Use codegen-units=1 in official builds. [chromium/src : main]
danakj (Gerrit)
Attention is currently required from: Nico Weber.
Patch set 1:Auto-Submit +1
1 comment:
Patchset:
I was learning about this on the weekend, so let's do it while it's fresh in my head
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danakj (Gerrit)
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danakj uploaded patch set #2 to this change.
Use codegen-units=1 in official builds.
The rust compiler does the best job of optimizing within a crate when
it compiles everything in a single unit. This is slower, so it's not
used by developers generally, but we should use it in official builds.
R=tha...@chromium.org
Change-Id: Id12861ae7a390f051b4d3f1069c07f745ee1fcfa
Cq-Include-Trybots: luci.chromium.try:linux-rust-x64-rel,android-rust-arm-rel
---
M build/config/compiler/BUILD.gn
1 file changed, 19 insertions(+), 0 deletions(-)
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Nico Weber (Gerrit)
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Patch set 2:Code-Review +1
2 comments:
Patchset:
looks fine.
i wonder if this actually helps much when we do LTO though.
File build/config/compiler/BUILD.gn:
Patch Set #2, Line 837:
if (!use_thin_lto) {unrelated, but we should only embed bitcode when using Chromium's rust toolchain, else it can't work. (that's one reason we're building that)
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danakj (Gerrit)
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Patch set 2:Auto-Submit +1Commit-Queue +1
2 comments:
Patchset:
looks fine. […]
It helps with other optimizations as well beyond what LTO will do, I believe. Like general compiler optimization.
File build/config/compiler/BUILD.gn:
Patch Set #2, Line 837:
if (!use_thin_lto) {
unrelated, but we should only embed bitcode when using Chromium's rust toolchain, else it can't work […]
Will send another CL
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danakj (Gerrit)
Attention is currently required from: Nico Weber.
Patch set 2:Commit-Queue +2
danakj (Gerrit)
1 comment:
File build/config/compiler/BUILD.gn:
Patch Set #2, Line 837:
if (!use_thin_lto) {
Will send another CL
Done: https://chromium-review.googlesource.com/c/chromium/src/+/3498523
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Nico Weber (Gerrit)
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1 comment:
Patchset:
It helps with other optimizations as well beyond what LTO will do, I believe. […]
like what? the value is probably that it can optimize across all files in the crate since they go in a single o file, but LTO can optimize across .o files.
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danakj (Gerrit)
1 comment:
Patchset:
like what? the value is probably that it can optimize across all files in the crate since they go in […]
That's right the value proposition is that rustc sees all of the crate and generates a single .o file for it.
But I thought LTO was more or less about inlining code and maybe I misunderstood that. I thought there's more information around at compile time or more possible optimizations, like loop unrolling. One benefit of codegen-units=1 is that all uses of foo<Bar>() get turned into a single function, but I think LTO should solve that too.
FWIW rustc by default does LTO _inside_ a crate, while compiling with codegen-units=16 in release builds. But avoids LTO across crates because that's slow. This was explained in "Rust for Rustaceans" as a compromise, as codegen-units=16 speeds up the build but makes for worse code, and LTO inside the crate helps to mitigate that.
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Nico Weber (Gerrit)
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1 comment:
Patchset:
That's right the value proposition is that rustc sees all of the crate and generates a single . […]
we use "thinlto" to make LTO less slow. as long as rustc can output .o files with bitcode, that should transparently work hopefully. (rustc shouldn't be involved in the link invocations, and lld will thinlto fine as long as it gets a .o file with bitcode in the right version.)
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Chromium LUCI CQ (Gerrit)
Chromium LUCI CQ submitted this change.
Use codegen-units=1 in official builds.
The rust compiler does the best job of optimizing within a crate when
it compiles everything in a single unit. This is slower, so it's not
used by developers generally, but we should use it in official builds.
R=tha...@chromium.org
Change-Id: Id12861ae7a390f051b4d3f1069c07f745ee1fcfa
Cq-Include-Trybots: luci.chromium.try:linux-rust-x64-rel,android-rust-arm-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3498521
Reviewed-by: Nico Weber <tha...@chromium.org>
Auto-Submit: danakj <dan...@chromium.org>
Commit-Queue: danakj <dan...@chromium.org>
Cr-Commit-Position: refs/heads/main@{#976365}
---
M build/config/compiler/BUILD.gn
1 file changed, 24 insertions(+), 0 deletions(-)
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 7315319..9d4ab29 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -838,6 +838,9 @@
# Optimization - don't include bitcode if it won't be used.
rustflags += [ "-Cembed-bitcode=no" ]
}
+ if (is_official_build) {
+ rustflags += [ "-Ccodegen-units=1" ]
+ }
}
# The BUILDCONFIG file sets this config on targets by default, which means when
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