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[Support] Make line-number cache robust against access patterns.

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Summary:
The LLVM SourceMgr class (which is used indirectly by Swift, though not Clang)
has a routine for looking up line numbers of SMLocs. This routine uses a
shared, special-purpose cache that handles exactly one access pattern
efficiently: looking up the line number of an SMLoc that points into the same
buffer as the last query made to the SourceMgr, at a location in the buffer at
or ahead of the last query.

When this works it's fine, but when it fails it's catastrophic for performancer:
one recent out-of-order access from a Swift utility routine ran for tens of
seconds, spending 99% of its time repeatedly scanning buffers for '\n'.

This change removes the shared cache from the SourceMgr and installs a new
cache in each SrcBuffer. The per-SrcBuffer caches are also "full", in the sense
that rather than caching a single last-query pointer, they cache _all_ the
line-ending offsets, in a binary-searchable array, such that once it's
populated (on first access), all subsequent access patterns run at the same
speed.

Performance measurements I've done show this is actually a little bit faster on
real codebases (though only a couple fractions of a percent). Memory usage is
up by a few tens to hundreds of bytes per SrcBuffer that has a line lookup done
on it; I've attempted to minimize this by using dynamic selection of integer
sized when storing offset arrays. But the main motive here is to
make-impossible the cases we don't always see, that show up by surprise when
there is an out-of-order access pattern.

Reviewers: jordan_rose

Reviewed By: jordan_rose

Subscribers: probinson, llvm-commits

Differential Revision: https://reviews.llvm.org/D45003

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@329470 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Graydon Hoare
2018-04-07 00:44:02 +00:00
parent d1d162f3a7
commit 141766acc5
4 changed files with 421 additions and 77 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lib/Support/SourceMgr.cpp
+73 -50
View File
@@ -28,6 +28,7 @@
#include <algorithm>
#include <cassert>
#include <cstddef>
#include <limits>
#include <memory>
#include <string>
#include <utility>
@@ -36,24 +37,6 @@ using namespace llvm;
static const size_t TabStop = 8;
namespace {
struct LineNoCacheTy {
const char *LastQuery;
unsigned LastQueryBufferID;
unsigned LineNoOfQuery;
};
} // end anonymous namespace
static LineNoCacheTy *getCache(void *Ptr) {
return (LineNoCacheTy*)Ptr;
}
SourceMgr::~SourceMgr() {
delete getCache(LineNoCache);
}
unsigned SourceMgr::AddIncludeFile(const std::string &Filename,
SMLoc IncludeLoc,
std::string &IncludedFile) {
@@ -85,46 +68,86 @@ unsigned SourceMgr::FindBufferContainingLoc(SMLoc Loc) const {
return 0;
}
template <typename T>
unsigned SourceMgr::SrcBuffer::getLineNumber(const char *Ptr) const {
// Ensure OffsetCache is allocated and populated with offsets of all the
// '\n' bytes.
std::vector<T> *Offsets = nullptr;
if (OffsetCache.isNull()) {
Offsets = new std::vector<T>();
OffsetCache = Offsets;
size_t Sz = Buffer->getBufferSize();
assert(Sz <= std::numeric_limits<T>::max());
StringRef S = Buffer->getBuffer();
for (size_t N = 0; N < Sz; ++N) {
if (S[N] == '\n') {
Offsets->push_back(static_cast<T>(N));
}
}
} else {
Offsets = OffsetCache.get<std::vector<T> *>();
}
const char *BufStart = Buffer->getBufferStart();
assert(Ptr >= BufStart && Ptr <= Buffer->getBufferEnd());
ptrdiff_t PtrDiff = Ptr - BufStart;
assert(PtrDiff >= 0 && static_cast<size_t>(PtrDiff) <= std::numeric_limits<T>::max());
T PtrOffset = static_cast<T>(PtrDiff);
// std::lower_bound returns the first EOL offset that's not-less-than
// PtrOffset, meaning the EOL that _ends the line_ that PtrOffset is on
// (including if PtrOffset refers to the EOL itself). If there's no such
// EOL, returns end().
auto EOL = std::lower_bound(Offsets->begin(), Offsets->end(), PtrOffset);
// Lines count from 1, so add 1 to the distance from the 0th line.
return (1 + (EOL - Offsets->begin()));
}
SourceMgr::SrcBuffer::SrcBuffer(SourceMgr::SrcBuffer &&Other)
: Buffer(std::move(Other.Buffer)),
OffsetCache(Other.OffsetCache),
IncludeLoc(Other.IncludeLoc) {
Other.OffsetCache = nullptr;
}
SourceMgr::SrcBuffer::~SrcBuffer() {
if (!OffsetCache.isNull()) {
if (OffsetCache.is<std::vector<uint8_t>*>())
delete OffsetCache.get<std::vector<uint8_t>*>();
else if (OffsetCache.is<std::vector<uint16_t>*>())
delete OffsetCache.get<std::vector<uint16_t>*>();
else if (OffsetCache.is<std::vector<uint32_t>*>())
delete OffsetCache.get<std::vector<uint32_t>*>();
else
delete OffsetCache.get<std::vector<uint64_t>*>();
OffsetCache = nullptr;
}
}
std::pair<unsigned, unsigned>
SourceMgr::getLineAndColumn(SMLoc Loc, unsigned BufferID) const {
if (!BufferID)
BufferID = FindBufferContainingLoc(Loc);
assert(BufferID && "Invalid Location!");
const MemoryBuffer *Buff = getMemoryBuffer(BufferID);
auto &SB = getBufferInfo(BufferID);
const char *Ptr = Loc.getPointer();
// Count the number of \n's between the start of the file and the specified
// location.
unsigned LineNo = 1;
size_t Sz = SB.Buffer->getBufferSize();
assert(Sz <= std::numeric_limits<uint64_t>::max());
unsigned LineNo;
if (Sz <= std::numeric_limits<uint8_t>::max())
LineNo = SB.getLineNumber<uint8_t>(Ptr);
else if (Sz <= std::numeric_limits<uint16_t>::max())
LineNo = SB.getLineNumber<uint16_t>(Ptr);
else if (Sz <= std::numeric_limits<uint32_t>::max())
LineNo = SB.getLineNumber<uint32_t>(Ptr);
else
LineNo = SB.getLineNumber<uint64_t>(Ptr);
const char *BufStart = Buff->getBufferStart();
const char *Ptr = BufStart;
// If we have a line number cache, and if the query is to a later point in the
// same file, start searching from the last query location. This optimizes
// for the case when multiple diagnostics come out of one file in order.
if (LineNoCacheTy *Cache = getCache(LineNoCache))
if (Cache->LastQueryBufferID == BufferID &&
Cache->LastQuery <= Loc.getPointer()) {
Ptr = Cache->LastQuery;
LineNo = Cache->LineNoOfQuery;
}
// Scan for the location being queried, keeping track of the number of lines
// we see.
for (; SMLoc::getFromPointer(Ptr) != Loc; ++Ptr)
if (*Ptr == '\n') ++LineNo;
// Allocate the line number cache if it doesn't exist.
if (!LineNoCache)
LineNoCache = new LineNoCacheTy();
// Update the line # cache.
LineNoCacheTy &Cache = *getCache(LineNoCache);
Cache.LastQueryBufferID = BufferID;
Cache.LastQuery = Ptr;
Cache.LineNoOfQuery = LineNo;
const char *BufStart = SB.Buffer->getBufferStart();
size_t NewlineOffs = StringRef(BufStart, Ptr-BufStart).find_last_of("\n\r");
if (NewlineOffs == StringRef::npos) NewlineOffs = ~(size_t)0;
return std::make_pair(LineNo, Ptr-BufStart-NewlineOffs);