[PATCH] D14112: Fixed : rL251291: Loop Vectorizer - skipping "bitcast" before GEP

[Elena Demikhovsky via llvm-commits]

delena created this revision.
delena added reviewers: mzolotukhin, jmolloy.
delena added a subscriber: llvm-commits.
delena set the repository for this revision to rL LLVM.

As Michael Zolotukhin noticed, skipping bitcast is not always correct.
The 'bitcast' may be skipped when the size of the pointee is the same.

Bitcast between GEP and load can not be skipped, and the load can not be vectorized in the following case:

  %arrayidx = getelementptr inbounds double*, double** %in, i64 %indvars.iv
  %tmp53 = bitcast double** %arrayidx to i8*
  %tmp54 = load i8, i8* %tmp53, align 1

Skipping 'bitcast' in this case allows to vectorize load:
  %arrayidx = getelementptr inbounds double*, double** %in, i64 %indvars.iv
  %tmp53 = bitcast double** %arrayidx to i64*
  %tmp54 = load i64, i64* %tmp53, align 8



Repository:
  rL LLVM

http://reviews.llvm.org/D14112

Files:
  ../lib/Transforms/Vectorize/LoopVectorize.cpp
  ../test/Transforms/LoopVectorize/gep_with_bitcast.ll

Index: ../lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- ../lib/Transforms/Vectorize/LoopVectorize.cpp
+++ ../lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -254,16 +254,29 @@
   return VectorType::get(Scalar, VF);
 }
 
-/// A helper function that returns GEP instruction and knows to skip
-/// 'bitcast'.
+/// A helper function that returns GEP instruction and knows to skip a
+/// 'bitcast'. The 'bitcast' may be skipped if the source and the destination
+/// pointee types of the 'bitcast' have the same size.
+/// For example:
+///   bitcast double** %var to i64* - can be skipped
+///   bitcast double** %var to i8*  - can not
 static GetElementPtrInst *getGEPInstruction(Value *Ptr) {
 
   if (isa<GetElementPtrInst>(Ptr))
     return cast<GetElementPtrInst>(Ptr);
 
   if (isa<BitCastInst>(Ptr) &&
-      isa<GetElementPtrInst>(cast<BitCastInst>(Ptr)->getOperand(0)))
-    return cast<GetElementPtrInst>(cast<BitCastInst>(Ptr)->getOperand(0));
+      isa<GetElementPtrInst>(cast<BitCastInst>(Ptr)->getOperand(0))) {
+    Type *BitcastTy = Ptr->getType();
+    Type *GEPTy = cast<BitCastInst>(Ptr)->getSrcTy();
+    if (!isa<PointerType>(BitcastTy) || !isa<PointerType>(GEPTy))
+      return nullptr;
+    Type *Pointee1Ty = cast<PointerType>(BitcastTy)->getPointerElementType();
+    Type *Pointee2Ty = cast<PointerType>(GEPTy)->getPointerElementType();
+    const DataLayout &DL = cast<BitCastInst>(Ptr)->getModule()->getDataLayout();
+    if (DL.getTypeSizeInBits(Pointee1Ty) == DL.getTypeSizeInBits(Pointee2Ty))
+      return cast<GetElementPtrInst>(cast<BitCastInst>(Ptr)->getOperand(0));
+  }
   return nullptr;
 }
 
Index: ../test/Transforms/LoopVectorize/gep_with_bitcast.ll
===================================================================
--- ../test/Transforms/LoopVectorize/gep_with_bitcast.ll
+++ ../test/Transforms/LoopVectorize/gep_with_bitcast.ll
@@ -37,4 +37,31 @@
 
 for.end:
   ret void
+}
+
+; CHECK-LABEL: @bar
+; CHECK: vector.body
+; CHECK: bitcast double** {{.*}} to i8*
+; CHECK: load i8, i8*
+; CHECK: insertelement <4 x i8> undef, i8 {{.*}}
+
+define void @bar(double** noalias nocapture readonly %in, double** noalias nocapture readnone %out, i8* noalias nocapture %res) #0 {
+entry:
+  br label %for.body
+
+for.body:
+  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds double*, double** %in, i64 %indvars.iv
+  %tmp53 = bitcast double** %arrayidx to i8*
+  %tmp54 = load i8, i8* %tmp53, align 1
+  %cmp1 = icmp eq i8 %tmp54, 0
+  %arrayidx3 = getelementptr inbounds i8, i8* %res, i64 %indvars.iv
+  %frombool = zext i1 %cmp1 to i8
+  store i8 %frombool, i8* %arrayidx3, align 1
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv.next, 4096
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:
+  ret void
 }
\ No newline at end of file


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