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Allow 0-weight branches in BranchProbabilityInfo.

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Summary:
When computing branch weights in BPI, we used to disallow branches with
weight 0. This is a minor nuisance, because a branch with weight 0 is
different to "don't have information". In the context of
instrumentation, it may mean "never executed", in the context of
sampling, it means "never or seldom executed".

In allowing 0 weight branches, I ran into issues with the switch
expansion code in selection DAG. It is currently hardwired to not handle
branches with weight 0. To maintain the current behaviour, I changed it
to use 1 when it finds 0, but perhaps the algorithm needs changes to
tolerate branches with weight zero.

Reviewers: hansw

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9533

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236617 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Diego Novillo 2015-05-06 17:55:11 +00:00
parent 5a6ea245b0
commit 26e46f2283
5 changed files with 26 additions and 20 deletions
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12
lib/Analysis/BranchProbabilityInfo.cpp
View File

@ -201,8 +201,7 @@ bool BranchProbabilityInfo::calcMetadataWeights(BasicBlock *BB) {
mdconst::dyn_extract<ConstantInt>(WeightsNode->getOperand(i));
if (!Weight)
return false;
Weights.push_back(
std::max<uint32_t>(1, Weight->getLimitedValue(WeightLimit)));
Weights.push_back(Weight->getLimitedValue(WeightLimit));
}
assert(Weights.size() == TI->getNumSuccessors() && "Checked above");
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
@ -553,7 +552,7 @@ uint32_t BranchProbabilityInfo::getSumForBlock(const BasicBlock *BB) const {
uint32_t PrevSum = Sum;
Sum += Weight;
assert(Sum > PrevSum); (void) PrevSum;
assert(Sum >= PrevSum); (void) PrevSum;
}
return Sum;
@ -616,14 +615,17 @@ uint32_t BranchProbabilityInfo::getEdgeWeight(const BasicBlock *Src,
uint32_t BranchProbabilityInfo::
getEdgeWeight(const BasicBlock *Src, const BasicBlock *Dst) const {
uint32_t Weight = 0;
bool FoundWeight = false;
DenseMap<Edge, uint32_t>::const_iterator MapI;
for (succ_const_iterator I = succ_begin(Src), E = succ_end(Src); I != E; ++I)
if (*I == Dst) {
MapI = Weights.find(std::make_pair(Src, I.getSuccessorIndex()));
if (MapI != Weights.end())
if (MapI != Weights.end()) {
FoundWeight = true;
Weight += MapI->second;
}
}
return (Weight == 0) ? DEFAULT_WEIGHT : Weight;
return (!FoundWeight) ? DEFAULT_WEIGHT : Weight;
}
/// Set the edge weight for a given edge specified by PredBlock and an index

6
lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
View File

@ -8009,7 +8009,11 @@ void SelectionDAGBuilder::visitSwitch(const SwitchInst &SI) {
const ConstantInt *CaseVal = I.getCaseValue();
uint32_t Weight = 1;
if (BPI) {
Weight = BPI->getEdgeWeight(SI.getParent(), I.getSuccessorIndex());
// TODO - BPI used to guarantee non-zero weights, but this produces
// information loss (see PR 22718). Since we can't handle zero weights
// here, use the same flooring mechanism previously used by BPI.
Weight = std::max(
1u, BPI->getEdgeWeight(SI.getParent(), I.getSuccessorIndex()));
assert(Weight <= UINT32_MAX / SI.getNumSuccessors());
}
Clusters.push_back(CaseCluster::range(CaseVal, CaseVal, Succ, Weight));

12
test/Transforms/SampleProfile/branch.ll
View File

@ -36,8 +36,8 @@ entry:
tail call void @llvm.dbg.value(metadata i8** %argv, i64 0, metadata !14, metadata !DIExpression()), !dbg !27
%cmp = icmp slt i32 %argc, 2, !dbg !28
br i1 %cmp, label %return, label %if.end, !dbg !28
; CHECK: edge entry -> return probability is 1 / 2 = 50%
; CHECK: edge entry -> if.end probability is 1 / 2 = 50%
; CHECK: edge entry -> return probability is 0 / 1 = 0%
; CHECK: edge entry -> if.end probability is 1 / 1 = 100%
if.end: ; preds = %entry
%arrayidx = getelementptr inbounds i8*, i8** %argv, i64 1, !dbg !30
@ -46,8 +46,8 @@ if.end: ; preds = %entry
tail call void @llvm.dbg.value(metadata i32 %call, i64 0, metadata !17, metadata !DIExpression()), !dbg !30
%cmp1 = icmp sgt i32 %call, 100, !dbg !35
br i1 %cmp1, label %for.body, label %if.end6, !dbg !35
; CHECK: edge if.end -> for.body probability is 1 / 2 = 50%
; CHECK: edge if.end -> if.end6 probability is 1 / 2 = 50%
; CHECK: edge if.end -> for.body probability is 0 / 1 = 0%
; CHECK: edge if.end -> if.end6 probability is 1 / 1 = 100%
for.body: ; preds = %if.end, %for.body
%u.016 = phi i32 [ %inc, %for.body ], [ 0, %if.end ]
@ -65,8 +65,8 @@ for.body: ; preds = %if.end, %for.body
tail call void @llvm.dbg.value(metadata i32 %inc, i64 0, metadata !21, metadata !DIExpression()), !dbg !38
%exitcond = icmp eq i32 %inc, %call, !dbg !38
br i1 %exitcond, label %if.end6, label %for.body, !dbg !38
; CHECK: edge for.body -> if.end6 probability is 1 / 10227 = 0.00977804
; CHECK: edge for.body -> for.body probability is 10226 / 10227 = 99.9902% [HOT edge]
; CHECK: edge for.body -> if.end6 probability is 0 / 10226 = 0%
; CHECK: edge for.body -> for.body probability is 10226 / 10226 = 100% [HOT edge]
if.end6: ; preds = %for.body, %if.end
%result.0 = phi double [ 0.000000e+00, %if.end ], [ %sub, %for.body ]

8
test/Transforms/SampleProfile/calls.ll
View File

@ -52,8 +52,8 @@ while.cond: ; preds = %if.end, %entry
store i32 %inc, i32* %i, align 4, !dbg !14
%cmp = icmp slt i32 %0, 400000000, !dbg !14
br i1 %cmp, label %while.body, label %while.end, !dbg !14
; CHECK: edge while.cond -> while.body probability is 5391 / 5392 = 99.9815% [HOT edge]
; CHECK: edge while.cond -> while.end probability is 1 / 5392 = 0.018546%
; CHECK: edge while.cond -> while.body probability is 5391 / 5391 = 100% [HOT edge]
; CHECK: edge while.cond -> while.end probability is 0 / 5391 = 0%
while.body: ; preds = %while.cond
%1 = load i32, i32* %i, align 4, !dbg !16
@ -63,8 +63,8 @@ while.body: ; preds = %while.cond
; both branches out of while.body had the same weight. In reality,
; the edge while.body->if.then is taken most of the time.
;
; CHECK: edge while.body -> if.then probability is 5752 / 5753 = 99.9826% [HOT edge]
; CHECK: edge while.body -> if.else probability is 1 / 5753 = 0.0173822%
; CHECK: edge while.body -> if.then probability is 5752 / 5752 = 100% [HOT edge]
; CHECK: edge while.body -> if.else probability is 0 / 5752 = 0%
if.then: ; preds = %while.body

8
test/Transforms/SampleProfile/propagate.ll
View File

@ -73,8 +73,8 @@ for.cond: ; preds = %for.inc16, %if.else
%5 = load i64, i64* %N.addr, align 8, !dbg !15
%cmp1 = icmp slt i64 %4, %5, !dbg !15
br i1 %cmp1, label %for.body, label %for.end18, !dbg !15
; CHECK: edge for.cond -> for.body probability is 10 / 11 = 90.9091% [HOT edge]
; CHECK: edge for.cond -> for.end18 probability is 1 / 11 = 9.09091%
; CHECK: edge for.cond -> for.body probability is 10 / 10 = 100% [HOT edge]
; CHECK: edge for.cond -> for.end18 probability is 0 / 10 = 0%
for.body: ; preds = %for.cond
%6 = load i64, i64* %i, align 8, !dbg !18
@ -119,8 +119,8 @@ for.cond8: ; preds = %for.inc, %if.else7
%14 = load i64, i64* %i, align 8, !dbg !28
%cmp10 = icmp slt i64 %conv9, %14, !dbg !28
br i1 %cmp10, label %for.body11, label %for.end, !dbg !28
; CHECK: edge for.cond8 -> for.body11 probability is 16191 / 16192 = 99.9938% [HOT edge]
; CHECK: edge for.cond8 -> for.end probability is 1 / 16192 = 0.00617589%
; CHECK: edge for.cond8 -> for.body11 probability is 16191 / 16191 = 100% [HOT edge]
; CHECK: edge for.cond8 -> for.end probability is 0 / 16191 = 0%
for.body11: ; preds = %for.cond8
%15 = load i32, i32* %j, align 4, !dbg !31