[PATCH] D14797: [BranchFolding] Merge MMOs during tail merge

[Chad Rosier via llvm-commits]

mcrosier added inline comments.

================
Comment at: lib/CodeGen/BranchFolding.cpp:747
@@ -746,2 +746,3 @@
 
-static bool hasIdenticalMMOs(const MachineInstr *MI1, const MachineInstr *MI2) {
+// Add MI1's MMOs to MI2's.
+// If there are duplicated MMOs, we don't add.
----------------
How about:

// Add MI1's MMOs to MI2's MMOs while excluding any duplicates.  The MI scheduler currently doesn't handle multiple MMOs, so duplicates would likely pessimize the scheduler.

================
Comment at: lib/CodeGen/BranchFolding.cpp:756
@@ +755,3 @@
+  // Mostly, MI1's MMO count is 1 or zero. So we don't have to use
+  // SmallSet.
+  for (; I1 != E1; ++I1) {
----------------
gberry wrote:
> flyingforyou wrote:
> > When I wrote the code with N^2 search, there is a reason that most of cases, MI only has a one or zero MMO.
> > So, This routine is faster than using SmallSet.
> > 
> > In order to support that most of MI has one or zero MMO, I tested some benchmarks app and got a data.
> > 
> > When I investigated 12354 MI which is used by mergeMMOs function,
> > MI(MMO is 0) == 1595, MI(MMO is 1) == 10514. Sum of two cases is 12109 and it's 98%.
> > 
> > So, I don't think Smallset is useful in this case. 
> > But there is no explain why I use N^2 search, so I added the comment.
> > 
> > I want you agree with this, please.
> I believe a SmallSet will have almost exactly the same performance as the code you have written here when N is small, since it just uses a SmallVector and does a linear search.  If N ever gets large though it will switch to using a std::set which should have log(N) lookup.  I'd like to hear from another reviewer on whether this is warranted in this case.
I believe the number of MMO's on a MI is target-dependent.  For AArch64 it tends to be 1, but for x86 it can be more, IIRC.  Therefore, I tend to agree with Geoff; we should consider using a SmallSet.  @gberry, do you have a suggestion on how the compare function could be written?


http://reviews.llvm.org/D14797