[PATCH] D16300: Enable LoopLoadElimination by default

[Adam Nemet via llvm-commits]

anemet created this revision.
anemet added reviewers: dberlin, hfinkel.
anemet added a subscriber: llvm-commits.
Herald added subscribers: joker.eph, aemerson.

If you're interested to benchmark this yourself, you can enable the pass
with -mllvm -enable-loop-load-elim currently.  Please let me know if you
see any regressions.

I re-benchmarked this and results are similar to original results in
D13259:

On ARM64:
  SingleSource/Benchmarks/Polybench/linear-algebra/solvers/dynprog -59.27%
  SingleSource/Benchmarks/Polybench/stencils/adi                   -19.78%

On x86:
  SingleSource/Benchmarks/Polybench/linear-algebra/solvers/dynprog  -27.14%

And of course the original ~20% gain on SPECint_2006/456.hmmer with Loop
Distribution.

In terms of compile time, there is ~5% increase on both
SingleSource/Benchmarks/Misc/oourafft and
SingleSource/Benchmarks/Linkpack/linkpack-pc.  These are both very tiny
loop-intensive programs where SCEV computations dominates compile time.

The reason that time spent in SCEV increases has to do with the design
of the old pass manager.  If a transform pass does not preserve an
analysis we *invalidate* the analysis even if there was *no*
modification made by the transform pass.

This means that currently we don't take advantage of LLE and LV sharing
the same analysis (LAA) and unfortunately we recompute LAA *and* SCEV
for LLE.

(There should be a way to work around this limitation in the case of
SCEV and LAA since both compute things on demand and internally cache
their result.  Thus we could pretend that transform passes preserve
these analyses and manually invalidate them upon actual modification.
On the other hand the new pass manager is supposed to solve so I am not
sure if this is worthwhile.)

http://reviews.llvm.org/D16300

Files:
  lib/Transforms/IPO/PassManagerBuilder.cpp

Index: lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- lib/Transforms/IPO/PassManagerBuilder.cpp
+++ lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -102,8 +102,8 @@
         "Enable the GlobalsModRef AliasAnalysis outside of the LTO pipeline."));
 
 static cl::opt<bool> EnableLoopLoadElim(
-    "enable-loop-load-elim", cl::init(false), cl::Hidden,
-    cl::desc("Enable the new, experimental LoopLoadElimination Pass"));
+    "enable-loop-load-elim", cl::init(true), cl::Hidden,
+    cl::desc("Enable the LoopLoadElimination Pass"));
 
 PassManagerBuilder::PassManagerBuilder() {
     OptLevel = 2;


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