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Use ICF (identical code folding) for building rustc #99062

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Jul 18, 2022
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Use ICF (identical code folding) for building rustc #99062

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6 changes: 6 additions & 0 deletions src/bootstrap/compile.rs
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Expand Up @@ -651,6 +651,12 @@ impl Step for Rustc {
panic!("Cannot use and generate PGO profiles at the same time");
}

// With LLD, we can use ICF (identical code folding) to reduce the executable size
// of librustc_driver/rustc and to improve i-cache utilization.
if builder.config.use_lld {
cargo.rustflag("-Clink-args=-Wl,--icf=all");
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This is specific to the compiler; it doesn't affect tools or the standard library. Is that intentionall?

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My main motivation was to try to reduce the size of librustc_driver, the largest dynamic library. But we can also try it for libstd.

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icf=all is a bit zealous, should be safe?

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In my experiments, safe didn't really do anything in terms of binary size. But we can try it.

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#99455 let's see :)

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icf=safe doesn't improve binary sizes at all, sadly. There are still nice cycle wins, but these come mostly from using lld.

}

let is_collecting = if let Some(path) = &builder.config.rust_profile_generate {
if compiler.stage == 1 {
cargo.rustflag(&format!("-Cprofile-generate={}", path));
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3 changes: 2 additions & 1 deletion src/ci/docker/host-x86_64/dist-x86_64-linux/Dockerfile
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Expand Up @@ -122,7 +122,8 @@ ENV RUST_CONFIGURE_ARGS \
--set target.x86_64-unknown-linux-gnu.ranlib=/rustroot/bin/llvm-ranlib \
--set llvm.thin-lto=true \
--set llvm.ninja=false \
--set rust.jemalloc
--set rust.jemalloc \
--set rust.use-lld=true
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This only affects Linux; is that intentional? Or should we use lld on other Unix platforms too?

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Indeed I wanted to only try it first for Linux x64, as the most common toolchain. I usually test CI compilation optimization flags and similar things for it first, since it tends to have the best support.

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@bjorn3 bjorn3 Jul 17, 2022 •
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I'm guessing this option wouldn't work on windows anyway as we don't use a single section per function there because COFF has a 2^16 - something limit on the section count and it is rather easy to hit this limit if every function has it's own section.

ENV SCRIPT ../src/ci/pgo.sh python3 ../x.py dist \
--host $HOSTS --target $HOSTS \
--include-default-paths \
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4 changes: 4 additions & 0 deletions src/ci/pgo.sh
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Expand Up @@ -195,3 +195,7 @@ rm -r $BUILD_ARTIFACTS/llvm $BUILD_ARTIFACTS/lld
$@ \
--rust-profile-use=${RUSTC_PROFILE_MERGED_FILE} \
--llvm-profile-use=${LLVM_PROFILE_MERGED_FILE}

echo "Rustc binary size"
ls -la ./build/$PGO_HOST/stage2/bin
ls -la ./build/$PGO_HOST/stage2/lib
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