# Is there an equivalent to cross-language link time optimization via LLVM?

**URL:** <https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625>\
**Category:** General Usage\
**Created:** [February 13, 2026, 7:15pm UTC](https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625 "2026-02-13T19:15:48Z")\
**Posts on this page:** 11\
**Page:** 2

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**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [March 5, 2026, 7:21pm UTC](https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625/21 "2026-03-05T19:21:36Z")

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What’s happening here? Are we just spitting LLM text at each other here? We’re a community of humans, let’s please _talk_ to eachother.

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**Author:** ![obsidianjulua](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/obsidianjulua/32/221495_2.png) [@obsidianjulua](https://discourse.julialang.org/u/obsidianjulua)\
**Post date:** [March 5, 2026, 7:25pm UTC](https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625/22 "2026-03-05T19:25:23Z")

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Sorry there copied from the recent commit for the project that works with coss language lto and also the changelog refrences templates for c++ directly so i just pasted that too.

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**Author:** ![obsidianjulua](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/obsidianjulua/32/221495_2.png) [@obsidianjulua](https://discourse.julialang.org/u/obsidianjulua)\
**Post date:** [March 6, 2026, 3:45am UTC](https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625/23 "2026-03-06T03:45:39Z")

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```julia-auto
--- Per-call overhead (single invocation, 10000 samples) ---

  pure_julia scalar_add median=30.0 ns
  bare_ccall scalar_add median=30.0 ns
  wrapper_ccall scalar_add median=30.0 ns
  lto_llvmcall scalar_add median=30.0 ns
  pure_julia scalar_mul median=40.0 ns
  bare_ccall scalar_mul median=40.0 ns
  wrapper_ccall scalar_mul median=30.0 ns
  lto_llvmcall scalar_mul median=30.0 ns
  pure_julia make_point median=40.0 ns
  bare_ccall make_point median=40.0 ns
  wrapper_ccall make_point median=40.0 ns
  lto_llvmcall make_point median=40.0 ns
  bare_ccall_UNSAFE pack_record ⚠ skipped (packed struct ABI crash)
  wrapper_ccall pack_record median=80.0 ns

--- Hot loop: 1000000 iterations of add_to(acc, val) ---

  pure_julia total=0.68 ms 0.677 ns/iter
  bare_ccall_loop total=2.03 ms 2.026 ns/iter
  wrapper_ccall_loop total=23.85 ms 23.854 ns/iter
  lto_llvmcall_loop total=25.59 ms 25.594 ns/iter
  whole_loop_in_cpp total=1.0 ms 0.997 ns/iter 

```

So this is where optimizing the code comes in because I do have the llvmcall in place but im not currently matching the bare ccall. I also just got alot of this working in the last 4 months and it could be optimized for sure.

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**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [March 6, 2026, 4:07am UTC](https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625/24 "2026-03-06T04:07:00Z")

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Those medians all being divisible by 10.0ns suggests to me the single “invocation” (evaluation?) ran into timer resolution. More evaluations per sample like the hot loop benchmark would address that. The wrapper adding 20ns to a ccall is really bad. Have you checked if LLVM optimizes away the wrapper’s branch or if the wrapper’s calls get inlined to other expressions?

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**Author:** ![obsidianjulua](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/obsidianjulua/32/221495_2.png) [@obsidianjulua](https://discourse.julialang.org/u/obsidianjulua)\
**Post date:** [March 6, 2026, 4:07am UTC](https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625/25 "2026-03-06T04:07:42Z")

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ok i did have a issue in my wrapper and i fixed my code on the c++ side and i have better results and there very good

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<div class="post-metadata">

**Author:** ![obsidianjulua](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/obsidianjulua/32/221495_2.png) [@obsidianjulua](https://discourse.julialang.org/u/obsidianjulua)\
**Post date:** [March 6, 2026, 4:15am UTC](https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625/26 "2026-03-06T04:15:29Z")

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## Per-Call Overhead (vs Bare `ccall`)

| Scenario | Tier | Median (ns) | Vs Bare `ccall` | Note |
| --- | --- | --- | --- | --- |
| `scalar_add` | `pure_julia` | 31.0 | 1.0x | Julia native `a+b` |
| `scalar_add` | `bare_ccall` | 31.0 | 1.0x | Hand-written `ccall` (community baseline) |
| `scalar_add` | `wrapper_ccall` | 31.0 | 1.0x | RepliBuild generated `ccall` wrapper |
| `scalar_add` | **`lto_llvmcall`** | **31.0** | **1.0x** | **RepliBuild LTO: `Base.llvmcall` (Julia JIT inlines C++ IR)** |
| `scalar_mul` | `pure_julia` | 41.0 | 1.0x | Julia native `a*b` |
| `scalar_mul` | `bare_ccall` | 41.0 | 1.0x | Hand-written `ccall` |
| `scalar_mul` | `wrapper_ccall` | 61.0 | 1.5x | RepliBuild generated `ccall` wrapper |
| `scalar_mul` | **`lto_llvmcall`** | **41.0** | **1.0x** | **RepliBuild LTO: `Base.llvmcall`** |
| `make_point` | `pure_julia` | 41.0 | 1.0x | Julia native struct construction |
| `make_point` | `bare_ccall` | 41.0 | 1.0x | Hand-written `ccall` (struct return, manual layout) |
| `make_point` | `wrapper_ccall` | 41.0 | 1.0x | RepliBuild generated wrapper (ABI-verified layout) |
| `make_point` | **`lto_llvmcall`** | **41.0** | **1.0x** | **RepliBuild LTO: `Base.llvmcall`** |
| `pack_record` | `bare_ccall_UNSAFE` | NaN | — | ⚠ Naive `ccall` — packed struct return crashes; cannot safely benchmark |
| `pack_record` | `wrapper_ccall` | 81.0 | — | RepliBuild generated wrapper (DWARF-verified packed layout) |

* * *

## Hot Loop (2,000,000 Iterations)

_Running `add_to(acc, val)` continuously across the FFI boundary._

| Tier | Median (ns) | ns / iter | Note |
| --- | --- | --- | --- |
| `pure_julia` | 677,784.0 | **0.677** | Julia `@inbounds` loop with native add |
| `bare_ccall_loop` | 2,800,660.0 | **1.801** | Julia loop — bare `ccall` in a typed function |
| `wrapper_ccall_loop` | 677,889.0 | **0.677** | Julia loop calling RepliBuild `ccall` wrapper each iteration |
| **`lto_llvmcall_loop`** | **677,624.0** | **0.677** | **Julia loop with LTO: Julia JIT inlines C++ `add_to` across FFI boundary** |
| `whole_loop_in_cpp` | 997,968.0 | **0.997** | Single `ccall` to C++ `accumulate_array` (entire loop in C++) |

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<div class="post-metadata">

**Author:** ![obsidianjulua](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/obsidianjulua/32/221495_2.png) [@obsidianjulua](https://discourse.julialang.org/u/obsidianjulua)\
**Post date:** [March 6, 2026, 4:20am UTC](https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625/27 "2026-03-06T04:20:13Z")

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That just made c++ faster than c++

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<div class="post-metadata">

**Author:** ![Benny](https://avatars.discourse-cdn.com/v4/letter/b/49beb7/32.png) [@Benny](https://discourse.julialang.org/u/Benny)\
**Post date:** [March 6, 2026, 4:27am UTC](https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625/28 "2026-03-06T04:27:32Z")

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Can you put these benchmarks on Github and just share links with short introductions here? Pasting a couple snippets is fine but having to scroll through a table is a bit much. Same with docs sections, that was a bit confusing to some people without much explanation.

As for the benchmark, those numbers fit the goal, but how is the `ccall`-wrapper doing as well as pure Julia or the `llvmcall` wrapper? I’d expect there to be a `ccall` overhead each iteration like the bare `ccall` run.

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<div class="post-metadata">

**Author:** ![obsidianjulua](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/obsidianjulua/32/221495_2.png) [@obsidianjulua](https://discourse.julialang.org/u/obsidianjulua)\
**Post date:** [March 6, 2026, 4:32am UTC](https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625/29 "2026-03-06T04:32:45Z")

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the repo is public and i commit regularly, all the test are also included inside the repo, RepliBuild.jl is registered but you will need linux, llvm 21.1.8 or any major 21 and aur mlir 21 version to match because RepliBuild uses newer jit features not in mlir.jl, so there is a custom c api bridge for that, basically any linux with up to date julia and llvm and mlir will work

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<div class="post-metadata">

**Author:** ![obsidianjulua](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/obsidianjulua/32/221495_2.png) [@obsidianjulua](https://discourse.julialang.org/u/obsidianjulua)\
**Post date:** [March 6, 2026, 4:39am UTC](https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625/30 "2026-03-06T04:39:38Z")

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also you caught something else i forgot to take `enable_lto = true` and RepliBuild optimized the ccall to dispacth it as llvmcall… so im going to run the test and take the flag out of the toml for the ccall and re run the test to see what un opted ccall generates compared, but yeah replibuild did what it was supposed to and julia just dispatched it to llvmcall because if the flag

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<div class="post-metadata">

**Author:** ![obsidianjulua](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/obsidianjulua/32/221495_2.png) [@obsidianjulua](https://discourse.julialang.org/u/obsidianjulua)\
**Post date:** [March 6, 2026, 4:47am UTC](https://discourse.julialang.org/t/is-there-an-equivalent-to-cross-language-link-time-optimization-via-llvm/135625/31 "2026-03-06T04:47:21Z")

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_Running `add_to(acc, val)` continuously across the FFI boundary._

| Tier | Median (ns) | ns / iter | Note |
| --- | --- | --- | --- |
| `pure_julia` | 676,738.0 | **0.677** | Julia `@inbounds` loop with native add |
| `bare_ccall_loop` | 1,800,310.0 | **1.800** | Julia loop — bare `ccall` in a typed function |
| `wrapper_ccall_loop` | 2,025,930.0 | **2.026** | Julia loop calling RepliBuild `ccall` wrapper (LTO disabled fallback) |
| **`lto_llvmcall_loop`** | **677,078.0** | **0.677** | **Julia loop with LTO: Julia JIT inlines C++ `add_to` across FFI boundary** |
| `whole_loop_in_cpp` | 997,147.0 | **0.997** | Single `ccall` to C++ `accumulate_array` (entire loop in C++) |

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