[TableGen] Change std::vector to SmallVector

[nadav]

Summary

The size of VTList that is pushed into this container is usually 1, but often 6 or 7. Change the vector to SmallVector to eliminate frequent mallocs.
This happens hundreds of thousands of times in each tablegen execution during the LLVM/clang build.

[wenlei]

Reserve space in a vector to reduce the number of mallocs. The size of VTList is usually 1, but often 6 or 7.

It seems to me that SmallVector fits the need better than std::vector here?

Also curious how did you discover this?

[wenlei]

LGTM. Thanks!

[nadav]

@wenlei You are right. SmallVector is a better fit. I updated the patch.

This patch, and D83920. D83797, D83788 that I sent today were all discovered by an internal performance tool.

[hiraditya]

Nice catch!

llvm/utils/TableGen/CodeGenDAGPatterns.cpp

 // inference will apply to each mode separately.
 
 TypeSetByHwMode::TypeSetByHwMode(ArrayRef<ValueTypeByHwMode> VTList) {
+  AddrSpaces.reserve(VTList.size());
   for (const ValueTypeByHwMode &VVT : VTList) {
     insert(VVT);
     AddrSpaces.push_back(VVT.PtrAddrSpace);