[PATCH] InstCombine: extract instead of shuffle when performing vector/array type punning

[Chandler Carruth]

================
Comment at: lib/Transforms/InstCombine/InstCombineVectorOps.cpp:862
@@ +861,3 @@
+//                 +--+--+--+--+
+static bool ShuffleIsExtractingFromLHS(ShuffleVectorInst &SVI,
+                                       SmallVector<int, 16> &Mask) {
----------------
should be 'isShuffle...'

================
Comment at: lib/Transforms/InstCombine/InstCombineVectorOps.cpp:963-965
@@ +962,5 @@
+  //
+  // If the shuffle is extracting contiguous range of values from the input
+  // vector then each use which is a bitcast of the extracted size can be
+  // replaced. This will work if the vector types are compatible, and the begin
+  // index is aligned to a value in the casted vector type. If the begin index
----------------
The first sentence here doesn't read cleanly. It's important to emphasize that we only do this when the extracted sub-vector is bitcast to an non-vector type that we could extract.

================
Comment at: lib/Transforms/InstCombine/InstCombineVectorOps.cpp:973-980
@@ +972,10 @@
+  //
+  // Example of <16 x i8>, target type i32:
+  // Index range [4,8):         v-----------v Will work.
+  //                +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+  //     <16 x i8>: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
+  //     <4 x i32>: |           |           |           |           |
+  //                +-----------+-----------+-----------+-----------+
+  // Index range [6,10):              ^-----------^ Needs an extra shuffle.
+  // Target type i40:           ^--------------^ Won't work, bail.
+  if (ShuffleIsExtractingFromLHS(SVI, Mask)) {
----------------
Fabulous ascii art! Thanks!

================
Comment at: lib/Transforms/InstCombine/InstCombineVectorOps.cpp:1009-1027
@@ +1008,21 @@
+      VectorType *CastSrcTy = VectorType::get(TgtTy, TgtNumElems);
+      if (!BegIsAligned) {
+        // Shuffle the input so [0,NumElements) contains the output, and
+        // [NumElems,SrcNumElems) is undef.
+        Constant *Undef = ConstantInt::get(Int32Ty, SrcNumElems);
+        SmallVector<Constant *, 16> ShuffleMask(SrcNumElems, Undef);
+        for (unsigned I = 0, E = MaskElems, Idx = BegIdx; I != E; ++Idx, ++I)
+          ShuffleMask[I] = ConstantInt::get(Int32Ty, Idx);
+        LHS = new ShuffleVectorInst(LHS, UndefValue::get(LHS->getType()),
+                                    ConstantVector::get(ShuffleMask),
+                                    SVI.getName() + ".extract", BC);
+        BegIdx = 0;
+      }
+      unsigned SrcElemsPerTgtElem = TgtElemBitWidth / SrcElemBitWidth;
+      assert(SrcElemsPerTgtElem);
+      BegIdx /= SrcElemsPerTgtElem;
+      Instruction *Ext = ExtractElementInst::Create(
+          CastInst::Create(Instruction::BitCast, LHS, CastSrcTy,
+                           SVI.getName() + ".bc", BC),
+          ConstantInt::get(Int32Ty, BegIdx), SVI.getName() + ".extract", BC);
+      // The shufflevector isn't being replace: the bitcast that used it
----------------
Please use the InstCombine IR builder throughout. That will ensure these new instructions are visited.

Also, please create a single extract element instruction and replace all the bitcasts with it.

http://reviews.llvm.org/D7734

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