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

[Jinyang He]

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