Fix linker-plugin-lto only doing thin lto #136840
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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 |
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Unfortunately I have no clue either, so r? compiler |
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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
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Feb 11, 2025
| // 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); | ||
| } |
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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.
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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.)
bjorn3
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Feb 11, 2025
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SparrowLii
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Feb 17, 2025
| @@ -290,6 +290,11 @@ pub(crate) unsafe fn create_module<'ll>( | |||
| ); | |||
| } | |||
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| // Disable ThinLTO if fat lto is requested. Otherwise lld defaults to thin lto. | |||
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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
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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 :))
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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.
dianqk
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Feb 17, 2025
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Makes sense, I didn’t look into what
Yeah, I think it’s unused now. |
dianqk
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Mar 27, 2025
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Thanks! |
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Fix linker-plugin-lto only doing thin lto 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
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Fix the test for #84395 by removing the |
dianqk
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Mar 27, 2025
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Fix the test for #84395 by removing the
-Zemit-thin-lto=noflag and instead setting-Clto=fat(diff to previous version). As mentioned in that issue, clang also requires setting-fltowhen using-lto-embed-bitcode=optimizedand otherwise runs into the same issue, so I guess that’s another improvement.
Hmm, I think -Zemit-thin-lto=no and -Clto=fat have different meanings, one is not submitting the Thin LTO buffer, while the other is performing Rust's Fat LTO, but -Clto=fat should imply -Zemit-thin-lto=no.
Btw, -Zemit-thin-lto=no should also fix your issue?
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
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Keep check for
Should they be different? As far as I understand #84395,
It does, but I think that’s a workaround that should not be needed :) |
This will be useful when we don't want to perform Rust's own LTO. |
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Fix linker-plugin-lto only doing thin lto 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
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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=0module flag tosignal 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