[MBP] Add flags to disable the BadCFGConflict check in MachineBlockPlacement.
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
llvm-svn: 226034
diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index aaa7d91..779b84e 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -60,6 +60,17 @@
"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 @@
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;
+ }
}
}