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Revive http://reviews.llvm.org/D12778 to handle forward-hot-prob and backward-hot-prob consistently.

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Summary:
Consider the following diamond CFG:

 A
/ \
B C
 \/
 D

Suppose A->B and A->C have probabilities 81% and 19%. In block-placement, A->B is called a hot edge and the final placement should be ABDC. However, the current implementation outputs ABCD. This is because when choosing the next block of B, it checks if Freq(C->D) > Freq(B->D) * 20%, which is true (if Freq(A) = 100, then Freq(B->D) = 81, Freq(C->D) = 19, and 19 > 81*20%=16.2). Actually, we should use 25% instead of 20% as the probability here, so that we have 19 < 81*25%=20.25, and the desired ABDC layout will be generated.

Reviewers: djasper, davidxl

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20989

llvm-svn: 272203
This commit is contained in:
Dehao Chen 2016-06-08 21:30:12 +00:00
parent b6385a607e
commit 521d44df2a
3 changed files with 44 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
lib/CodeGen/MachineBlockPlacement.cpp
View File

@ -496,8 +496,7 @@ MachineBlockPlacement::selectBestSuccessor(MachineBasicBlock *BB,
// Make sure that a hot successor doesn't have a globally more
// important predecessor.
auto RealSuccProb = MBPI->getEdgeProbability(BB, Succ);
BlockFrequency CandidateEdgeFreq =
MBFI->getBlockFreq(BB) * RealSuccProb * HotProb.getCompl();
BlockFrequency CandidateEdgeFreq = MBFI->getBlockFreq(BB) * RealSuccProb;
bool BadCFGConflict = false;
for (MachineBasicBlock *Pred : Succ->predecessors()) {
if (Pred == Succ || BlockToChain[Pred] == &SuccChain ||
@ -506,7 +505,15 @@ MachineBlockPlacement::selectBestSuccessor(MachineBasicBlock *BB,
continue;
BlockFrequency PredEdgeFreq =
MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, Succ);
if (PredEdgeFreq >= CandidateEdgeFreq) {
// A B
// \ /
// C
// We layout ACB iff A.freq > C.freq * HotProb
// i.e. A.freq > A.freq * HotProb + B.freq * HotProb
// i.e. A.freq * (1 - HotProb) > B.freq * HotProb
// A: CandidateEdge
// B: PredEdge
if (PredEdgeFreq * HotProb >= CandidateEdgeFreq * HotProb.getCompl()) {
BadCFGConflict = true;
break;
}

4
test/CodeGen/AArch64/fast-isel-branch-cond-split.ll
View File

@ -19,8 +19,8 @@ bb4:
}
; CHECK-LABEL: test_and
; CHECK: cbz w0, {{LBB[0-9]+_2}}
; CHECK: cbnz w1, {{LBB[0-9]+_3}}
; CHECK: cbnz w0, {{LBB[0-9]+_2}}
; CHECK: cbz w1, {{LBB[0-9]+_1}}
define i64 @test_and(i32 %a, i32 %b) {
bb1:
%0 = icmp ne i32 %a, 0

32
test/CodeGen/X86/block-placement.ll
View File

@ -1176,3 +1176,35 @@ innercleanup:
call void @clean()
br label %outercleanup
}
declare void @hot_function()
define void @test_hot_branch(i32* %a) {
; Test that a hot branch that has a probability a little larger than 80% will
; break CFG constrains when doing block placement.
; CHECK-LABEL: test_hot_branch:
; CHECK: %entry
; CHECK: %then
; CHECK: %exit
; CHECK: %else
entry:
%gep1 = getelementptr i32, i32* %a, i32 1
%val1 = load i32, i32* %gep1
%cond1 = icmp ugt i32 %val1, 1
br i1 %cond1, label %then, label %else, !prof !5
then:
call void @hot_function()
br label %exit
else:
call void @cold_function()
br label %exit
exit:
call void @hot_function()
ret void
}
!5 = !{!"branch_weights", i32 84, i32 16}