403d61aedd
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
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ReStructuredText
===========================
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lld |release| Release Notes
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===========================
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.. contents::
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:local:
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.. only:: PreRelease
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.. warning::
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These are in-progress notes for the upcoming LLVM |release| release.
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Release notes for previous releases can be found on
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`the Download Page <https://releases.llvm.org/download.html>`_.
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Introduction
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============
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This document contains the release notes for the lld linker, release |release|.
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Here we describe the status of lld, including major improvements
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from the previous release. All lld releases may be downloaded
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from the `LLVM releases web site <https://llvm.org/releases/>`_.
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Non-comprehensive list of changes in this release
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=================================================
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ELF Improvements
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----------------
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* ``-z pack-relative-relocs`` is now available to support ``DT_RELR`` for glibc 2.36+.
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(`D120701 <https://reviews.llvm.org/D120701>`_)
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* ``--no-fortran-common`` (pre 12.0.0 behavior) is now the default.
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* ``FORCE_LLD_DIAGNOSTICS_CRASH`` environment variable is now available to force LLD to crash.
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(`D128195 <https://reviews.llvm.org/D128195>`_)
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Breaking changes
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----------------
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* The GNU ld incompatible ``--no-define-common`` has been removed.
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* The obscure ``-dc``/``-dp`` options have been removed.
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* ``-d`` is now ignored.
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* If a prevailing COMDAT group defines STB_WEAK symbol, having a STB_GLOBAL symbol in a non-prevailing group is now rejected with a diagnostic.
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(`D120626 <https://reviews.llvm.org/D120626>`_)
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* Support for the legacy ``.zdebug`` format has been removed. Run
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``objcopy --decompress-debug-sections`` in case old object files use ``.zdebug``.
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(`D126793 <https://reviews.llvm.org/D126793>`_)
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* ``--time-trace-file=<file>`` has been removed.
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Use ``--time-trace=<file>`` instead.
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(`D128451 <https://reviews.llvm.org/D128451>`_)
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COFF Improvements
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-----------------
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* Added autodetection of MSVC toolchain, a la clang-cl. Also added
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``/winsysroot:`` support for explicit specification of MSVC toolchain
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location, similar to clang-cl's ``/winsysroot``. For now,
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``/winsysroot:`` requires also passing in an explicit ``/machine:`` flag.
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(`D118070 <https://reviews.llvm.org/D118070>`_)
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* ...
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MinGW Improvements
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------------------
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* ...
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MachO Improvements
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------------------
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* We now support proper relocation and pruning of EH frames. **Note:** this
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comes at some performance overhead on x86_64 builds, and we recommend adding
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the ``-femit-compact-unwind=no-compact-unwind`` compile flag to avoid it.
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(`D129540 <https://reviews.llvm.org/D129540>`_,
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`D122258 <https://reviews.llvm.org/D122258>`_)
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WebAssembly Improvements
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------------------------
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