[PATCH] D76983: [InstCombine] Transform extractelement-trunc -> bitcast-extractelement

[Daan Sprenkels via Phabricator via llvm-commits]

dsprenkels updated this revision to Diff 253452.
dsprenkels added a comment.

The new preliminary tests have been submitted in https://reviews.llvm.org/D77024. I removed the test for now and will submit another update when the other diff has been committed.

I also updated the diff in line with the inline comments.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D76983/new/

https://reviews.llvm.org/D76983

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp


Index: llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -690,6 +690,7 @@
 
   Value *Src = CI.getOperand(0);
   Type *DestTy = CI.getType(), *SrcTy = Src->getType();
+  ConstantInt *Cst;
 
   // Attempt to truncate the entire input expression tree to the destination
   // type.   Only do this if the dest type is a simple type, don't convert the
@@ -758,7 +759,7 @@
   // more efficiently. Support vector types. Cleanup code by using m_OneUse.
 
   // Transform trunc(lshr (zext A), Cst) to eliminate one type conversion.
-  Value *A = nullptr; ConstantInt *Cst = nullptr;
+  Value *A = nullptr;
   if (Src->hasOneUse() &&
       match(Src, m_LShr(m_ZExt(m_Value(A)), m_ConstantInt(Cst)))) {
     // We have three types to worry about here, the type of A, the source of
@@ -843,6 +844,38 @@
   if (Instruction *I = foldVecTruncToExtElt(CI, *this))
     return I;
 
+  // Whenever an element is extracted from a vector, and then truncated,
+  // canonicalize by converting it to a bitcast followed by an
+  // extractelement.
+  //
+  // Example (little endian):
+  //   trunc (extractelement <4 x i64> %X, 0) to i32
+  //   --->
+  //   extractelement <8 x i32> (bitcast <4 x i64> %X to <8 x i32>), i32 0
+  Value *VecOp;
+  if (match(Src,
+            m_OneUse(m_ExtractElement(m_Value(VecOp), m_ConstantInt(Cst))))) {
+    Type *VecOpTy = VecOp->getType();
+    unsigned DestScalarSize = DestTy->getScalarSizeInBits();
+    unsigned VecOpScalarSize = VecOpTy->getScalarSizeInBits();
+    unsigned VecNumElts = VecOpTy->getVectorNumElements();
+
+    // A badly fit destination size would result in an invalid cast.
+    if (VecOpScalarSize % DestScalarSize == 0) {
+      unsigned TruncRatio = VecOpScalarSize / DestScalarSize;
+      unsigned BitCastNumElts = VecNumElts * TruncRatio;
+      unsigned VecOpIdx = Cst->getZExtValue();
+      unsigned NewIdx =
+          DL.isBigEndian()
+              ? (VecOpIdx + 1) * TruncRatio - 1
+              : VecOpIdx * TruncRatio;
+
+      Type *BitCastTo = VectorType::get(DestTy, BitCastNumElts);
+      Value *BitCast = Builder.CreateBitCast(VecOp, BitCastTo);
+      return ExtractElementInst::Create(BitCast, Builder.getInt32(NewIdx));
+    }
+  }
+
   return nullptr;
 }
 


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D76983.253452.patch
Type: text/x-patch
Size: 2416 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20200329/150d6369/attachment-0001.bin>