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[MBP] Add flags to disable the BadCFGConflict check in MachineBlockPlacement.

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Some benchmarks have shown that this could lead to a potential
performance benefit, and so adding some flags to try to help measure the
difference.

A possible explanation. In diamond-shaped CFGs (A followed by either
B or C both followed by D), putting B and C both in between A and
D leads to the code being less dense than it could be. Always either
B or C have to be skipped increasing the chance of cache misses etc.
Moving either B or C to after D might be beneficial on average.

In the long run, but we should probably do a better job of analyzing the
basic block and branch probabilities to move the correct one of B or
C to after D. But even if we don't use this in the long run, it is
a good baseline for benchmarking.

Original patch authored by Daniel Jasper with test tweaks and a second
flag added by me.

Differential Revision: http://reviews.llvm.org/D6969

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226034 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chandler Carruth
2015-01-14 20:19:29 +00:00
parent 11abe69e98
commit 8af8091ef5
2 changed files with 148 additions and 21 deletions
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57
lib/CodeGen/MachineBlockPlacement.cpp
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@ -60,6 +60,17 @@ static cl::opt<unsigned> AlignAllBlock("align-all-blocks",
"blocks in the function."),
cl::init(0), cl::Hidden);
static cl::opt<bool> OnlyHotBadCFGConflictCheck(
"only-hot-bad-cfg-conflict-check",
cl::desc("Only check that a hot successor doesn't have a hot predecessor."),
cl::init(false), cl::Hidden);
static cl::opt<bool> NoBadCFGConflictCheck(
"no-bad-cfg-conflict-check",
cl::desc("Don't check whether a hot successor has a more important "
"predecessor."),
cl::init(false), cl::Hidden);
// FIXME: Find a good default for this flag and remove the flag.
static cl::opt<unsigned>
ExitBlockBias("block-placement-exit-block-bias",
@ -374,28 +385,32 @@ MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
continue;
}
// Make sure that a hot successor doesn't have a globally more important
// predecessor.
BlockFrequency CandidateEdgeFreq
= MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
bool BadCFGConflict = false;
for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
PE = (*SI)->pred_end();
PI != PE; ++PI) {
if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
BlockToChain[*PI] == &Chain)
continue;
BlockFrequency PredEdgeFreq
= MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
if (PredEdgeFreq >= CandidateEdgeFreq) {
BadCFGConflict = true;
break;
if (!NoBadCFGConflictCheck) {
// Make sure that a hot successor doesn't have a globally more
// important predecessor.
BlockFrequency CandidateEdgeFreq =
OnlyHotBadCFGConflictCheck
? MBFI->getBlockFreq(BB) * SuccProb
: MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
bool BadCFGConflict = false;
for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
PE = (*SI)->pred_end();
PI != PE; ++PI) {
if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
BlockToChain[*PI] == &Chain)
continue;
BlockFrequency PredEdgeFreq =
MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
if (PredEdgeFreq >= CandidateEdgeFreq) {
BadCFGConflict = true;
break;
}
}
if (BadCFGConflict) {
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
<< " (prob) (non-cold CFG conflict)\n");
continue;
}
}
if (BadCFGConflict) {
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
<< " (prob) (non-cold CFG conflict)\n");
continue;
}
}