Fix linker-plugin-lto only doing thin lto #136840

Flakebi · 2025-02-10T23:45:01Z

When rust provides LLVM bitcode files to lld and the bitcode contains
function summaries as used for thin lto, lld defaults to using thin lto.
This prevents some optimizations that are only applied for fat lto.

We solve this by not creating function summaries when fat lto is
enabled. The bitcode for the module is just directly written out.

An alternative solution would be to set the ThinLTO=0 module flag to
signal lld to do fat lto.
The code in clang that sets this flag is here:
https://github.com/llvm/llvm-project/blob/560149b5e3c891c64899e9912e29467a69dc3a4c/clang/lib/CodeGen/BackendUtil.cpp#L1150
The code in LLVM that queries the flag and defaults to thin lto if not
set is here:
https://github.com/llvm/llvm-project/blob/e258bca9505f35e0a22cb213a305eea9b76d11ea/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp#L4441-L4446

try-job: x86_64-gnu-debug
try-job: aarch64-gnu-debug

rustbot · 2025-02-10T23:45:04Z

Could not assign reviewer from: workingjubilee.
User(s) workingjubilee are either the PR author, already assigned, or on vacation. Please use r? to specify someone else to assign.

rustbot · 2025-02-10T23:45:10Z

r? @jieyouxu

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rustbot · 2025-02-10T23:45:12Z

This PR modifies tests/run-make/. If this PR is trying to port a Makefile
run-make test to use rmake.rs, please update the
run-make port tracking issue
so we can track our progress. You can either modify the tracking issue
directly, or you can comment on the tracking issue and link this PR.

cc @jieyouxu

workingjubilee · 2025-02-11T00:09:30Z

I have barely any idea about LTO besides "it happens and it involves dlopening a compiler and shoving its serialized data back in it" tbh soo

jieyouxu · 2025-02-11T00:35:31Z

Unfortunately I have no clue either, so

r? compiler

Flakebi · 2025-02-11T12:59:05Z

For reference, the code that switches to thin lto when the flag is not set is here: https://github.com/llvm/llvm-project/blob/e258bca9505f35e0a22cb213a305eea9b76d11ea/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp#L4441-L4446

  // By default we compile with ThinLTO if the module has a summary, but the
  // client can request full LTO with a module flag.
  bool IsThinLTO = true;
  if (auto *MD =
          mdconst::extract_or_null<ConstantInt>(M.getModuleFlag("ThinLTO")))
    IsThinLTO = MD->getZExtValue();

The code in clang that sets the flag, which is replicated here for Rust is here: https://github.com/llvm/llvm-project/blob/560149b5e3c891c64899e9912e29467a69dc3a4c/clang/lib/CodeGen/BackendUtil.cpp#L1150

        if (!TheModule->getModuleFlag("ThinLTO") && !CodeGenOpts.UnifiedLTO)
          TheModule->addModuleFlag(llvm::Module::Error, "ThinLTO", uint32_t(0));

bjorn3 · 2025-02-11T17:39:59Z

compiler/rustc_codegen_llvm/src/context.rs

+    // Disable ThinLTO if fat lto is requested. Otherwise lld defaults to thin lto.
+    if sess.lto() == config::Lto::Fat {
+        llvm::add_module_flag_u32(llmod, llvm::ModuleFlagMergeBehavior::Override, "ThinLTO", 0);
+    }


What if a dependency is built with lto=true (aka lto=fat), but then the user wants to use thinLTO? I'm pretty sure the standard library is built with lto=true for example, but that shouldn't prevent thinLTO from ever working.

Good question, it seems to change somewhat, but still work in general. I added a test for this.
What changes: Without this change, the test passes when
lib is compiled with O0 and main with O3 and

lib uses lto=thin and main uses lto=thin

lib uses lto=thin and main uses lto=fat

lib uses lto=fat and main uses lto=thin

lib uses lto=fat and main uses lto=fat

With this change, all of these keep passing except for case 3 (lib uses lto=fat and main uses lto=thin).
When lib is compiled with O1, O2 or O3, case 3 passes as well.
I assume this is the important case, as the standard library is compiled with optimizations.
(And lto with O0 is kinda questionable, except maybe for nvptx and amdgpu, but they require lto=fat anyway.)

compiler/rustc_codegen_ssa/src/back/link.rs

fee1-dead · 2025-02-16T10:27:53Z

r? compiler

SparrowLii

I know little about lto, either. I think it would be much more acceptable if this PR could limit the change to amdhsa conditions.

r? compiler

SparrowLii · 2025-02-17T01:21:21Z

compiler/rustc_codegen_llvm/src/context.rs

@@ -290,6 +290,11 @@ pub(crate) unsafe fn create_module<'ll>(
        );
    }

+    // Disable ThinLTO if fat lto is requested. Otherwise lld defaults to thin lto.


That sounds counterintuitive. Can you explain the relationship between the user's lto option and llvm's lto in the comments?

And I think it needs a individual test to ensure that the previous lto=fat option is not affected

I changed the comment, is it clearer now?

(I want to affect the current lto=fat option, as it currently does thin lto, which I think is not intended and a bug :))

I can confirm that the current behavior is surprising, since I ran into this issue with autodiff and it took multiple months to find out that despite selecting lto=fat there is still a thin-lto module in the compilation pipeline.

Kobzol · 2025-02-17T09:33:30Z

@dianqk Does this interact with your recent patch?

dianqk · 2025-02-17T10:06:31Z

@dianqk Does this interact with your recent patch?

IMO, they aren't directly related.

compiler/rustc_codegen_ssa/src/back/linker.rs

Nadrieril · 2025-02-17T21:59:33Z

r? codegen

GuillaumeGomez · 2025-05-09T12:18:15Z

Since it failed CI, let's remove it from the merge queue.

@bors r-

jieyouxu · 2025-05-09T12:25:56Z

Since it failed CI, let's remove it from the merge queue.

Weird, that should've been kicked out automatically because full CI failed.

GuillaumeGomez · 2025-05-09T12:30:45Z

I think bors is a bit behind. There were 3 merged PRs still in the queue. I ran a synchronization and then these two poped up. So anyway. :')

bors · 2025-07-10T17:15:40Z

☔ The latest upstream changes (presumably #143731) made this pull request unmergeable. Please resolve the merge conflicts.

Flakebi · 2025-07-29T22:24:30Z

I finally found some time to look at this again. Debugging the aarch64 test failure from an x86 system was not the most straightforward thing 😅

I rebased to fix conflicts, new changes are in the second commit b5b0282.

In CI, the cross-lang-lto-clang passed on x86_64 but not on aarch64. I don’t have a good way to debug this, but with some local changes (my clang to cross-compiling to aarch64 uses different target features, so nothing gets inlined by default; and I wasn’t able to link a rust std for aarch64), I think I found the culprit.
With fat lto on aarch64, the inline(never) rust function gets inlined. This is the same behavior as with linking a C library into a Rust binary, just that it’s different between aarch64 and x86 (fat lto on x86 inlines the noinline C function into the Rust binary but it does not inline the inline(never) Rust function into the C binary).

Flakebi · 2025-07-30T18:15:17Z

PR CI passed (though it doesn’t run the interesting tests). If a person of power can start a bors check run, that would be great.

@rustbot ready

ZuseZ4 · 2025-07-30T18:19:00Z

@Flakebi I haven't hearedof check runs, but I can start a try run for you if you want. But you would need to specify first for which target you want to run it, see here: https://rustc-dev-guide.rust-lang.org/tests/ci.html?highlight=bors%20try#try-builds

Flakebi · 2025-07-30T20:13:29Z

Aah yes, thanks, I meant a try run.
The two configurations for the cross-lang-lto-clang test are most interesting, that should be x86_64-gnu-debug and aarch64-gnu-debug, I added them to the PR description

ZuseZ4 · 2025-07-30T20:36:39Z

@bors try

Fix linker-plugin-lto only doing thin lto try-job: x86_64-gnu-debug try-job: aarch64-gnu-debug

rust-bors · 2025-07-30T20:36:43Z

⌛ Trying commit 6a1ed2f with merge cc3bd03…

To cancel the try build, run the command @bors try cancel.

rust-bors · 2025-07-30T22:21:16Z

☀️ Try build successful (CI)
Build commit: cc3bd03 (cc3bd03ea8b217fbd191dee23e1cf35df02e6025, parent: e5e79f8bd428d0b8d26e8240d718b134ef297459)

dianqk · 2025-07-31T06:53:09Z

@ZuseZ4 FYI, you can use @bors try jobs=*-gnu-debug now. The new command is from #t-compiler > Testing try builds with new bors @ 💬.

rust-bors · 2025-07-31T06:53:11Z

Unknown argument "now.". Run @bors2 help to see available commands.

dianqk · 2025-07-31T06:53:22Z

@bors try jobs=*-gnu-debug

rust-bors · 2025-07-31T06:53:27Z

⌛ Trying commit 6a1ed2f with merge d17dd8b…

To cancel the try build, run the command @bors try cancel.

Fix linker-plugin-lto only doing thin lto try-job: *-gnu-debug

dianqk · 2025-07-31T06:54:45Z

@bors try cancel

rust-bors · 2025-07-31T06:54:48Z

Try build cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/16642078994

dianqk

The new changes look good to me. Please squash your commits.

compiler/rustc_codegen_llvm/src/back/write.rs

tests/run-make/cross-lang-lto-clang/rmake.rs

When rust provides LLVM bitcode files to lld and the bitcode contains function summaries as used for thin lto, lld defaults to using thin lto. This prevents some optimizations that are only applied for fat lto. We solve this by not creating function summaries when fat lto is enabled. The bitcode for the module is just directly written out. An alternative solution would be to set the `ThinLTO=0` module flag to signal lld to do fat lto. The code in clang that sets this flag is here: https://github.com/llvm/llvm-project/blob/560149b5e3c891c64899e9912e29467a69dc3a4c/clang/lib/CodeGen/BackendUtil.cpp#L1150 The code in LLVM that queries the flag and defaults to thin lto if not set is here: https://github.com/llvm/llvm-project/blob/e258bca9505f35e0a22cb213a305eea9b76d11ea/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp#L4441-L4446

rustbot · 2025-07-31T08:39:08Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

rustbot assigned jieyouxu Feb 10, 2025

rustbot added A-run-make Area: port run-make Makefiles to rmake.rs S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Feb 10, 2025

Flakebi mentioned this pull request Feb 10, 2025

Tracking Issue for amdgcn target #135024

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Rollup of 40 pull requests #140573

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amdgcn target failling to build compiler_builtins (and thus all examples) #144381

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Flakebi force-pushed the linker-plugin-lto-fat branch from 660d1e2 to b5b0282 Compare July 29, 2025 22:20

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rust-bors bot added a commit that referenced this pull request Jul 30, 2025

Auto merge of #136840 - Flakebi:linker-plugin-lto-fat, r=<try>

cc3bd03

Fix linker-plugin-lto only doing thin lto try-job: x86_64-gnu-debug try-job: aarch64-gnu-debug

rust-bors bot added a commit that referenced this pull request Jul 31, 2025

Auto merge of #136840 - Flakebi:linker-plugin-lto-fat, r=<try>

d17dd8b

Fix linker-plugin-lto only doing thin lto try-job: *-gnu-debug

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Flakebi force-pushed the linker-plugin-lto-fat branch from 6a1ed2f to 7a127fb Compare July 31, 2025 08:39

Fix linker-plugin-lto only doing thin lto #136840

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