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Subject: Re: [PATCH] loongarch: Only select HAVE_OBJTOOL and allow ORC
 unwinder if the inline assembler supports R_LARCH_{32,64}_PCREL
From: Xi Ruoyao <xry111@xry111.site>
To: Jinyang He <hejinyang@loongson.cn>, Nathan Chancellor
 <nathan@kernel.org>,  Peter Zijlstra <peterz@infradead.org>
Cc: Huacai Chen <chenhuacai@kernel.org>, WANG Xuerui <kernel@xen0n.name>, 
 Tiezhu Yang <yangtiezhu@loongson.cn>, Nick Desaulniers
 <ndesaulniers@google.com>, Bill Wendling <morbo@google.com>, Justin Stitt
 <justinstitt@google.com>, Youling Tang <tangyouling@kylinos.cn>,
 loongarch@lists.linux.dev,  linux-kernel@vger.kernel.org,
 llvm@lists.linux.dev, mengqinggang@loongson.cn,  cailulu@loongson.cn,
 wanglei@loongson.cn, luweining@loongson.cn, Yujie Liu
 <yujie.liu@intel.com>, Heng Qi <hengqi@linux.alibaba.com>, Tejun Heo
 <tj@kernel.org>
Date: Thu, 06 Jun 2024 03:05:41 +0800
In-Reply-To: <82b7e6ea-c2cb-6364-ebe9-bff928028408@loongson.cn>
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On Wed, 2024-06-05 at 23:47 +0800, Jinyang He wrote:
> In our 419 repo this func has been renamed arch_update_insn_state (, now =
it
> should be arch_update_cfi_state) and give some actions to deal with the
> frame pointer case. If we support it we may deal with some case but for=
=20
> clang...
>=20
> .global test
> .type test,@function
> test:
>=20
> addi.d=C2=A0 $sp,$sp,-448
> st.d=C2=A0=C2=A0=C2=A0 $ra,$sp,440
> st.d=C2=A0=C2=A0=C2=A0 $fp,$sp,432
> addi.d=C2=A0 $fp,$sp,448
>=20
> # do something=C2=A0 <- not change $sp

This is simplified.  In the real code $sp is changed, something like:

bstrins.d $sp, $zero, 5, 0

$fp is needed to allow restoring $sp after realigning $sp for some local
over-aligned variables, as demonstrated by this example:

struct x { _Alignas(64) char buf[128]; };

void f(struct x *p);
void g()
{
	struct x x =3D { .buf =3D "1145141919810" };
	f(&x);
}

GCC does not align $sp, it produces the aligned address for x from an
unaligned $sp instead:

addi.d $a0, $sp, 63
srli.d $a0, $a0, 6
slli.d $a0, $a0, 6

thus there's no need to use $fp.

/* snip */

> <- restore regs from non-cfa ?
> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 ld.d=C2=A0=C2=A0=C2=A0 $=
ra, $sp, 8=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=
=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 # 8-byte Folded R=
eload=C2=A0=20

/* snip */

> Maybe Clang have anything wrong?

I don't think we must restore regs based on $fp just because CFA is
based on $fp.  The .cfi directives only provides *one* possible way to
restore the regs.  This way is convenient to the unwinder, but not
necessarily convenient to the CPU thus the real instruction sequence can
use a different way.  They only need to be "equivalent", not necessarily
"exactly same."

Also note that .cfi_* directives are completely irrelevant for objtool.
THe objtool synthesizes the ORC unwind info from the machine
instructions, not the DWARF unwind info coded with .cfi_*.

The entire point of ORC is avoiding DWARF.  It's even named ORC because
ORC and DWARF are sworn enemies in some tales.

--=20
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University