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match clang's `va_arg` assembly on arm targets tracking issue: rust-lang#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#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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