[llvm] r226034 - [MBP] Add flags to disable the BadCFGConflict check in MachineBlockPlacement.
Chandler Carruth
chandlerc at gmail.com
Wed Jan 14 12:19:29 PST 2015
Author: chandlerc
Date: Wed Jan 14 14:19:29 2015
New Revision: 226034
URL: http://llvm.org/viewvc/llvm-project?rev=226034&view=rev
Log:
[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
Added:
llvm/trunk/test/CodeGen/X86/code_placement_bad_cfg_check.ll
Modified:
llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
Modified: llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp?rev=226034&r1=226033&r2=226034&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp (original)
+++ llvm/trunk/lib/CodeGen/MachineBlockPlacement.cpp Wed Jan 14 14:19:29 2015
@@ -60,6 +60,17 @@ static cl::opt<unsigned> AlignAllBlock("
"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",
@@ -374,29 +385,33 @@ MachineBasicBlock *MachineBlockPlacement
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)
+ 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;
- 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;
- }
}
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
Added: llvm/trunk/test/CodeGen/X86/code_placement_bad_cfg_check.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/code_placement_bad_cfg_check.ll?rev=226034&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/X86/code_placement_bad_cfg_check.ll (added)
+++ llvm/trunk/test/CodeGen/X86/code_placement_bad_cfg_check.ll Wed Jan 14 14:19:29 2015
@@ -0,0 +1,112 @@
+; RUN: llc -mcpu=corei7 -mtriple=x86_64-linux < %s | FileCheck %s -check-prefix=CHECK-BAD-CFG
+; RUN: llc -mcpu=corei7 -mtriple=x86_64-linux -no-bad-cfg-conflict-check < %s | FileCheck %s -check-prefix=CHECK-NO-BAD-CFG
+; RUN: llc -mcpu=corei7 -mtriple=x86_64-linux -only-hot-bad-cfg-conflict-check < %s | FileCheck %s -check-prefix=CHECK-HOT-BAD-CFG
+
+define void @foo(i32 %t) {
+; Test that we lift the call to 'c' up to immediately follow the call to 'b'
+; when we disable the cfg conflict check.
+;
+; CHECK-BAD-CFG-LABEL: foo:
+; CHECK-BAD-CFG: callq b
+; CHECK-BAD-CFG: callq a
+; CHECK-BAD-CFG: callq c
+;
+; CHECK-NO-BAD-CFG-LABEL: foo:
+; CHECK-NO-BAD-CFG: callq b
+; CHECK-NO-BAD-CFG: callq c
+; CHECK-NO-BAD-CFG: callq a
+;
+; CHECK-HOT-BAD-CFG-LABEL: foo:
+; CHECK-HOT-BAD-CFG: callq b
+; CHECK-HOT-BAD-CFG: callq c
+; CHECK-HOT-BAD-CFG: callq a
+
+entry:
+ %cmp = icmp eq i32 %t, 0
+ br i1 %cmp, label %if.then, label %if.else
+
+if.then:
+ call void @a()
+ br label %if.end
+
+if.else:
+ call void @b()
+ br label %if.end
+
+if.end:
+ call void @c()
+ ret void
+}
+
+define void @bar(i32 %t1, i32 %t2, i32 %t3) {
+; Test that we lift the call to 'c' up to immediately follow the call to 'b'
+; when we disable the cfg conflict check.
+;
+; CHECK-BAD-CFG-LABEL: bar:
+; CHECK-BAD-CFG: callq a
+; CHECK-BAD-CFG: callq c
+; CHECK-BAD-CFG: callq d
+; CHECK-BAD-CFG: callq f
+; CHECK-BAD-CFG: callq b
+; CHECK-BAD-CFG: callq e
+; CHECK-BAD-CFG: callq g
+;
+; CHECK-NO-BAD-CFG-LABEL: bar:
+; CHECK-NO-BAD-CFG: callq a
+; CHECK-NO-BAD-CFG: callq c
+; CHECK-NO-BAD-CFG: callq g
+; CHECK-NO-BAD-CFG: callq d
+; CHECK-NO-BAD-CFG: callq f
+; CHECK-NO-BAD-CFG: callq b
+; CHECK-NO-BAD-CFG: callq e
+;
+; CHECK-HOT-BAD-CFG-LABEL: bar:
+; CHECK-HOT-BAD-CFG: callq a
+; CHECK-HOT-BAD-CFG: callq c
+; CHECK-HOT-BAD-CFG: callq d
+; CHECK-HOT-BAD-CFG: callq f
+; CHECK-HOT-BAD-CFG: callq g
+; CHECK-HOT-BAD-CFG: callq b
+; CHECK-HOT-BAD-CFG: callq e
+
+entry:
+ br i1 undef, label %if1.then, label %if1.else
+
+if1.then:
+ call void @a()
+ %cmp2 = icmp eq i32 %t2, 0
+ br i1 %cmp2, label %if2.then, label %if2.else
+
+if2.then:
+ call void @b()
+ br label %if.end
+
+if2.else:
+ call void @c()
+ br label %if.end
+
+if1.else:
+ call void @d()
+ %cmp3 = icmp eq i32 %t3, 0
+ br i1 %cmp3, label %if3.then, label %if3.else
+
+if3.then:
+ call void @e()
+ br label %if.end
+
+if3.else:
+ call void @f()
+ br label %if.end
+
+if.end:
+ call void @g()
+ ret void
+}
+
+declare void @a()
+declare void @b()
+declare void @c()
+declare void @d()
+declare void @e()
+declare void @f()
+declare void @g()
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