[MCA] Avoid an InstrDesc copy in mca::LSUnit::reserve.

[orodley]

Summary

InstrDesc contains 4 vectors (as well as some other data), so it's
expensive to copy.

[mattd]

Thanks for the patch @orodley. Good catch! LGTM.

[orodley]

Thanks for the review.

[mattd]

@orodley No problem. I don't know if you have commit access (your account looks pretty new), but I'm happy to land the patch for you later if you wish.

[dberris]

LGTM, I'll land for @orodley until he gets llvm commit access.

[orodley]

@mattd Dean's in my time zone so I asked him before I saw your comment, but thanks for the offer.

[mattd]

In D49775#1176080, @orodley wrote:

@mattd Dean's in my time zone so I asked him before I saw your comment, but thanks for the offer.

Awesome. Thanks for looking into this! I'm curious, did you discover this through some performance measuring, or did it just catch your eye when reading over the source?

[orodley]

@mattd

Yep, it was through profiling. I've been working on a new scheduler using MCA, as Clement alluded to in https://bugs.llvm.org/show_bug.cgi?id=37696.
mca::Pipeline::run ends up being the hot part, and this was a simple, easily upstreamable 5% improvement within that :)

[mattd]

In D49775#1176089, @orodley wrote:

@mattd

Yep, it was through profiling. I've been working on a new scheduler using MCA, as Clement alluded to in https://bugs.llvm.org/show_bug.cgi?id=37696.
mca::Pipeline::run ends up being the hot part, and this was a simple, easily upstreamable 5% improvement within that :)

Thanks!

llvm/trunk/tools/llvm-mca/LSUnit.cpp

void LSUnit::assignSQSlot(unsigned Index) {
   assert(StoreQueue.count(Index) == 0);
 
   LLVM_DEBUG(dbgs() << "[LSUnit] - AssignSQSlot <Idx=" << Index
                     << ",slot=" << StoreQueue.size() << ">\n");
   StoreQueue.insert(Index);
 }
 
 bool LSUnit::reserve(const InstRef &IR) {
-  const InstrDesc Desc = IR.getInstruction()->getDesc();
+  const InstrDesc &Desc = IR.getInstruction()->getDesc();
   unsigned MayLoad = Desc.MayLoad;
   unsigned MayStore = Desc.MayStore;
   unsigned IsMemBarrier = Desc.HasSideEffects;
   if (!MayLoad && !MayStore)
     return false;
 
   const unsigned Index = IR.getSourceIndex();
   if (MayLoad) {