[PATCH] D60413: SExt -> Zext when no sign bits is used

[Diogo N. Sampaio via Phabricator via llvm-commits]

dnsampaio updated this revision to Diff 194166.
dnsampaio added a comment.

Added transformation for comparing.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D60413/new/

https://reviews.llvm.org/D60413

Files:
  lib/Transforms/InstCombine/InstCombineCasts.cpp
  test/Transforms/InstCombine/cast.ll


Index: test/Transforms/InstCombine/cast.ll
===================================================================
--- test/Transforms/InstCombine/cast.ll
+++ test/Transforms/InstCombine/cast.ll
@@ -1559,3 +1559,21 @@
   %5 = zext i8 %4 to i32
   ret i32 %5
 }
+
+; We should be able to convert the sext to zext here.
+; CHECK-LABEL: define i16 @foo(i16 %a) {
+; CHECK:       %and = and i16 %a, -256
+; CHECK-NEXT:  %lsr = lshr i16 %a, 8
+; CHECK-NEXT:  %or = or i16 %and, %lsr
+; CHECK-NEXT:  ret i16 %or
+
+define i16 @foo(i16 %a) {
+entry:
+  %ext = sext i16 %a to i32
+  %and = and i32 %ext, 65280
+  %lsr = lshr i32 %ext, 8
+  %and2 = and i32 %lsr, 255
+  %or = or i32 %and, %and2
+  %ret = trunc i32 %or to i16
+  ret i16 %ret
+}
Index: lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -14,6 +14,7 @@
 #include "llvm/ADT/SetVector.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/Analysis/DemandedBits.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/PatternMatch.h"
@@ -1441,7 +1442,16 @@
     A = Builder.CreateShl(A, ShAmtV, CI.getName());
     return BinaryOperator::CreateAShr(A, ShAmtV);
   }
+  // If we are not using any of the sign bits, convert to a zext
+  DemandedBits DB(*CI.getParent()->getParent(), AC, DT);
+  APInt Demanded = DB.getDemandedBits(&CI);
+  uint32_t SrcBitSize = SrcTy->getScalarSizeInBits();
+  uint32_t DestBitSize = DestTy->getScalarSizeInBits();
 
+  if(Demanded.countLeadingZeros() >= (DestBitSize - SrcBitSize)){
+    Value *ZExt = Builder.CreateZExt(Src, DestTy);
+    return replaceInstUsesWith(CI, ZExt);
+  }
   return nullptr;
 }
 


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