Fix `-Zregparm` for LLVM builtins #145309

winstonallo · 2025-08-12T13:16:14Z

This fixes the issue where -Zregparm=N was not working correctly when calling LLVM intrinsics

By default on x86-32, arguments are passed on the stack. The -Zregparm=N flag allows the first N arguments to be passed in registers instead.

When calling intrinsics like memset, LLVM still passes parameters on the stack, which prevents optimizations like tail calls.

As proposed by @tgross35, I fixed this by setting the NumRegisterParameters LLVM module flag to N when the -Zregparm=N is set.

// compiler/rust_codegen_llvm/src/context.rs#375-382
if let Some(regparm_count) = sess.opts.unstable_opts.regparm {
    llvm::add_module_flag_u32(
        llmod,
        llvm::ModuleFlagMergeBehavior::Error,
        "NumRegisterParameters",
        regparm_count,
    );
}

Here is a before/after compiler explorer.

Here is the final result for the code snippet in the original issue:

entrypoint:
        push    esi
        mov     esi, eax
        mov     eax, ecx
        mov     ecx, esi
        pop     esi
        jmp     memset   ; Tail call parameters in registers

Fixes: #145271

winstonallo · 2025-08-12T13:16:49Z

I think this should fix it already, let me know if I am missing something! Working on the tests now.

rustbot · 2025-08-12T13:32:15Z

r? @petrochenkov

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winstonallo · 2025-08-12T13:33:12Z

I added a test verifying that the memset parameters are passed in registers as discussed.

tests/assembly-llvm/regparm-module-flag.rs

petrochenkov · 2025-08-12T15:43:03Z

r? nikic

tests/assembly-llvm/regparm-module-flag.rs

tgross35

Thank you for doing a quick fix here!

Just a few requests to make sure we're testing what we expect

tests/assembly-llvm/regparm-module-flag.rs

rustbot · 2025-08-12T20:29:29Z

This PR modifies tests/auxiliary/minicore.rs.

cc @jieyouxu

tgross35

Two small nits but this looks good to me once CI passes, please squash

@nikic anything else?

tests/assembly-llvm/regparm-module-flag.rs

winstonallo · 2025-08-13T04:41:43Z

I squashed, let me know if there is anything else I can do!

tgross35 · 2025-08-13T07:11:40Z

Looks great! Thanks for making this happen, and welcome :)

@bors r+

bors · 2025-08-13T07:11:44Z

📌 Commit 02d4666 has been approved by tgross35

It is now in the queue for this repository.

winstonallo · 2025-08-13T07:18:56Z

Looks great! Thanks for making this happen, and welcome :)

Thank you very much for all the help! I'll keep an eye on the issue tracker, but if you come across anything you think would be a good fit, please let me know!

tgross35 · 2025-08-13T07:30:13Z

If you want something tiny then as Miguel mentioned, -Zreg-struct-return could use an assembly test (now that you're an expert at those). It does basically the same thing as regparm, except for return types.

If you're looking for something in particular or just looking to get more involved, https://rust-lang.zulipchat.com/ is a great place to stop by.

winstonallo · 2025-08-13T08:54:01Z

Should I create a separate issue for the asm tests, or just submit a PR when I'm done?

tgross35 · 2025-08-13T08:57:21Z

No need for a new issue!

…ross35 Fix `-Zregparm` for LLVM builtins This fixes the issue where `-Zregparm=N` was not working correctly when calling LLVM intrinsics By default on `x86-32`, arguments are passed on the stack. The `-Zregparm=N` flag allows the first `N` arguments to be passed in registers instead. When calling intrinsics like `memset`, LLVM still passes parameters on the stack, which prevents optimizations like tail calls. As proposed by `@tgross35,` I fixed this by setting the `NumRegisterParameters` LLVM module flag to `N` when the `-Zregparm=N` is set. ```rust // compiler/rust_codegen_llvm/src/context.rs#375-382 if let Some(regparm_count) = sess.opts.unstable_opts.regparm { llvm::add_module_flag_u32( llmod, llvm::ModuleFlagMergeBehavior::Error, "NumRegisterParameters", regparm_count, ); } ``` [Here](https://rust.godbolt.org/z/YMezreo48) is a before/after compiler explorer. Here is the final result for the code snippet in the original issue: ```asm entrypoint: push esi mov esi, eax mov eax, ecx mov ecx, esi pop esi jmp memset ; Tail call parameters in registers ``` Fixes: rust-lang#145271

tests/assembly-llvm/regparm-module-flag.rs

…ross35 Fix `-Zregparm` for LLVM builtins This fixes the issue where `-Zregparm=N` was not working correctly when calling LLVM intrinsics By default on `x86-32`, arguments are passed on the stack. The `-Zregparm=N` flag allows the first `N` arguments to be passed in registers instead. When calling intrinsics like `memset`, LLVM still passes parameters on the stack, which prevents optimizations like tail calls. As proposed by `@tgross35,` I fixed this by setting the `NumRegisterParameters` LLVM module flag to `N` when the `-Zregparm=N` is set. ```rust // compiler/rust_codegen_llvm/src/context.rs#375-382 if let Some(regparm_count) = sess.opts.unstable_opts.regparm { llvm::add_module_flag_u32( llmod, llvm::ModuleFlagMergeBehavior::Error, "NumRegisterParameters", regparm_count, ); } ``` [Here](https://rust.godbolt.org/z/YMezreo48) is a before/after compiler explorer. Here is the final result for the code snippet in the original issue: ```asm entrypoint: push esi mov esi, eax mov eax, ecx mov ecx, esi pop esi jmp memset ; Tail call parameters in registers ``` Fixes: rust-lang#145271

Rollup of 9 pull requests Successful merges: - #122661 (Change the desugaring of `assert!` for better error output) - #138736 (Sanitizers target modificators) - #144955 (search graph: lazily update parent goals) - #145120 (llvm: Accept new LLVM lifetime format) - #145153 (Handle macros with multiple kinds, and improve errors) - #145189 (Weekly `cargo update`) - #145250 (Add regression test for former ICE involving malformed meta items containing interpolated tokens) - #145309 (Fix `-Zregparm` for LLVM builtins) - #145331 (Make std use the edition 2024 prelude) r? `@ghost` `@rustbot` modify labels: rollup

fmease · 2025-08-13T15:27:10Z

#145309 (comment)
@bors r-

set * Enforce the `-Zregparm=N` flag by setting the NumRegisterParameters LLVM module flag * Add assembly tests verifying that the parameters are passed in registers for reparm values 1, 2, and 3, for both LLVM intrinsics and non-builtin functions * Add c_void type to minicore

winstonallo · 2025-08-13T15:39:49Z

Fixed the typo, thank you!

nikic · 2025-08-13T16:47:59Z

@bors r=tgross35 rollup=iffy assembly test

bors · 2025-08-13T16:48:02Z

📌 Commit 04ff144 has been approved by tgross35

It is now in the queue for this repository.

winstonallo marked this pull request as ready for review August 12, 2025 13:32

rustbot assigned petrochenkov Aug 12, 2025

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winstonallo mentioned this pull request Aug 12, 2025

-Zregparm does not work correctly for LLVM builtins #145271

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Rust unstable features needed for the kernel Rust-for-Linux/linux#2

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winstonallo force-pushed the issue-145271-fix branch from 74a664c to 2e24b51 Compare August 13, 2025 04:40

winstonallo force-pushed the issue-145271-fix branch from 2e24b51 to 02d4666 Compare August 13, 2025 06:52

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Rollup of 10 pull requests #145346

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Rollup of 9 pull requests #145351

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winstonallo force-pushed the issue-145271-fix branch from 02d4666 to 04ff144 Compare August 13, 2025 15:38

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Fix -Zregparm for LLVM builtins #145309

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Fix `-Zregparm` for LLVM builtins #145309

Fix `-Zregparm` for LLVM builtins #145309

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