[PATCH] D99205: Add jump-threading optimization for deterministic finite automata

[Justin Kreiner via Phabricator via llvm-commits]

jkreiner added a comment.

In D99205#2784375 <https://reviews.llvm.org/D99205#2784375>, @dnsampaio wrote:

> For our downstream arch we're seeing gains up to 27%.

That's great to hear!

In D99205#2784461 <https://reviews.llvm.org/D99205#2784461>, @foad wrote:

> Naive question: does your new pass "result in irreducible control flow that harms other optimizations"? From the ASCII art diagram in the commit message it looks like it does. Why is that OK?

It could produce irreducible control flow like that, but the pass is late enough in the pipeline that it won't have a negative impact. I experimented with putting it early in the pipeline and the gains I measured weren't great, but it seems to be profitable where it is right now.



================
Comment at: llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp:363-366
+inline raw_ostream &operator<<(raw_ostream &OS, const ThreadingPath &TPath) {
+  TPath.print(OS);
+  return OS;
+}
----------------
dnsampaio wrote:
> In release mode I get warnings this is not used (we use -Werror):
> llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp:364:21: error: unused function 'operator<<' [-Werror,-Wunused-function]
> 
> Perhaps put it around #ifndef NDEBUG ?
I see, I'll add that #ifndef NDEBUG check.


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