrC, 0, nullptr);
-      if (KnownLShrc.getMaxValue().ult(Width)) {
-        // iff C1,C3 is pow2 and C2 + cttz(C3) < BitWidth:
-        // ((C1 << X) >> C2) & C3 -> X == (cttz(C3)+C2-cttz(C1)) ? C3 : 0
-        Constant *CmpC = ConstantExpr::getSub(LshrC, Log2C1);
-        Value *Cmp = Builder.CreateICmpEQ(X, CmpC);
-        return SelectInst::Create(Cmp, ConstantInt::get(Ty, *C3),
-                                  ConstantInt::getNullValue(Ty));
+      if (match(Op0, m_OneUse(m_LShr(m_Shl(m_ImmConstant(C1), m_Value(X)),
+                                     m_ImmConstant(C2)))) &&
+          match(C1, m_Power2())) {
+        Constant *Log2C1 = ConstantExpr::getExactLogBase2(C1);
+        Constant *LshrC = ConstantExpr::getAdd(C2, Log2C3);
+        KnownBits KnownLShrc = computeKnownBits(LshrC, 0, nullptr);
+        if (KnownLShrc.getMaxValue().ult(Width)) {
+          // iff C1,C3 is pow2 and C2 + cttz(C3) < BitWidth:
+          // ((C1 << X) >> C2) & C3 -> X == (cttz(C3)+C2-cttz(C1)) ? C3 : 0
+          Constant *CmpC = ConstantExpr::getSub(LshrC, Log2C1);
+          Value *Cmp = Builder.CreateICmpEQ(X, CmpC);
+          return SelectInst::Create(Cmp, ConstantInt::get(Ty, *C3),
+                                    ConstantInt::getNullValue(Ty));
+        }
+      }
+
+      if (match(Op0, m_OneUse(m_Shl(m_LShr(m_ImmConstant(C1), m_Value(X)),
+                                    m_ImmConstant(C2)))) &&
+          match(C1, m_Power2())) {
+        Constant *Log2C1 = ConstantExpr::getExactLogBase2(C1);
+        Constant *Cmp =
+            ConstantExpr::getCompare(ICmpInst::ICMP_ULT, Log2C3, C2);
+        if (Cmp->isZeroValue()) {
+          // iff C1,C3 is pow2 and Log2(C3) >= C2:
+          // ((C1 >> X) << C2) & C3 -> X == (cttz(C1)+C2-cttz(C3)) ? C3 : 0
+          Constant *ShlC = ConstantExpr::getAdd(C2, Log2C1);
+          Constant *CmpC = ConstantExpr::getSub(ShlC, Log2C3);
+          Value *Cmp = Builder.CreateICmpEQ(X, CmpC);
+          return SelectInst::Create(Cmp, ConstantInt::get(Ty, *C3),
+                                    ConstantInt::getNullValue(Ty));
+        }
       }
-      // TODO: Symmetrical case
-      // iff C1,C3 is pow2 and Log2(C3) >= C2:
-      // ((C1 >> X) << C2) & C3 -> X == (cttz(C1)+C2-cttz(C3)) ? C3 : 0
     }
   }
 


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