[llvm] 4c8c101 - [InstCombine] try to narrow more shifted bswap-of-zext

[Chenbing Zheng via llvm-commits]

Author: Chenbing Zheng
Date: 2022-05-06T10:45:10+08:00
New Revision: 4c8c101b496431c468825518e9bd4375e8209ae9

URL: https://github.com/llvm/llvm-project/commit/4c8c101b496431c468825518e9bd4375e8209ae9
DIFF: https://github.com/llvm/llvm-project/commit/4c8c101b496431c468825518e9bd4375e8209ae9.diff

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

Try to narrow more bswap, if the shift amount is less than the zext
(bswap (zext X)) >> C --> (zext (bswap X)) << C'

https://alive2.llvm.org/ce/z/i7ddjn

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D124598

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index a0a35147ba33b..e5b0e32aa85ea 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -1232,19 +1232,25 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
       }
     }
 
-    // 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 
diff erence, 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);
+        }
       }
     }
 

diff  --git a/llvm/test/Transforms/InstCombine/lshr.ll b/llvm/test/Transforms/InstCombine/lshr.ll
index e4e869b921edd..2545b86c090bf 100644
--- a/llvm/test/Transforms/InstCombine/lshr.ll
+++ b/llvm/test/Transforms/InstCombine/lshr.ll
@@ -864,18 +864,16 @@ define i128 @narrow_bswap_extra_wide(i16 %x) {
   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
 }