Magic-Mirror/llvm-capstone
1
0
Fork 0
mirror of https://github.com/capstone-engine/llvm-capstone.git synced 2025-01-19 14:44:39 +00:00
Code Issues Actions 2 Packages Projects Releases Wiki Activity

[BOLT] Align BranchInfo and FuncBranchData in DataAggregator::recordTrace

Browse Source 
`DataAggregator::recordTrace` serves two purposes:
  - Attaching LBR fallthrough ("trace") information to CFG (`getBranchInfo`),
    which eventually gets emitted as YAML profile.
  - Populating vector of offsets that gets added to `FuncBranchData`, which
    eventually gets emitted as fdata profile.

`recordTrace` is invoked from `getFallthroughsInTrace` which checks its return
status and passes on the collected vector of offsets to `doTrace`.

However, if a malformed trace is passed to `recordTrace` it might partially
attach the profile to CFG and exit with false, not propagating the vector of
offsets to `doTrace`. This leads to a difference between fdata and yaml profile
collected from the same binary and the same perf file.

(Skylake LBR errata might produce such malformed traces where the last entry
is duplicated, resulting in invalid fallthrough path between the last two
entries).

There are two ways to handle this mismatch: conservative (aligned with fdata),
or aggressive (aligned with yaml). Conservative approach would discard the
trace entirely, buffering the CFG updates until all fallthroughs are confirmed.
Aggressive approach would apply CFG updates and return the matching
fallthroughs in the vector even if the trace is invalid (doesn't correspond to
a valid fallthrough path). I chose to go with the former (conservative/fdata)
approach which produces more accurate profile.

We can't rely on pre-filtering such traces early (in LBR sample processing) as
DataAggregator is used for both perf samples and pre-aggregated perf information
which loses branch stack information.

Test Plan: https://github.com/rafaelauler/bolt-tests/pull/22

Reviewed By: #bolt, rafauler

Differential Revision: https://reviews.llvm.org/D151614
This commit is contained in:
Amir Ayupov 2023-05-30 17:28:22 -07:00
parent fef23e8d87
commit bce889c8df
2 changed files with 23 additions and 22 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
bolt/include/bolt/Profile/DataAggregator.h
View File

@ -199,10 +199,10 @@ private:
/// execution order.
///
/// Return true if the trace is valid, false otherwise.
bool recordTrace(
BinaryFunction &BF, const LBREntry &First, const LBREntry &Second,
uint64_t Count = 1,
SmallVector<std::pair<uint64_t, uint64_t>, 16> *Branches = nullptr) const;
bool
recordTrace(BinaryFunction &BF, const LBREntry &First, const LBREntry &Second,
uint64_t Count,
SmallVector<std::pair<uint64_t, uint64_t>, 16> &Branches) const;
/// Return a vector of offsets corresponding to a trace in a function
/// (see recordTrace() above).

37
bolt/lib/Profile/DataAggregator.cpp
View File

@ -838,11 +838,9 @@ bool DataAggregator::doTrace(const LBREntry &First, const LBREntry &Second,
}
bool DataAggregator::recordTrace(
BinaryFunction &BF,
const LBREntry &FirstLBR,
const LBREntry &SecondLBR,
BinaryFunction &BF, const LBREntry &FirstLBR, const LBREntry &SecondLBR,
uint64_t Count,
SmallVector<std::pair<uint64_t, uint64_t>, 16> *Branches) const {
SmallVector<std::pair<uint64_t, uint64_t>, 16> &Branches) const {
BinaryContext &BC = BF.getBinaryContext();
if (!BF.isSimple())
@ -902,24 +900,27 @@ bool DataAggregator::recordTrace(
return false;
}
// Record fall-through jumps
BinaryBasicBlock::BinaryBranchInfo &BI = BB->getBranchInfo(*NextBB);
BI.Count += Count;
const MCInst *Instr = BB->getLastNonPseudoInstr();
uint64_t Offset = 0;
if (Instr)
Offset = BC.MIB->getOffsetWithDefault(*Instr, 0);
else
Offset = BB->getOffset();
if (Branches) {
const MCInst *Instr = BB->getLastNonPseudoInstr();
uint64_t Offset = 0;
if (Instr)
Offset = BC.MIB->getOffsetWithDefault(*Instr, 0);
else
Offset = BB->getOffset();
Branches->emplace_back(Offset, NextBB->getOffset());
}
Branches.emplace_back(Offset, NextBB->getOffset());
BB = NextBB;
}
// Record fall-through jumps
for (const auto &[FromOffset, ToOffset] : Branches) {
BinaryBasicBlock *FromBB = BF.getBasicBlockContainingOffset(FromOffset);
BinaryBasicBlock *ToBB = BF.getBasicBlockAtOffset(ToOffset);
assert(FromBB && ToBB);
BinaryBasicBlock::BinaryBranchInfo &BI = FromBB->getBranchInfo(*ToBB);
BI.Count += Count;
}
return true;
}
@ -930,7 +931,7 @@ DataAggregator::getFallthroughsInTrace(BinaryFunction &BF,
uint64_t Count) const {
SmallVector<std::pair<uint64_t, uint64_t>, 16> Res;
if (!recordTrace(BF, FirstLBR, SecondLBR, Count, &Res))
if (!recordTrace(BF, FirstLBR, SecondLBR, Count, Res))
return std::nullopt;
return Res;