[PATCH] D124598: [InstCombine] try to narrow more shifted bswap-of-zext

[Chenbing.Zheng via Phabricator via llvm-commits]

This revision was landed with ongoing or failed builds.
This revision was automatically updated to reflect the committed changes.
Closed by commit rG4c8c101b4964: [InstCombine] try to narrow more shifted bswap-of-zext (authored by Chenbing.Zheng).

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D124598/new/

https://reviews.llvm.org/D124598

Files:
  llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
  llvm/test/Transforms/InstCombine/lshr.ll


Index: llvm/test/Transforms/InstCombine/lshr.ll
===================================================================
--- llvm/test/Transforms/InstCombine/lshr.ll
+++ llvm/test/Transforms/InstCombine/lshr.ll
@@ -864,18 +864,16 @@
   ret i128 %s
 }
 
-; TODO: The bswap can be narrowed followed by shl.
-
 define i32 @narrow_bswap_undershift(i16 %x) {
 ; CHECK-LABEL: @narrow_bswap_undershift(
-; CHECK-NEXT:    [[Z:%.*]] = zext i16 [[X:%.*]] to i32
-; CHECK-NEXT:    [[B:%.*]] = call i32 @llvm.bswap.i32(i32 [[Z]])
-; CHECK-NEXT:    [[S:%.*]] = lshr exact i32 [[B]], 8
+; CHECK-NEXT:    [[TMP1:%.*]] = call i16 @llvm.bswap.i16(i16 [[X:%.*]])
+; CHECK-NEXT:    [[TMP2:%.*]] = zext i16 [[TMP1]] to i32
+; CHECK-NEXT:    [[S:%.*]] = shl nuw nsw i32 [[TMP2]], 7
 ; CHECK-NEXT:    ret i32 [[S]]
 ;
   %z = zext i16 %x to i32
   %b = call i32 @llvm.bswap.i32(i32 %z)
-  %s = lshr i32 %b, 8
+  %s = lshr i32 %b, 9
   ret i32 %s
 }
 
Index: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -1232,19 +1232,25 @@
       }
     }
 
-    // Try to narrow a bswap:
-    // (bswap (zext X)) >> C --> zext (bswap X >> C')
+    // Try to narrow bswap.
     // In the case where the shift amount equals the bitwidth difference, the
     // shift is eliminated.
     if (match(Op0, m_OneUse(m_Intrinsic<Intrinsic::bswap>(
                        m_OneUse(m_ZExt(m_Value(X))))))) {
-      // TODO: If the shift amount is less than the zext, we could shift left.
       unsigned SrcWidth = X->getType()->getScalarSizeInBits();
       unsigned WidthDiff = BitWidth - SrcWidth;
-      if (SrcWidth % 16 == 0 && ShAmtC >= WidthDiff) {
+      if (SrcWidth % 16 == 0) {
         Value *NarrowSwap = Builder.CreateUnaryIntrinsic(Intrinsic::bswap, X);
-        Value *NewShift = Builder.CreateLShr(NarrowSwap, ShAmtC - WidthDiff);
-        return new ZExtInst(NewShift, Ty);
+        if (ShAmtC >= WidthDiff) {
+          // (bswap (zext X)) >> C --> zext (bswap X >> C')
+          Value *NewShift = Builder.CreateLShr(NarrowSwap, ShAmtC - WidthDiff);
+          return new ZExtInst(NewShift, Ty);
+        } else {
+          // (bswap (zext X)) >> C --> (zext (bswap X)) << C'
+          Value *NewZExt = Builder.CreateZExt(NarrowSwap, Ty);
+          Constant *ShiftDiff = ConstantInt::get(Ty, WidthDiff - ShAmtC);
+          return BinaryOperator::CreateShl(NewZExt, ShiftDiff);
+        }
       }
     }
 


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