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[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
This commit is contained in:
Chandler Carruth
parent
6d9693229c
commit
ba862f13cb
@ -60,6 +60,17 @@ static cl::opt<unsigned> AlignAllBlock("align-all-blocks",
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"blocks in the function."),
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cl::init(0), cl::Hidden);
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static cl::opt<bool> OnlyHotBadCFGConflictCheck(
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"only-hot-bad-cfg-conflict-check",
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cl::desc("Only check that a hot successor doesn't have a hot predecessor."),
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cl::init(false), cl::Hidden);
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static cl::opt<bool> NoBadCFGConflictCheck(
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"no-bad-cfg-conflict-check",
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cl::desc("Don't check whether a hot successor has a more important "
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"predecessor."),
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cl::init(false), cl::Hidden);
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// FIXME: Find a good default for this flag and remove the flag.
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static cl::opt<unsigned>
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ExitBlockBias("block-placement-exit-block-bias",
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@ -374,28 +385,32 @@ MachineBasicBlock *MachineBlockPlacement::selectBestSuccessor(
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continue;
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}
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// Make sure that a hot successor doesn't have a globally more important
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// predecessor.
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BlockFrequency CandidateEdgeFreq
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= MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
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bool BadCFGConflict = false;
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for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
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PE = (*SI)->pred_end();
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PI != PE; ++PI) {
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if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
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BlockToChain[*PI] == &Chain)
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continue;
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BlockFrequency PredEdgeFreq
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= MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
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if (PredEdgeFreq >= CandidateEdgeFreq) {
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BadCFGConflict = true;
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break;
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if (!NoBadCFGConflictCheck) {
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// Make sure that a hot successor doesn't have a globally more
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// important predecessor.
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BlockFrequency CandidateEdgeFreq =
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OnlyHotBadCFGConflictCheck
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? MBFI->getBlockFreq(BB) * SuccProb
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: MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
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bool BadCFGConflict = false;
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for (MachineBasicBlock::pred_iterator PI = (*SI)->pred_begin(),
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PE = (*SI)->pred_end();
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PI != PE; ++PI) {
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if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
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BlockToChain[*PI] == &Chain)
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continue;
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BlockFrequency PredEdgeFreq =
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MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
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if (PredEdgeFreq >= CandidateEdgeFreq) {
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BadCFGConflict = true;
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break;
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}
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}
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if (BadCFGConflict) {
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DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
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<< " (prob) (non-cold CFG conflict)\n");
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continue;
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}
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}
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if (BadCFGConflict) {
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DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
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<< " (prob) (non-cold CFG conflict)\n");
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continue;
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}
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}
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112
test/CodeGen/X86/code_placement_bad_cfg_check.ll
Normal file
112
test/CodeGen/X86/code_placement_bad_cfg_check.ll
Normal file
@ -0,0 +1,112 @@
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; RUN: llc -mcpu=corei7 -mtriple=x86_64-linux < %s | FileCheck %s -check-prefix=CHECK-BAD-CFG
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; RUN: llc -mcpu=corei7 -mtriple=x86_64-linux -no-bad-cfg-conflict-check < %s | FileCheck %s -check-prefix=CHECK-NO-BAD-CFG
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; RUN: llc -mcpu=corei7 -mtriple=x86_64-linux -only-hot-bad-cfg-conflict-check < %s | FileCheck %s -check-prefix=CHECK-HOT-BAD-CFG
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define void @foo(i32 %t) {
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; Test that we lift the call to 'c' up to immediately follow the call to 'b'
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; when we disable the cfg conflict check.
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;
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; CHECK-BAD-CFG-LABEL: foo:
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; CHECK-BAD-CFG: callq b
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; CHECK-BAD-CFG: callq a
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; CHECK-BAD-CFG: callq c
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;
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; CHECK-NO-BAD-CFG-LABEL: foo:
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; CHECK-NO-BAD-CFG: callq b
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; CHECK-NO-BAD-CFG: callq c
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; CHECK-NO-BAD-CFG: callq a
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;
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; CHECK-HOT-BAD-CFG-LABEL: foo:
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; CHECK-HOT-BAD-CFG: callq b
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; CHECK-HOT-BAD-CFG: callq c
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; CHECK-HOT-BAD-CFG: callq a
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entry:
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%cmp = icmp eq i32 %t, 0
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br i1 %cmp, label %if.then, label %if.else
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if.then:
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call void @a()
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br label %if.end
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if.else:
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call void @b()
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br label %if.end
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if.end:
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call void @c()
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ret void
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}
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define void @bar(i32 %t1, i32 %t2, i32 %t3) {
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; Test that we lift the call to 'c' up to immediately follow the call to 'b'
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; when we disable the cfg conflict check.
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;
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; CHECK-BAD-CFG-LABEL: bar:
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; CHECK-BAD-CFG: callq a
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; CHECK-BAD-CFG: callq c
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; CHECK-BAD-CFG: callq d
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; CHECK-BAD-CFG: callq f
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; CHECK-BAD-CFG: callq b
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; CHECK-BAD-CFG: callq e
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; CHECK-BAD-CFG: callq g
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;
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; CHECK-NO-BAD-CFG-LABEL: bar:
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; CHECK-NO-BAD-CFG: callq a
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; CHECK-NO-BAD-CFG: callq c
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; CHECK-NO-BAD-CFG: callq g
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; CHECK-NO-BAD-CFG: callq d
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; CHECK-NO-BAD-CFG: callq f
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; CHECK-NO-BAD-CFG: callq b
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; CHECK-NO-BAD-CFG: callq e
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;
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; CHECK-HOT-BAD-CFG-LABEL: bar:
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; CHECK-HOT-BAD-CFG: callq a
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; CHECK-HOT-BAD-CFG: callq c
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; CHECK-HOT-BAD-CFG: callq d
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; CHECK-HOT-BAD-CFG: callq f
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; CHECK-HOT-BAD-CFG: callq g
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; CHECK-HOT-BAD-CFG: callq b
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; CHECK-HOT-BAD-CFG: callq e
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entry:
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br i1 undef, label %if1.then, label %if1.else
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if1.then:
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call void @a()
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%cmp2 = icmp eq i32 %t2, 0
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br i1 %cmp2, label %if2.then, label %if2.else
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if2.then:
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call void @b()
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br label %if.end
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if2.else:
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call void @c()
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br label %if.end
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if1.else:
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call void @d()
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%cmp3 = icmp eq i32 %t3, 0
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br i1 %cmp3, label %if3.then, label %if3.else
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if3.then:
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call void @e()
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br label %if.end
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if3.else:
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call void @f()
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br label %if.end
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if.end:
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call void @g()
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ret void
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}
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declare void @a()
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declare void @b()
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declare void @c()
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declare void @d()
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declare void @e()
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declare void @f()
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declare void @g()
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