rustc: Persist LLVM's Linker in Fat LTO
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This commit updates our Fat LTO logic to tweak our custom wrapper around LLVM's "link modules" functionality. Previously whenever the `LLVMRustLinkInExternalBitcode` function was called it would call LLVM's `Linker::linkModules` wrapper. Internally this would crate an instance of a `Linker` which internally creates an instance of an `IRMover`. Unfortunately for us the creation of `IRMover` is somewhat O(n) with the input module. This means that every time we linked a module it was O(n) with respect to the entire module we had built up! Now the modules we build up during LTO are quite large, so this quickly started creating an O(n^2) problem for us! Discovered in rust-lang#48025 it turns out this has always been a problem and we just haven't noticed it. It became particularly worse recently though due to most libraries having 16x more object files than they previously did (1 -> 16). This commit fixes this performance issue by preserving the `Linker` instance across all links into the main LLVM module. This means we only create one `IRMover` and allows LTO to progress much speedier. From the `cargo-cache` project in rust-lang#48025 a **full build** locally when from 5m15s to 2m24s. Looking at the timing logs each object file was linked in in single-digit millisecond rather than hundreds, clearly being a nice improvement! Closes rust-lang#48025
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…kruppe rustc: Persist LLVM's `Linker` in Fat LTO This commit updates our Fat LTO logic to tweak our custom wrapper around LLVM's "link modules" functionality. Previously whenever the `LLVMRustLinkInExternalBitcode` function was called it would call LLVM's `Linker::linkModules` wrapper. Internally this would crate an instance of a `Linker` which internally creates an instance of an `IRMover`. Unfortunately for us the creation of `IRMover` is somewhat O(n) with the input module. This means that every time we linked a module it was O(n) with respect to the entire module we had built up! Now the modules we build up during LTO are quite large, so this quickly started creating an O(n^2) problem for us! Discovered in rust-lang#48025 it turns out this has always been a problem and we just haven't noticed it. It became particularly worse recently though due to most libraries having 16x more object files than they previously did (1 -> 16). This commit fixes this performance issue by preserving the `Linker` instance across all links into the main LLVM module. This means we only create one `IRMover` and allows LTO to progress much speedier. From the `cargo-cache` project in rust-lang#48025 a **full build** locally went from 5m15s to 2m24s. Looking at the timing logs each object file was linked in in single-digit millisecond rather than hundreds, clearly being a nice improvement! Closes rust-lang#48025
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…kruppe rustc: Persist LLVM's `Linker` in Fat LTO This commit updates our Fat LTO logic to tweak our custom wrapper around LLVM's "link modules" functionality. Previously whenever the `LLVMRustLinkInExternalBitcode` function was called it would call LLVM's `Linker::linkModules` wrapper. Internally this would crate an instance of a `Linker` which internally creates an instance of an `IRMover`. Unfortunately for us the creation of `IRMover` is somewhat O(n) with the input module. This means that every time we linked a module it was O(n) with respect to the entire module we had built up! Now the modules we build up during LTO are quite large, so this quickly started creating an O(n^2) problem for us! Discovered in rust-lang#48025 it turns out this has always been a problem and we just haven't noticed it. It became particularly worse recently though due to most libraries having 16x more object files than they previously did (1 -> 16). This commit fixes this performance issue by preserving the `Linker` instance across all links into the main LLVM module. This means we only create one `IRMover` and allows LTO to progress much speedier. From the `cargo-cache` project in rust-lang#48025 a **full build** locally went from 5m15s to 2m24s. Looking at the timing logs each object file was linked in in single-digit millisecond rather than hundreds, clearly being a nice improvement! Closes rust-lang#48025
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This commit updates our Fat LTO logic to tweak our custom wrapper around LLVM's
"link modules" functionality. Previously whenever the
LLVMRustLinkInExternalBitcodefunction was called it would call LLVM'sLinker::linkModuleswrapper. Internally this would crate an instance of aLinkerwhich internally creates an instance of anIRMover. Unfortunately forus the creation of
IRMoveris somewhat O(n) with the input module. This meansthat every time we linked a module it was O(n) with respect to the entire module
we had built up!
Now the modules we build up during LTO are quite large, so this quickly started
creating an O(n^2) problem for us! Discovered in #48025 it turns out this has
always been a problem and we just haven't noticed it. It became particularly
worse recently though due to most libraries having 16x more object files than
they previously did (1 -> 16).
This commit fixes this performance issue by preserving the
Linkerinstanceacross all links into the main LLVM module. This means we only create one
IRMoverand allows LTO to progress much speedier.From the
cargo-cacheproject in #48025 a full build locally went from5m15s to 2m24s. Looking at the timing logs each object file was linked in in
single-digit millisecond rather than hundreds, clearly being a nice improvement!
Closes #48025