[LoopUnroll] Enable advanced unrolling analysis by default.

[mzolotukhin]

Summary

This patch turns on LoopUnrollAnalyzer by default. To mitigate compile
time regressions, I chose very conservative thresholds for now. Later we
can make them more aggressive, but it might require being smarter in
which loops we're optimizing. E.g. currently the biggest issue is that
with more agressive thresholds we unroll many cold loops, which
increases compile time for no performance benefit (performance of those
loops is improved, but it doesn't matter since they are cold).

Test results for compile time(using 4 samples to reduce noise):

MultiSource/Benchmarks/VersaBench/ecbdes/ecbdes 5.19%
SingleSource/Benchmarks/Polybench/medley/reg_detect/reg_detect  4.19%
MultiSource/Benchmarks/FreeBench/fourinarow/fourinarow  3.39%
MultiSource/Applications/JM/lencod/lencod 1.47%
MultiSource/Benchmarks/Fhourstones-3_1/fhourstones3_1 -6.06%

I didn't see any performance changes in the testsuite, but it improves
some internal tests.

[chandlerc]

LGTM.

Maybe work on a patch to use profile info to adjust the thresholds? Once we have that, I think we can help benchmarking a range of thresholds and produce some data about where the sweet spot lies.

[mzolotukhin]

Hi Chandler,

Thanks for LGTM, I'll commit the patch on Monday as I won't be able to check bots on this weekend.

As for the use of profiling info - it indeed looks like the best way to handle this. Also, I think it would make sense to use it more extensively everywhere, not only in loop-unrolling. This way we can save some compile time by not doing expensive optimizations on code that doesn't matter, and then spend more time optimizing relevant pieces of the code. For now it's just general ideas, but I hope to get to something more real soon.

Thanks,
Michael

llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp

 
 #define DEBUG_TYPE "loop-unroll"
 
 static cl::opt<unsigned>
     UnrollThreshold("unroll-threshold", cl::Hidden,
                     cl::desc("The baseline cost threshold for loop unrolling"));
 
 static cl::opt<unsigned> UnrollPercentDynamicCostSavedThreshold(
-    "unroll-percent-dynamic-cost-saved-threshold", cl::Hidden,
+    "unroll-percent-dynamic-cost-saved-threshold", cl::init(50), cl::Hidden,
     cl::desc("The percentage of estimated dynamic cost which must be saved by "
              "unrolling to allow unrolling up to the max threshold."));
 
 static cl::opt<unsigned> UnrollDynamicCostSavingsDiscount(
-    "unroll-dynamic-cost-savings-discount", cl::Hidden,
+    "unroll-dynamic-cost-savings-discount", cl::init(100), cl::Hidden,
     cl::desc("This is the amount discounted from the total unroll cost when "
              "the unrolled form has a high dynamic cost savings (triggered by "
              "the '-unroll-perecent-dynamic-cost-saved-threshold' flag)."));
 
 static cl::opt<unsigned> UnrollMaxIterationsCountToAnalyze(
-    "unroll-max-iteration-count-to-analyze", cl::init(0), cl::Hidden,
+    "unroll-max-iteration-count-to-analyze", cl::init(10), cl::Hidden,
     cl::desc("Don't allow loop unrolling to simulate more than this number of"
              "iterations when checking full unroll profitability"));
 
 static cl::opt<unsigned> UnrollCount(
     "unroll-count", cl::Hidden,
     cl::desc("Use this unroll count for all loops including those with "
              "unroll_count pragma values, for testing purposes"));
 

llvm/trunk/test/Transforms/LoopUnroll/partial-unroll-const-bounds.ll

-; RUN: opt < %s -S -unroll-threshold=20 -loop-unroll -unroll-allow-partial -unroll-runtime | FileCheck %s
+; RUN: opt < %s -S -unroll-threshold=20 -loop-unroll -unroll-allow-partial -unroll-runtime -unroll-dynamic-cost-savings-discount=0 | FileCheck %s
 
 ; The Loop TripCount is 9. However unroll factors 3 or 9 exceed given threshold.
 ; The test checks that we choose a smaller, power-of-two, unroll count and do not give up on unrolling.
 
 ; CHECK: for.body:
 ; CHECK: store
 ; CHECK: for.body.1:
 ; CHECK: store