eddyz87 wrote:
@AaronBallman,
> But a mismatch can still potentially result in a miscompilation, right? e.g.,
> you have a function in a header file with the `bfp_fastcall` attribute on it.
> The definition of the function is compiled into a library with Clang 16 and
> ignores the unknown attribute, so the callee will clobber registers. But the
> declaration is used by an application compiled with Clang 20 and assumes the
> attribute means the callee won't clobber registers, so doesn't generate the
> save/restore code. When the call resolves, the registers are clobbered
> unexpectedly, right? But if the attribute was part of the function type, this
> program presumably would not link due to type mismatch.
Not quite, the feature is a bit unusual and pursues several goals:
- allow bpf programs run on new kernels (those that support `bpf_fastcall`) to
gain some performance by clobbering less registers;
- allow same code to run unmodified on old kernels (those that do not support
`bpf_fastcall`);
- allow `bpf_fastcall` only for "builtin" functions, implemented by kernel, not
written in BPF;
- allow new kernels to provide different sets of `bpf_fastcall` builtin
functions depending on configuration and architecture;
- do not complicate kernel side implementation much (read as: do not do
register allocation in kernel jit).
Suppose there is a function `void foo(void)` marked with `bpf_fastcall` and the
following IR:
%v = ... define %v somehow ...
call void @foo
... use %v somehow ...
In such a case:
- only `@foo` prototype is visible to BPF program, not it's body;
- for the BPF backend the attribute serves as a hint for register allocation,
the BPF code like below might be generated:
```
r1 = ... definition of %v ...
fp[-32] = r1
call foo
r1 = fp[-32]
... use r1 as %v ...
```
Kernels supporting `bpf_fastcall` for function `foo` would remove `r1` spill
and fill, kernels not supporting `bpf_fastcall` would just work as before.
https://github.com/llvm/llvm-project/pull/101228
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