[all-commits] [llvm/llvm-project] d17561: [lld-macho][arm64] Enhance safe ICF with thunk-bas...

[alx32 via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: d1756165a9066f907b88d51dd8e3ffee15a8cc1e
      https://github.com/llvm/llvm-project/commit/d1756165a9066f907b88d51dd8e3ffee15a8cc1e
  Author: alx32 <103613512+alx32 at users.noreply.github.com>
  Date:   2024-09-05 (Thu, 05 Sep 2024)

  Changed paths:
    M lld/MachO/Arch/ARM64.cpp
    M lld/MachO/Config.h
    M lld/MachO/Driver.cpp
    M lld/MachO/ICF.cpp
    M lld/MachO/InputSection.cpp
    M lld/MachO/InputSection.h
    M lld/MachO/MapFile.cpp
    M lld/MachO/Symbols.cpp
    M lld/MachO/Symbols.h
    M lld/MachO/SyntheticSections.cpp
    M lld/MachO/Target.h
    A lld/test/MachO/icf-safe-thunks.ll

  Log Message:
  -----------
  [lld-macho][arm64] Enhance safe ICF with thunk-based deduplication (#106573)

Currently, our `safe` ICF mode only merges non-address-significant code,
leaving duplicate address-significant functions in the output. This
patch introduces `safe_thunks` ICF mode, which keeps a single master
copy of each function and replaces address-significant duplicates with
thunks that branch to the master copy.
Currently `--icf=safe_thunks` is only supported for `arm64`
architectures.

**Perf stats for a large binary:**
| ICF Option | Total Size | __text Size | __unwind_info | % total |

|-------------------|------------|-------------|---------------------|---------------------------|
| `--icf=none` | 91.738 MB | 55.220 MB | 1.424 MB | 0% |
| `--icf=safe` | 85.042 MB | 49.572 MB | 1.168 MB | 7.30% |
| `--icf=safe_thunks` | 84.650 MB | 49.219 MB | 1.143 MB | 7.72% |
| `--icf=all` | 82.060 MB | 48.726 MB | 1.111 MB | 10.55% |

So overall we can expect a `~0.45%` binary size reduction for a typical
large binary compared to the `--icf=safe` option.

**Runtime:**
Linking the above binary took ~10 seconds. Comparing the link
performance of --icf=safe_thunks vs --icf=safe, a ~2% slowdown was
observed.



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