Reduce usage of AddressSpace::ZERO#12
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xdoardo merged 1 commit intoCHERIoT-Platform:betafrom Sep 2, 2025
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seharris
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Looks good to me! Just make sure you're okay with my comment on the CI config. If you think messing with that later will make too much mess then we might want to reconsider that change.
| pull_master_script: git fetch origin master | ||
| tidy_script: CC="clang" CXX="clang++" ./x test tidy | ||
| check_script: CC="clang" CXX="clang++" ./x check | ||
| ui_test_script: CC="clang" CXX="clang++" ./x test ui |
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This should be fine at this stage, but may need fiddling with once we start adding new targets (the ui suite includes tests that run code, and I think it will default to running for all targets enabled in config.toml).
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Yep, could be especially pesky for CHERIoT. I think it's okay for now, and we can adapt it to our needs later.
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match clang's `va_arg` assembly on arm targets tracking issue: rust-lang/rust#44930 For this example ```rust #![feature(c_variadic)] #[unsafe(no_mangle)] unsafe extern "C" fn variadic(a: f64, mut args: ...) -> f64 { let b = args.arg::<f64>(); let c = args.arg::<f64>(); a + b + c } ``` We currently generate (via llvm): ```asm variadic: sub sp, sp, #12 stmib sp, {r2, r3} vmov d0, r0, r1 add r0, sp, #4 vldr d1, [sp, #4] add r0, r0, #15 bic r0, r0, #7 vadd.f64 d0, d0, d1 add r1, r0, #8 str r1, [sp] vldr d1, [r0] vadd.f64 d0, d0, d1 vmov r0, r1, d0 add sp, sp, #12 bx lr ``` LLVM is not doing a good job. In fact, it's well-known that LLVM's implementation of `va_arg` is kind of bad, and we implement it ourselves (based on clang) for many targets already. For arm, our own `emit_ptr_va_arg` saves 3 instructions. Next, it turns out it's important for LLVM to explicitly start and end the lifetime of the `va_list`. In rust-lang/rust#146059 I already end the lifetime, but when looking at this again, I noticed that it is important to also start it, see https://godbolt.org/z/EGqvKTTsK: failing to explicitly start the lifetime uses an extra register. So, the combination of `emit_ptr_va_arg` with starting/ending the lifetime makes rustc emit exactly the instructions that clang generates:: ```asm variadic: sub sp, sp, #12 stmib sp, {r2, r3} vmov d16, r0, r1 vldr d17, [sp, #4] vadd.f64 d16, d16, d17 vldr d17, [sp, #12] vadd.f64 d16, d16, d17 vmov r0, r1, d16 add sp, sp, #12 bx lr ``` The arguments to `emit_ptr_va_arg` are based on [the clang implementation](https://github.com/llvm/llvm-project/blob/03dc2a41f3d9a500e47b513de5c5008c06860d65/clang/lib/CodeGen/Targets/ARM.cpp#L798-L844). r? ``@workingjubilee`` (I can re-roll if your queue is too full, but you do seem like the right person here) try-job: armhf-gnu
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PR rust-lang/rust#143182 added the ability to set the default address space for targets. This is of course useful for CHERI; however, some pieces of code still use the hard-coded
AddressSpace::ZERO. This PR tries to reduce the usage of that constant in relevant places and when possible (that is, there is a way to get a reference to theTargetDataLayoutfor the target).Most notably, this PR moves
fn type_ptrfrom theBaseTypeCodegenMethodstrait, which has no access to theTargetDataLayoutfor the current target, to theLayoutTypeCodegenMethodstrait.About CI
Piggy-backing on this change there's a change to CI to run `./x test ui`.