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Parse and set Mach-O section type and attributes - #1648

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Parse and set Mach-O section type and attributes#1648
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@madsmtm madsmtm commented Apr 19, 2026 •
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Mach-O sections have a 32-bit "flags" parameter, which contains 8 bits for a "section type", and 24 bits for "attributes". These can be controlled with #[link_section = ...] like so:

// Sets the section type to S_MOD_INIT_FUNC_POINTERS
#[unsafe(link_section = "__DATA,__mod_init_func,mod_init_funcs")]
static CONSTRUCTOR: extern "C" fn() = ...;

// Sets the section type to S_CSTRING_LITERALS
#[link_section = "__TEXT,__cstring,cstring_literals"]
static NAME_DATA: [u8; 4] = b"foo\0";

#[unsafe(link_section = "__TEXT,__text,regular,pure_instructions+no_dead_strip")]
static LINKER_USED_INSTRUCTIONS: [u8; 10] = ...;

The default section type is "regular" (S_REGULAR).

This PR parses the section type names and attribute names understood by LLVM, and convert them to the corresponding Mach-O flags.

Fixes #1588.
Fixes bevyengine/bevy#20785.
Depends on bytecodealliance/wasmtime#13137.

Related to rust-lang/rust#155065, we could also put this parsing logic higher up, to rely less on LLVM / get diagnostics with spans for this on all backends?

@madsmtm madsmtm added the O-macos Operating system: MacOS label Apr 19, 2026
@madsmtm madsmtm mentioned this pull request Apr 19, 2026
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madsmtm commented Apr 19, 2026
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Comment thread src/constant.rs Outdated
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JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request Apr 20, 2026
@JonathanBrouwer
Rollup merge of rust-lang#155517 - madsmtm:test-macho-link-section-at…
356fdf0 
…trs, r=bjorn3

Add a test for Mach-O `#[link_section]` API inherited from LLVM

The format of the `#[link_section]` attribute is under-documented, but on Mach-O, I think it's roughly the following BNF:
```
LinkSection -> Segment `,` Section (`,` (SectionType (`,` (SectionAttributes)?)?)?)?
Segment -> <0 to 16 bytes>
Section -> <0 to 16 bytes>
SectionType -> `regular` | `zerofill` | `cstring_literals` | `4byte_literals` | `8byte_literals` | `literal_pointers` | `non_lazy_symbol_pointers` | `lazy_symbol_pointers` | `symbol_stubs` | `mod_init_funcs` | `mod_term_funcs` | `coalesced` | `interposing` | `16byte_literals` | `thread_local_regular` | `thread_local_zerofill` | `thread_local_variables` | `thread_local_variable_pointers` | `thread_local_init_function_pointers`
SectionAttributes -> SectionAttribute (`+` SectionAttribute)*
SectionAttribute -> `pure_instructions` | `no_toc` | `strip_static_syms` | `no_dead_strip` | `live_support`, `self_modifying_code` | `debug`
```

This PR adds a small test for a little part of this.

Once rust-lang#154429 is resolved, this should make it possible to test rust-lang/rustc_codegen_cranelift#1648 end-to-end.

r? bjorn3
rust-timer added a commit to rust-lang/rust that referenced this pull request Apr 20, 2026
@rust-timer
Unrolled build for #155517
cc2fe00 
Rollup merge of #155517 - madsmtm:test-macho-link-section-attrs, r=bjorn3

Add a test for Mach-O `#[link_section]` API inherited from LLVM

The format of the `#[link_section]` attribute is under-documented, but on Mach-O, I think it's roughly the following BNF:
```
LinkSection -> Segment `,` Section (`,` (SectionType (`,` (SectionAttributes)?)?)?)?
Segment -> <0 to 16 bytes>
Section -> <0 to 16 bytes>
SectionType -> `regular` | `zerofill` | `cstring_literals` | `4byte_literals` | `8byte_literals` | `literal_pointers` | `non_lazy_symbol_pointers` | `lazy_symbol_pointers` | `symbol_stubs` | `mod_init_funcs` | `mod_term_funcs` | `coalesced` | `interposing` | `16byte_literals` | `thread_local_regular` | `thread_local_zerofill` | `thread_local_variables` | `thread_local_variable_pointers` | `thread_local_init_function_pointers`
SectionAttributes -> SectionAttribute (`+` SectionAttribute)*
SectionAttribute -> `pure_instructions` | `no_toc` | `strip_static_syms` | `no_dead_strip` | `live_support`, `self_modifying_code` | `debug`
```

This PR adds a small test for a little part of this.

Once #154429 is resolved, this should make it possible to test rust-lang/rustc_codegen_cranelift#1648 end-to-end.

r? bjorn3
github-actions Bot pushed a commit to rust-lang/rustc-dev-guide that referenced this pull request Apr 21, 2026
@JonathanBrouwer
Rollup merge of #155517 - madsmtm:test-macho-link-section-attrs, r=bj…
a444181 
…orn3

Add a test for Mach-O `#[link_section]` API inherited from LLVM

The format of the `#[link_section]` attribute is under-documented, but on Mach-O, I think it's roughly the following BNF:
```
LinkSection -> Segment `,` Section (`,` (SectionType (`,` (SectionAttributes)?)?)?)?
Segment -> <0 to 16 bytes>
Section -> <0 to 16 bytes>
SectionType -> `regular` | `zerofill` | `cstring_literals` | `4byte_literals` | `8byte_literals` | `literal_pointers` | `non_lazy_symbol_pointers` | `lazy_symbol_pointers` | `symbol_stubs` | `mod_init_funcs` | `mod_term_funcs` | `coalesced` | `interposing` | `16byte_literals` | `thread_local_regular` | `thread_local_zerofill` | `thread_local_variables` | `thread_local_variable_pointers` | `thread_local_init_function_pointers`
SectionAttributes -> SectionAttribute (`+` SectionAttribute)*
SectionAttribute -> `pure_instructions` | `no_toc` | `strip_static_syms` | `no_dead_strip` | `live_support`, `self_modifying_code` | `debug`
```

This PR adds a small test for a little part of this.

Once rust-lang/rust#154429 is resolved, this should make it possible to test rust-lang/rustc_codegen_cranelift#1648 end-to-end.

r? bjorn3
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bjorn3
bjorn3 reviewed May 24, 2026
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Comment thread src/constant.rs Outdated
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bjorn3 commented May 24, 2026

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I updated Cranelift earlier today, so it should work after a rebase.

@bjorn3

bjorn3 commented Jun 24, 2026

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@madsmtm did you intend to work on this further or do you want me to pick it up?

@madsmtm madsmtm mentioned this pull request Jul 13, 2026
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madsmtm force-pushed the madsmtm/mach-o-section-flags branch from 2783559 to 9230b1a Compare July 13, 2026 14:03
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madsmtm commented Jul 13, 2026

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I've opened bytecodealliance/wasmtime#13869 instead, since on further reflection I think it makes more sense to have the parsing in cranelift-object (it's useful for other consumers of cranelift-object). I've updated this PR to what it'd look like after that.

@bjorn3

bjorn3 commented Aug 6, 2026

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Looks like this change will have to be merged at the same time as updating Cranelift. I've added it to the update_cranelift branch and added you as co-author. The branch will be merged in about two weeks when the next Cranelift version is released.

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madsmtm commented Aug 6, 2026

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Coolio!

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