Re: [PATCH] loongarch: Only select HAVE_OBJTOOL and allow ORC unwinder if the inline assembler supports R_LARCH_{32,64}_PCREL

[Xi Ruoyao <xry111@xry111.site>]

On Mon, 2024-06-17 at 21:11 +0800, Huacai Chen wrote:



> > select HAVE_OBJTOOL if AS_HAS_EXPLICIT_RELOCS && AS_HAS_THIN_ADD_SUB && !CC_IS_CLANG && !RUST
> Maybe we needn't consider RUST here?

Rustc does use $fp that objtool cannot handle as at now.  It can be
demonstrated with an over-aligned type, similar to Clang:

$ cat t.rs
#[repr(C, align(64))]
struct X(i32);

extern { fn f(x: &X) -> i64; }

#[no_mangle]
fn g() -> i64 {
    let x = X(114514);
    unsafe {f(&x)}
}
$ rustc t.rs --emit=asm --crate-type=staticlib -O
$ grep fp t.s
	st.d	$fp, $sp, 112
	addi.d	$fp, $sp, 128
	addi.d	$sp, $fp, -128
	ld.d	$fp, $sp, 112

The kernel uses rust-bindgen to generate some .rs file from C headers. 
And __attribute__((aligned(x))) is directly translated to
repr(align(x)).  As __attribute__((aligned(x))) is very common in the
kernel I expect objtool will fail to handle some object code from rustc.

> And can we think AS_HAS_THIN_ADD_SUB always imply
> AS_HAS_EXPLICIT_RELOCS here?

Maybe, AFAIK there's no assembler using thin add-sub but not explicit
relocs.

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University