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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
To: Xi Ruoyao <xry111@xry111.site>, 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>,
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 <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>
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From: Jinyang He <hejinyang@loongson.cn>
Message-ID: <2bf11cd2-8449-acda-f5ad-659c38cb018e@loongson.cn>
Date: Thu, 6 Jun 2024 10:10:05 +0800
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On 2024-06-06 03:05, Xi Ruoyao wrote:

> 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
>> clang...
>>
>> .global test
>> .type test,@function
>> test:
>>
>> addi.d  $sp,$sp,-448
>> st.d    $ra,$sp,440
>> st.d    $fp,$sp,432
>> addi.d  $fp,$sp,448
>>
>> # do something  <- 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 = { .buf = "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 ?
>>           ld.d    $ra, $sp, 8                     # 8-byte Folded Reload
> /* snip */
>
>> Maybe Clang have anything wrong?
> I don't think we must restore regs based on $fp just because CFA is
> based on $fp.

You are right.


> The .cfi directives only provides *one* possible way to
> restore the regs.
What I just confused is that there is no ".cfi_*"
in the eplogue by clang, which may cause wrong backtrace if gdb set

breakpoint there and backtrace. (But this is out of this topic.)


> 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.
Yes.
The yestorday attachment has something wrong.
Enmmmm.... (It seems I'm careless.) diff is,

                 switch (op->src.type) {
                 case OP_SRC_ADD:
                         if (op->dest.reg == CFI_SP && op->src.reg == 
CFI_SP) {
-                               /* addi.d sp, sp, imm */
-                               cfi->stack_size -= op->src.offset;
-                               if (cfa->base == CFI_SP)
                                 /* addi.d sp, sp, imm */
                                 cfi->stack_size -= op->src.offset;
                                 if (cfa->base == CFI_SP)

Or you can get the tmp fix by [1].

[1]: 
https://github.com/MQ-mengqing/linux/commit/c1f7df3eb2a2bb7a1c10c2aa6e0e3d585b457352