[PATCH] D124590: [InstCombine] add casts from splat-a-bit pattern if necessary

[Chenbing.Zheng via Phabricator via llvm-commits]

Chenbing.Zheng created this revision.
Chenbing.Zheng added reviewers: spatel, RKSimon, benshi001.
Chenbing.Zheng added a project: LLVM.
Herald added a subscriber: hiraditya.
Herald added a project: All.
Chenbing.Zheng requested review of this revision.
Herald added subscribers: llvm-commits, jacquesguan.

Splatting a bit of constant-index across a value:
sext (ashr (trunc iN X to iM), M-1) to iN --> ashr (shl X, N-M), N-1
If the dest type is different, use a cast (adjust use check).

https://alive2.llvm.org/ce/z/acAan3


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D124590

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
  llvm/test/Transforms/InstCombine/sext.ll


Index: llvm/test/Transforms/InstCombine/sext.ll
===================================================================
--- llvm/test/Transforms/InstCombine/sext.ll
+++ llvm/test/Transforms/InstCombine/sext.ll
@@ -381,13 +381,11 @@
   ret i32 %s
 }
 
-; TODO: this could be mask+compare+sext or shifts+trunc
-
 define i16 @smear_set_bit_different_dest_type(i32 %x) {
 ; CHECK-LABEL: @smear_set_bit_different_dest_type(
-; CHECK-NEXT:    [[T:%.*]] = trunc i32 [[X:%.*]] to i8
-; CHECK-NEXT:    [[A:%.*]] = ashr i8 [[T]], 7
-; CHECK-NEXT:    [[S:%.*]] = sext i8 [[A]] to i16
+; CHECK-NEXT:    [[TMP1:%.*]] = shl i32 [[X:%.*]], 24
+; CHECK-NEXT:    [[TMP2:%.*]] = ashr i32 [[TMP1]], 31
+; CHECK-NEXT:    [[S:%.*]] = trunc i32 [[TMP2]] to i16
 ; CHECK-NEXT:    ret i16 [[S]]
 ;
   %t = trunc i32 %x to i8
Index: llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1592,14 +1592,23 @@
 
   // Splatting a bit of constant-index across a value:
   // sext (ashr (trunc iN X to iM), M-1) to iN --> ashr (shl X, N-M), N-1
-  // TODO: If the dest type is different, use a cast (adjust use check).
+  // If the dest type is different, use a cast (adjust use check).
   if (match(Src, m_OneUse(m_AShr(m_Trunc(m_Value(X)),
-                                 m_SpecificInt(SrcBitSize - 1)))) &&
-      X->getType() == DestTy) {
-    Constant *ShlAmtC = ConstantInt::get(DestTy, DestBitSize - SrcBitSize);
-    Constant *AshrAmtC = ConstantInt::get(DestTy, DestBitSize - 1);
-    Value *Shl = Builder.CreateShl(X, ShlAmtC);
-    return BinaryOperator::CreateAShr(Shl, AshrAmtC);
+                                 m_SpecificInt(SrcBitSize - 1))))) {
+    Type *XTy = X->getType();
+    if (XTy == DestTy) {
+      Constant *ShlAmtC = ConstantInt::get(DestTy, DestBitSize - SrcBitSize);
+      Constant *AshrAmtC = ConstantInt::get(DestTy, DestBitSize - 1);
+      Value *Shl = Builder.CreateShl(X, ShlAmtC);
+      return BinaryOperator::CreateAShr(Shl, AshrAmtC);
+    } else {
+      unsigned XBitSize = XTy->getScalarSizeInBits();
+      Constant *ShlAmtC = ConstantInt::get(XTy, XBitSize - SrcBitSize);
+      Constant *AshrAmtC = ConstantInt::get(XTy, XBitSize - 1);
+      Value *Shl = Builder.CreateShl(X, ShlAmtC);
+      Value *Ashr = Builder.CreateAShr(Shl, AshrAmtC);
+      return CastInst::CreateIntegerCast(Ashr, DestTy, /* isSigned */ true);
+    }
   }
 
   if (match(Src, m_VScale(DL))) {


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