mold does not accelerate the compile process compare to normal config

[Mon-ius]

DEV_ROOT=$HOME/.dev
DEV_DOWNLOAD=$DEV_ROOT/download
DEV_INSTALL=$DEV_ROOT/usr
PATH=$PATH:$DEV_ROOT/usr/bin

sudo apt-get install -y cmake gcc g++
git clone --depth 1 --branch stable https://github.com/rui314/mold.git

cmake -DCMAKE_INSTALL_PREFIX="$DEV_INSTALL" -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_COMPILER=c++ -B build
cmake --build build -j$(nproc)
cmake --build build --target install --parallel "$(nproc)"

RUSTFLAGS="-C linker=clang -C link-arg=-fuse-ld=mold -Z threads=32" cargo +nightly build --release
# vs
RUSTFLAGS="-Z threads=32" cargo +nightly build --release

And the result of readelf -p .comment $bin:

String dump of section '.comment':
  [     0]  rustc version 1.80.0-nightly (ada5e2c7b 2024-05-31)
  [    34]  GCC: (Ubuntu 12.3.0-1ubuntu1~22.04) 12.3.0
  [    5f]  mold 2.32.0 (ec04bfda9d01a18f4738f53d37756e99c648600d; compatible with GNU ld)
  [    af]  GCC: (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0

[rui314]

If that's the case it is what it is. If linking is not the bottleneck of your build process, mold can't make it faster. Maybe your binary is relatively small?

[Mon-ius]

I am aware if the process I have done for configure the mold is right, the linking part normally spent over 1min to process on Intel(R) Xeon(R) Platinum 8360Y CPU @ 2.40GHz in this case.

[rui314]

What is the overall wallclock time to build the entire source tree? What if you change a single file and rebuild it?

[Mon-ius]

If change single file and rebuild it when RUSTFLAGS="-C linker=clang -C link-arg=-fuse-ld=mold" cargo build --release, it spent 34.20s/1m 11s, 34.20s is for change a single file and rebuild it, 1m 11s is overall spend.

However, when pure perform cargo build --release without any FLAGs, it spent the similar, 34.66s/1m 45s.

When perform RUSTFLAGS="-C linker=clang -C link-arg=-fuse-ld=mold -Z threads=32" cargo +nightly build --release, 34.63s/1m 42s

Each build test after cargo clean.

[rui314]

You don't want to run cargo clean after making a change to a file. Just change a single file and rebuild it, which is a typical scenario, and measure how much time does each build takes.

[Mon-ius]

As mentioned, 34.20s, 34.66s and 34.63s are the time spent by change a single file and rebuild it. So, wired is, the three build process make no different

[rui314]

What if you do not change anything but just delete the generated executable and rebuild it?

[Mon-ius]

@rui314 as you request,

• 0.14s for standard cargo build --release
• 0.14s for RUSTFLAGS="-Z threads=32" cargo +nightly build --release
• 0.11s for RUSTFLAGS="-C linker=clang -C link-arg=-fuse-ld=mold" cargo build --release
• 0.11s for RUSTFLAGS="-C linker=clang -C link-arg=-fuse-ld=mold -Z threads=32" cargo +nightly build --release

[rui314]

I'm not familiar with the rust build system, but aren't that the time the linkers take to link your program? If that's the case, the faster linker can't make any difference.

[Mon-ius]

it takes about 0.03s faster in this case. One more question, does mold compatible with https://github.com/rust-lang/rustc_codegen_cranelift

[rui314]

Yes

[Mon-ius]

Hi @rui314

Today, I tested a small program in Rust, which shows mold slow down about 60 ~ 200 percent compared to standard build.

3.96s, RUSTFLAGS="-Zcodegen-backend=cranelift -Z threads=8" cargo +nightly build --release, 6.19s, RUSTFLAGS="-Zcodegen-backend=cranelift -C linker=clang -C link-arg=-fuse-ld=mold -Z threads=8" cargo +nightly build --release 7.99s, RUSTFLAGS="-Zcodegen-backend=cranelift" cargo +nightly build --release,

11.52s, cargo +nightly build --release, 10.23s, RUSTFLAGS="-Z threads=8" cargo +nightly build --release, 14.66s, RUSTFLAGS="-C linker=clang -C link-arg=-fuse-ld=mold -Z threads=8" cargo +nightly build --release, 18.94s, RUSTFLAGS="-C linker=clang -C link-arg=-fuse-ld=mold" cargo +nightly build --release

---

19.95s, RUSTFLAGS="-C linker=clang -C link-arg=-fuse-ld=mold" cargo build --release 18.11s, RUSTFLAGS="-C linker=clang -C link-arg=-fuse-ld=mold -Z threads=8" cargo build --release

20.09s, cargo build --release 9.04s, RUSTFLAGS="-Z threads=8" cargo build --release

[rui314]

Instead of measuring only the total build time, can you measure how long does each internal step take?

[Mon-ius]

do you mean cargo build --timings?

[rui314]

I'm afraid I don't know. I've never used cargo personally.