[PATCH] D117680: [InstCombine] Fold bswap(shl(x, C)) -> and(x, 255)

[Paweł Bylica via Phabricator via llvm-commits]

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If bswap(shl(x, C)) and the C is the x's number of bits - 8
(i.e. the least significant byte is shifted to the most significant
position) then fold it to least significant byte mask and(x, 255).


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D117680

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
  llvm/test/Transforms/InstCombine/bswap-fold.ll


Index: llvm/test/Transforms/InstCombine/bswap-fold.ll
===================================================================
--- llvm/test/Transforms/InstCombine/bswap-fold.ll
+++ llvm/test/Transforms/InstCombine/bswap-fold.ll
@@ -355,6 +355,66 @@
   ret i64 %t3
 }
 
+define i16 @bs_shl16(i16 %0) {
+; CHECK-LABEL: @bs_shl16(
+; CHECK-NEXT:    [[TMP2:%.*]] = and i16 [[TMP0:%.*]], 255
+; CHECK-NEXT:    ret i16 [[TMP2]]
+;
+  %2 = shl i16 %0, 8
+  %3 = call i16 @llvm.bswap.i16(i16 %2)
+  ret i16 %3
+}
+
+define i32 @bs_shl32(i32 %0) {
+; CHECK-LABEL: @bs_shl32(
+; CHECK-NEXT:    [[TMP2:%.*]] = and i32 [[TMP0:%.*]], 255
+; CHECK-NEXT:    ret i32 [[TMP2]]
+;
+  %2 = shl i32 %0, 24
+  %3 = call i32 @llvm.bswap.i32(i32 %2)
+  ret i32 %3
+}
+
+define i64 @bs_shl64(i64 %0) {
+; CHECK-LABEL: @bs_shl64(
+; CHECK-NEXT:    [[TMP2:%.*]] = and i64 [[TMP0:%.*]], 255
+; CHECK-NEXT:    ret i64 [[TMP2]]
+;
+  %2 = shl i64 %0, 56
+  %3 = call i64 @llvm.bswap.i64(i64 %2)
+  ret i64 %3
+}
+
+define <2 x i16> @bs_shlv2i16(<2 x i16> %0) {
+; CHECK-LABEL: @bs_shlv2i16(
+; CHECK-NEXT:    [[TMP2:%.*]] = and <2 x i16> [[TMP0:%.*]], <i16 255, i16 255>
+; CHECK-NEXT:    ret <2 x i16> [[TMP2]]
+;
+  %2 = shl <2 x i16> %0, <i16 8, i16 8>
+  %3 = call <2 x i16> @llvm.bswap.v2i16(<2 x i16> %2)
+  ret <2 x i16> %3
+}
+
+define <2 x i32> @bs_shlv2i32(<2 x i32> %0) {
+; CHECK-LABEL: @bs_shlv2i32(
+; CHECK-NEXT:    [[TMP2:%.*]] = and <2 x i32> [[TMP0:%.*]], <i32 255, i32 255>
+; CHECK-NEXT:    ret <2 x i32> [[TMP2]]
+;
+  %2 = shl <2 x i32> %0, <i32 24, i32 24>
+  %3 = call <2 x i32> @llvm.bswap.v2i32(<2 x i32> %2)
+  ret <2 x i32> %3
+}
+
+define <2 x i64> @bs_shlv2i64(<2 x i64> %0) {
+; CHECK-LABEL: @bs_shlv2i64(
+; CHECK-NEXT:    [[TMP2:%.*]] = and <2 x i64> [[TMP0:%.*]], <i64 255, i64 255>
+; CHECK-NEXT:    ret <2 x i64> [[TMP2]]
+;
+  %2 = shl <2 x i64> %0, <i64 56, i64 56>
+  %3 = call <2 x i64> @llvm.bswap.v2i64(<2 x i64> %2)
+  ret <2 x i64> %3
+}
+
 declare i16 @llvm.bswap.i16(i16)
 declare i32 @llvm.bswap.i32(i32)
 declare i64 @llvm.bswap.i64(i64)
Index: llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -1196,6 +1196,12 @@
       Value *V = Builder.CreateLShr(X, CV);
       return new TruncInst(V, IIOperand->getType());
     }
+
+    // bswap(shl(x, bits(x)-8) -> and(x, 0xff)
+    const APInt *C;
+    if (match(IIOperand, m_Shl(m_Value(X), m_APInt(C))) &&
+        *C == (X->getType()->getScalarSizeInBits() - 8))
+      return BinaryOperator::CreateAnd(X, ConstantInt::get(X->getType(), 0xff));
     break;
   }
   case Intrinsic::masked_load:


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