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[PDB] Enable parallel ghash type merging by default

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Ghashing is probably going to be faster in most cases, even without
precomputed ghashes in object files.

Here is my table of results linking clang.pdb:

-------------------------------
| threads | GHASH   | NOGHASH |
-------------------------------
|  j1     | 51.031s | 25.141s |
|  j2     | 31.079s | 22.109s |
|  j4     | 18.609s | 23.156s |
|  j8     | 11.938s | 21.984s |
| j28     |  8.375s | 18.391s |
-------------------------------

This shows that ghashing is faster if at least four cores are available.
This may make the linker slower if most cores are busy in the middle of
a build, but in that case, the linker probably isn't on the critical
path of the build. Incremental build performance is arguably more
important than highly contended batch build link performance.

The -time output indicates that ghash computation is the dominant
factor:

    Input File Reading:             924 ms (  1.8%)
    GC:                             689 ms (  1.3%)
    ICF:                            527 ms (  1.0%)
    Code Layout:                    414 ms (  0.8%)
    Commit Output File:              24 ms (  0.0%)
    PDB Emission (Cumulative):    49938 ms ( 94.8%)
      Add Objects:                46783 ms ( 88.8%)
        Global Type Hashing:      38983 ms ( 74.0%)
        GHash Type Merging:        5640 ms ( 10.7%)
        Symbol Merging:            2154 ms (  4.1%)
      Publics Stream Layout:        188 ms (  0.4%)
      TPI Stream Layout:             18 ms (  0.0%)
      Commit to Disk:              2818 ms (  5.4%)
  --------------------------------------------------
  Total Link Time:                52669 ms (100.0%)

We can speed that up with a faster content hash (not SHA1).

Differential Revision: https://reviews.llvm.org/D102888
This commit is contained in:
Reid Kleckner 2021-05-20 16:18:18 -07:00
parent 020df692d8
commit 109aac9212
2 changed files with 24 additions and 13 deletions
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35
lld/COFF/Driver.cpp
View File

@ -693,7 +693,16 @@ static std::string createResponseFile(const opt::InputArgList &args,
return std::string(data.str());
}
enum class DebugKind { Unknown, None, Full, FastLink, GHash, Dwarf, Symtab };
enum class DebugKind {
Unknown,
None,
Full,
FastLink,
GHash,
NoGHash,
Dwarf,
Symtab
};
static DebugKind parseDebugKind(const opt::InputArgList &args) {
auto *a = args.getLastArg(OPT_debug, OPT_debug_opt);
@ -703,14 +712,15 @@ static DebugKind parseDebugKind(const opt::InputArgList &args) {
return DebugKind::Full;
DebugKind debug = StringSwitch<DebugKind>(a->getValue())
.CaseLower("none", DebugKind::None)
.CaseLower("full", DebugKind::Full)
.CaseLower("fastlink", DebugKind::FastLink)
// LLD extensions
.CaseLower("ghash", DebugKind::GHash)
.CaseLower("dwarf", DebugKind::Dwarf)
.CaseLower("symtab", DebugKind::Symtab)
.Default(DebugKind::Unknown);
.CaseLower("none", DebugKind::None)
.CaseLower("full", DebugKind::Full)
.CaseLower("fastlink", DebugKind::FastLink)
// LLD extensions
.CaseLower("ghash", DebugKind::GHash)
.CaseLower("noghash", DebugKind::NoGHash)
.CaseLower("dwarf", DebugKind::Dwarf)
.CaseLower("symtab", DebugKind::Symtab)
.Default(DebugKind::Unknown);
if (debug == DebugKind::FastLink) {
warn("/debug:fastlink unsupported; using /debug:full");
@ -1393,7 +1403,7 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
// Handle /debug
DebugKind debug = parseDebugKind(args);
if (debug == DebugKind::Full || debug == DebugKind::Dwarf ||
debug == DebugKind::GHash) {
debug == DebugKind::GHash || debug == DebugKind::NoGHash) {
config->debug = true;
config->incremental = true;
}
@ -1416,7 +1426,8 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
// Handle /pdb
bool shouldCreatePDB =
(debug == DebugKind::Full || debug == DebugKind::GHash);
(debug == DebugKind::Full || debug == DebugKind::GHash ||
debug == DebugKind::NoGHash);
if (shouldCreatePDB) {
if (auto *arg = args.getLastArg(OPT_pdb))
config->pdbPath = arg->getValue();
@ -1749,7 +1760,7 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
config->terminalServerAware =
!config->dll && args.hasFlag(OPT_tsaware, OPT_tsaware_no, true);
config->debugDwarf = debug == DebugKind::Dwarf;
config->debugGHashes = debug == DebugKind::GHash;
config->debugGHashes = debug == DebugKind::GHash || debug == DebugKind::Full;
config->debugSymtab = debug == DebugKind::Symtab;
config->autoImport =
args.hasFlag(OPT_auto_import, OPT_auto_import_no, config->mingw);

2
lld/test/COFF/pdb-type-server-simple.test
View File

@ -20,7 +20,7 @@ RUN: rm -rf %t && mkdir -p %t && cd %t
RUN: yaml2obj %S/Inputs/pdb-type-server-simple-a.yaml -o a.obj
RUN: yaml2obj %S/Inputs/pdb-type-server-simple-b.yaml -o b.obj
RUN: llvm-pdbutil yaml2pdb %S/Inputs/pdb-type-server-simple-ts.yaml -pdb ts.pdb
RUN: lld-link a.obj b.obj -entry:main -debug -out:t.exe -pdb:t.pdb -nodefaultlib -summary | FileCheck %s -check-prefix SUMMARY
RUN: lld-link a.obj b.obj -entry:main -debug:noghash -out:t.exe -pdb:t.pdb -nodefaultlib -summary | FileCheck %s -check-prefix SUMMARY
RUN: llvm-pdbutil dump -symbols -types -ids -globals %t/t.pdb | FileCheck %s
Re-run with /DEBUG:GHASH