On Thu, 13 Feb 2025 at 16:46, Ralf Jung post@ralfj.de wrote:
Hi all,
We have to carefully make the distinction here between codegen and ABI.
The arm64 C code in the kernel is built with -mgeneral-regs-only because FP/SIMD registers are not preserved/restored like GPRs, and so they must be used only in carefully controlled circumstances, i.e., in assembler code called under kernel_neon_begin()/kernel_neon_end() [modulo some exceptions related to NEON intrinsics]
This does not impact the ABI, which remains hard-float [this was the only arm64 calling convention that existed until about a year ago]. Any function that takes or returns floats or doubles (or NEON intrinsic types) is simply rejected by the compiler.
That's how C works. It is not how Rust works. Rust does not reject using floats ever. Instead, Rust offers softfloat targets where you can still use floats, but it won't use float registers. Obviously, that needs to use a different ABI. As you said, aarch64 does not have an official softfloat ABI, but LLVM implements a de-facto softfloat ABI if you ask it to generate functions that take/return float types while disabling the relevant target features. (Maybe LLVM should just refuse to generate such code, and then Rust may have ended up with a different design. But now this would all be quite tricky to change.)
Changing this to softfloat for Rust modifies this calling convention, i.e., it will result in floats and doubles being accepted as function parameters and return values, but there is no code in the kernel that actually supports/implements that.
As explained above, f32/f64 were already accepted as function parameters and return values in Rust code before this change. So this patch does not change anything here. (In fact, the ABI used for these functions should be exactly the same before and after this patch.)
OK, so can I summarize the above as
- Rust calling Rust will work fine and happily use float types without using FP/SIMD registers in codegen; - Rust calling C or C calling Rust will not support float or double arguments or return values due to the restrictions imposed by the C compiler.
?
Also, it should be clarified whether using a softfloat ABI permits the compiler to use FP/SIMD registers in codegen. We might still need -Ctarget-feature="-neon" here afaict.
Rust's softfloat targets do not use FP/SIMD registers by default. Ideally these targets allow selectively using FP/SIMD registers within certain functions; for aarch64, this is not properly supported by LLVM and therefore Rust.
I read this as 'this default behavior might change in the future', and so -Ctarget-feature="-neon" should be added even if it is redundant at this point in time.
Ideally, we'd have a target/target-feature combo that makes this more explicit: no FP/SIMD codegen at all, without affecting the ABI, therefore making float/double types in function prototypes illegal. AIUI, this change does something different.
Having targets without float support would be a significant departure from past language decisions in Rust -- that doesn't mean it's impossible, but it would require a non-trivial effort (starting with an RFC to lay down the motivation and design).
Fair enough. The codegen is all that matters, and there are other cases (e.g., spilling) where the compiler may decide to use FP/SIMD registers without any floats or doubles in sight. In fact, there are swaths of non-performance critical floating point code in the AMDGPU driver where it would be useful to have float/double support using softfloat codegen too.