[PATCH] D126056: [InstCombine] [NFC] Move transforms for truncated shifts into narrowBinOp

[Chenbing.Zheng via Phabricator via llvm-commits]

Chenbing.Zheng updated this revision to Diff 431278.
Chenbing.Zheng added a comment.

address comments


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D126056/new/

https://reviews.llvm.org/D126056

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


Index: llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -636,10 +636,12 @@
 
 /// Try to narrow the width of math or bitwise logic instructions by pulling a
 /// truncate ahead of binary operators.
-/// TODO: Transforms for truncated shifts should be moved into here.
 Instruction *InstCombinerImpl::narrowBinOp(TruncInst &Trunc) {
   Type *SrcTy = Trunc.getSrcTy();
   Type *DestTy = Trunc.getType();
+  unsigned SrcWidth = SrcTy->getScalarSizeInBits();
+  unsigned DestWidth = DestTy->getScalarSizeInBits();
+
   if (!isa<VectorType>(SrcTy) && !shouldChangeType(SrcTy, DestTy))
     return nullptr;
 
@@ -682,7 +684,30 @@
     }
     break;
   }
-
+  case Instruction::LShr:
+  case Instruction::AShr: {
+    // trunc (*shr (trunc A), C) --> trunc(*shr A, C)
+    Value *A;
+    Constant *C;
+    if (match(BinOp0, m_Trunc(m_Value(A))) && match(BinOp1, m_Constant(C))) {
+      unsigned MaxShiftAmt = SrcWidth - DestWidth;
+      // If the shift is small enough, all zero/sign bits created by the shift
+      // are removed by the trunc.
+      if (match(C, m_SpecificInt_ICMP(ICmpInst::ICMP_ULE,
+                                      APInt(SrcWidth, MaxShiftAmt)))) {
+        auto *OldShift = cast<Instruction>(Trunc.getOperand(0));
+        bool IsExact = OldShift->isExact();
+        auto *ShAmt = ConstantExpr::getIntegerCast(C, A->getType(), true);
+        ShAmt = Constant::mergeUndefsWith(ShAmt, C);
+        Value *Shift =
+            OldShift->getOpcode() == Instruction::AShr
+                ? Builder.CreateAShr(A, ShAmt, OldShift->getName(), IsExact)
+                : Builder.CreateLShr(A, ShAmt, OldShift->getName(), IsExact);
+        return CastInst::CreateTruncOrBitCast(Shift, DestTy);
+      }
+    }
+    break;
+  }
   default: break;
   }
 
@@ -870,26 +895,6 @@
     // TODO: Mask high bits with 'and'.
   }
 
-  // trunc (*shr (trunc A), C) --> trunc(*shr A, C)
-  if (match(Src, m_OneUse(m_Shr(m_Trunc(m_Value(A)), m_Constant(C))))) {
-    unsigned MaxShiftAmt = SrcWidth - DestWidth;
-
-    // If the shift is small enough, all zero/sign bits created by the shift are
-    // removed by the trunc.
-    if (match(C, m_SpecificInt_ICMP(ICmpInst::ICMP_ULE,
-                                    APInt(SrcWidth, MaxShiftAmt)))) {
-      auto *OldShift = cast<Instruction>(Src);
-      bool IsExact = OldShift->isExact();
-      auto *ShAmt = ConstantExpr::getIntegerCast(C, A->getType(), true);
-      ShAmt = Constant::mergeUndefsWith(ShAmt, C);
-      Value *Shift =
-          OldShift->getOpcode() == Instruction::AShr
-              ? Builder.CreateAShr(A, ShAmt, OldShift->getName(), IsExact)
-              : Builder.CreateLShr(A, ShAmt, OldShift->getName(), IsExact);
-      return CastInst::CreateTruncOrBitCast(Shift, DestTy);
-    }
-  }
-
   if (Instruction *I = narrowBinOp(Trunc))
     return I;
 


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