[GISel]: While constructing the GISelWorklist make sure we reserve at least the required size to the underlying dense map.

[aditya_nandakumar]

Summary

This should save some unnecessary growing of the DenseMap.

Note: We should also provide a reserve interface to the GISelWorkList which can be runtime initialized to either the template parameter or the number of instructions in the MachineFunction. I can do this in a subsequent patch..

[arsenm]

LGTM

include/llvm/CodeGen/GlobalISel/GISelWorkList.h

32

If this matters, should this even be a SmallVector?

[aditya_nandakumar]

include/llvm/CodeGen/GlobalISel/GISelWorkList.h

32

Could you elaborate? Are you implying we replace this with something else (std::vector) or implying that the template parameter is probably not needed and we can always runtime reserve the size once..

[arsenm]

include/llvm/CodeGen/GlobalISel/GISelWorkList.h

32

Yes, if this is is big, there's no reason for the reserved small size so std::vector

[aditya_nandakumar]

include/llvm/CodeGen/GlobalISel/GISelWorkList.h

32

I thought I'd run some numbers just to be sure (the 512 number we use in most places is quite arbitrary) - however we do use the GISelWorkList in some passes (out of tree) for collecting some intermediate instructions, and it's set to 16 (and we avoid heap).
Either way, I can change that (and add the interface to reserve) in a subsequent patch.

[aditya_nandakumar]

r353498

[aditya_nandakumar]

Sorry - just realized that there was an LGTM, but was not accepted.

include/llvm/CodeGen/GlobalISel/GISelWorkList.h

 // MachineInstrs. The main difference with something like a SetVector is that
 // erasing an element doesn't move all elements over one place - instead just
 // nulls out the element of the vector.
 //
 // FIXME: Does it make sense to factor out common code with the
 // instcombinerWorkList?
 template<unsigned N>
 class GISelWorkList {
   SmallVector<MachineInstr *, N> Worklist;
   DenseMap<MachineInstr *, unsigned> WorklistMap;
 
 public:
-  GISelWorkList() {}
+  GISelWorkList() : WorklistMap(N) {}
 
   bool empty() const { return WorklistMap.empty(); }
 
   unsigned size() const { return WorklistMap.size(); }
 
   /// Add the specified instruction to the worklist if it isn't already in it.
   void insert(MachineInstr *I) {
     if (WorklistMap.try_emplace(I, Worklist.size()).second)