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

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Date: Fri, 07 Jun 2024 16:29:09 +0800
From: Xi Ruoyao <xry111@...111.site>
To: Jinyang He <hejinyang@...ngson.cn>, Nathan Chancellor
 <nathan@...nel.org>,  Peter Zijlstra <peterz@...radead.org>
Cc: Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>, 
 Tiezhu Yang <yangtiezhu@...ngson.cn>, Nick Desaulniers
 <ndesaulniers@...gle.com>, Bill Wendling <morbo@...gle.com>, Justin Stitt
 <justinstitt@...gle.com>, Youling Tang <tangyouling@...inos.cn>,
 loongarch@...ts.linux.dev,  linux-kernel@...r.kernel.org,
 llvm@...ts.linux.dev, mengqinggang@...ngson.cn,  cailulu@...ngson.cn,
 wanglei@...ngson.cn, luweining@...ngson.cn, Yujie Liu
 <yujie.liu@...el.com>, Heng Qi <hengqi@...ux.alibaba.com>, Tejun Heo
 <tj@...nel.org>
Subject: Re: [PATCH] loongarch: Only select HAVE_OBJTOOL and allow ORC
 unwinder if the inline assembler supports R_LARCH_{32,64}_PCREL

On Fri, 2024-06-07 at 15:14 +0800, Jinyang He wrote:
> >      Note: on RISC-V and LoongArch, the stack slot for the previous frame
> >      pointer is stored at fp[-2] instead of fp[0]. See [Consider
> >      standardising which stack slot fp points
> >      to](https://github.com/riscv-non-isa/riscv-elf-psabi-doc/issues/18)
> >      for the RISC-V discussion.
> 
> In most cases the $fp is saved at cfa-16. But for va args, something
> becomes different at LoongArch (I do not know the case of riscv), the
> $fp isn't saved at cfa-16. (e.g. printk?)

Oops indeed.  Even with a very simple case:

int sum(int a, int b) {
	return a + b;
}

with -fno-omit-frame-pointer we get:

sum:
	addi.d	$r3,$r3,-16
	st.d	$r22,$r3,8
	addi.d	$r22,$r3,16
	ld.d	$r22,$r3,8
	add.w	$r4,$r4,$r5
	addi.d	$r3,$r3,16
	jr	$r1

So for leaf functions (where we don't save $ra) $fp is saved at cfa-8.

> I feel that the update_cfi_state should be arch specific. I believe
> that some logic can be reused, but each arch may have its own logic.

I agree it now.

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University