[lld] 403d61a - [lld-macho] Enable EH frame relocation / pruning
Jez Ng via llvm-commits
llvm-commits at lists.llvm.org
Wed Jul 13 18:14:17 PDT 2022
Author: Jez Ng
Date: 2022-07-13T21:14:05-04:00
New Revision: 403d61aeddec87fd210ee1425658f22367fc088a
URL: https://github.com/llvm/llvm-project/commit/403d61aeddec87fd210ee1425658f22367fc088a
DIFF: https://github.com/llvm/llvm-project/commit/403d61aeddec87fd210ee1425658f22367fc088a.diff
LOG: [lld-macho] Enable EH frame relocation / pruning
This just removes the code that gates the logic. The main issue here is
perf impact: without {D122258}, LLD takes a significant perf hit because
it now has to do a lot more work in the input parsing phase. But with
that change to eliminate unnecessary EH frames from input object files,
the perf overhead here is minimal. Concretely, here are the numbers for
some builds as measured on my 16-core Mac Pro:
**chromium_framework**
This is without the use of `-femit-dwarf-unwind=no-compact-unwind`:
base diff difference (95% CI)
sys_time 1.826 ± 0.019 1.962 ± 0.034 [ +6.5% .. +8.4%]
user_time 9.306 ± 0.054 9.926 ± 0.082 [ +6.2% .. +7.1%]
wall_time 8.225 ± 0.068 8.947 ± 0.128 [ +8.0% .. +9.6%]
samples 15 22
With that flag enabled, the regression mostly disappears, as hoped:
base diff difference (95% CI)
sys_time 1.839 ± 0.062 1.866 ± 0.068 [ -0.9% .. +3.8%]
user_time 9.452 ± 0.068 9.490 ± 0.067 [ -0.1% .. +0.9%]
wall_time 8.383 ± 0.127 8.452 ± 0.114 [ -0.1% .. +1.8%]
samples 17 21
**Unnamed internal app**
Without `-femit-dwarf-unwind`, this is the perf hit:
base diff difference (95% CI)
sys_time 1.372 ± 0.029 1.317 ± 0.024 [ -4.6% .. -3.5%]
user_time 2.835 ± 0.028 2.980 ± 0.027 [ +4.8% .. +5.4%]
wall_time 3.205 ± 0.079 3.383 ± 0.066 [ +4.9% .. +6.2%]
samples 102 83
With `-femit-dwarf-unwind`, the perf hit almost disappears:
base diff difference (95% CI)
sys_time 1.274 ± 0.026 1.270 ± 0.025 [ -0.9% .. +0.3%]
user_time 2.812 ± 0.023 2.822 ± 0.035 [ +0.1% .. +0.7%]
wall_time 3.166 ± 0.047 3.174 ± 0.059 [ -0.2% .. +0.7%]
samples 95 97
Just for fun, I measured the impact of `-femit-dwarf-unwind` on ld64
(`base` has the extra DWARF unwind info in the input object files,
`diff` doesn't):
base diff difference (95% CI)
sys_time 1.128 ± 0.010 1.124 ± 0.023 [ -1.3% .. +0.6%]
user_time 7.176 ± 0.030 7.106 ± 0.094 [ -1.5% .. -0.4%]
wall_time 7.874 ± 0.041 7.795 ± 0.121 [ -1.7% .. -0.3%]
samples 16 25
And for LLD:
base diff difference (95% CI)
sys_time 1.315 ± 0.019 1.280 ± 0.019 [ -3.2% .. -2.0%]
user_time 2.980 ± 0.022 2.822 ± 0.016 [ -5.5% .. -5.0%]
wall_time 3.369 ± 0.038 3.175 ± 0.033 [ -6.2% .. -5.3%]
samples 47 47
So parsing the extra EH frames is a lot more expensive for us than for
ld64. But given that we are quite a lot faster than ld64 to begin with,
I guess this isn't entirely unexpected...
Reviewed By: #lld-macho, oontvoo
Differential Revision: https://reviews.llvm.org/D129540
Added:
Modified:
lld/MachO/Config.h
lld/MachO/Driver.cpp
lld/MachO/InputFiles.cpp
lld/docs/ReleaseNotes.rst
Removed:
################################################################################
diff --git a/lld/MachO/Config.h b/lld/MachO/Config.h
index b6c6abb44c656..ccf71b6535eaf 100644
--- a/lld/MachO/Config.h
+++ b/lld/MachO/Config.h
@@ -131,9 +131,6 @@ struct Configuration {
bool omitDebugInfo = false;
bool warnDylibInstallName = false;
bool ignoreOptimizationHints = false;
- // Temporary config flag that will be removed once we have fully implemented
- // support for __eh_frame.
- bool parseEhFrames = false;
uint32_t headerPad;
uint32_t dylibCompatibilityVersion = 0;
uint32_t dylibCurrentVersion = 0;
diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
index 708facd180baa..abfe381f41e0a 100644
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -1305,7 +1305,6 @@ bool macho::link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
config->callGraphProfileSort = args.hasFlag(
OPT_call_graph_profile_sort, OPT_no_call_graph_profile_sort, true);
config->printSymbolOrder = args.getLastArgValue(OPT_print_symbol_order);
- config->parseEhFrames = static_cast<bool>(getenv("LLD_IN_TEST"));
// FIXME: Add a commandline flag for this too.
config->zeroModTime = getenv("ZERO_AR_DATE");
diff --git a/lld/MachO/InputFiles.cpp b/lld/MachO/InputFiles.cpp
index 01726c1ed2a31..fda6900edabeb 100644
--- a/lld/MachO/InputFiles.cpp
+++ b/lld/MachO/InputFiles.cpp
@@ -347,7 +347,7 @@ void ObjFile::parseSections(ArrayRef<SectionHeader> sectionHeaders) {
section.subsections.push_back({0, isec});
} else if (auto recordSize = getRecordSize(segname, name)) {
splitRecords(*recordSize);
- } else if (config->parseEhFrames && name == section_names::ehFrame &&
+ } else if (name == section_names::ehFrame &&
segname == segment_names::text) {
splitEhFrames(data, *sections.back());
} else if (segname == segment_names::llvm) {
@@ -1117,7 +1117,7 @@ template <class LP> void ObjFile::parse() {
}
if (compactUnwindSection)
registerCompactUnwind(*compactUnwindSection);
- if (config->parseEhFrames && ehFrameSection)
+ if (ehFrameSection)
registerEhFrames(*ehFrameSection);
}
diff --git a/lld/docs/ReleaseNotes.rst b/lld/docs/ReleaseNotes.rst
index 29c069242991e..936d800cabc37 100644
--- a/lld/docs/ReleaseNotes.rst
+++ b/lld/docs/ReleaseNotes.rst
@@ -65,7 +65,11 @@ MinGW Improvements
MachO Improvements
------------------
-* Item 1.
+* We now support proper relocation and pruning of EH frames. **Note:** this
+ comes at some performance overhead on x86_64 builds, and we recommend adding
+ the ``-femit-compact-unwind=no-compact-unwind`` compile flag to avoid it.
+ (`D129540 <https://reviews.llvm.org/D129540>`_,
+ `D122258 <https://reviews.llvm.org/D122258>`_)
WebAssembly Improvements
------------------------
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