[mlir][Vector] Move temporary alloc to top of the function alloca when lowering vector_transfers

[nicolasvasilache]

Summary

Recently introduced allocation hoisting is quite conservative on the cases when it triggers.
This revision makes it such that the allocations for vector transfer lowerings are hoisted
to the top of the function.
This should be revisited in the context of parallelism and is a temporary workaround.

[ftynse]

mlir/lib/Conversion/VectorToSCF/VectorToSCF.cpp

200

static

[nicolasvasilache]

Address review.

[nicolasvasilache]

Fix test.

mlir/lib/Conversion/VectorToSCF/VectorToSCF.cpp

if (xferOp.isMaskedDim(leadingRank + idx)) {
       inBoundsCondition =
           (inBoundsCondition) ? (inBoundsCondition && inBounds) : inBounds;
     }
     ++idx;
   }
   return inBoundsCondition;
 }
 
+// TODO: Parallelism and threadlocal considerations.
+static Value setAllocAtFunctionEntry(MemRefType memRefMinorVectorType,
+                                     Operation *op) {
+  auto &b = ScopedContext::getBuilderRef();
+  OpBuilder::InsertionGuard guard(b);
+  b.setInsertionPointToStart(&op->getParentOfType<FuncOp>().front());
+  Value res =
+      std_alloca(memRefMinorVectorType, ValueRange{}, b.getI64IntegerAttr(128));
+  return res;
+}
+
 template <>
 LogicalResult NDTransferOpHelper<TransferReadOp>::doReplace() {
   Value alloc, result;
   if (options.unroll)
     result = std_splat(vectorType, xferOp.padding());
   else
-    alloc = std_alloc(memRefMinorVectorType);
+    alloc = setAllocAtFunctionEntry(memRefMinorVectorType, op);
 
   emitLoops([&](ValueRange majorIvs, ValueRange leadingOffsets,
                 ValueRange majorOffsets, ValueRange minorOffsets,
                 MemRefBoundsCapture &memrefBounds) {
     /// Lambda to load 1-D vector in the current loop ivs + offset context.
     auto load1DVector = [&](ValueRange majorIvsPlusOffsets) -> Value {
       SmallVector<Value, 8> indexing;
       indexing.reserve(leadingRank + majorRank + minorRank);
LogicalResult NDTransferOpHelper<TransferReadOp>::doReplace() {
 
   return success();
 }
 
 template <>
 LogicalResult NDTransferOpHelper<TransferWriteOp>::doReplace() {
   Value alloc;
   if (!options.unroll) {
-    alloc = std_alloc(memRefMinorVectorType);
+    alloc = setAllocAtFunctionEntry(memRefMinorVectorType, op);
     std_store(xferOp.vector(),
               vector_type_cast(MemRefType::get({}, vectorType), alloc));
   }
 
   emitLoops([&](ValueRange majorIvs, ValueRange leadingOffsets,
                 ValueRange majorOffsets, ValueRange minorOffsets,
                 MemRefBoundsCapture &memrefBounds) {
     // Lower to 1-D vector_transfer_write and let recursion handle it.