Re: [PATCH] LoongArch: BPF: Avoid declare variables in switch-case

[Tiezhu Yang]

On 10/14/2022 09:13 AM, Huacai Chen wrote:
> Hi, Xuerui,
>
> On Fri, Oct 14, 2022 at 12:43 AM WANG Xuerui <kernel@xxxxxxxxxx> wrote:
> > On 10/13/22 23:40, Huacai Chen wrote:
> > > Not all compilers support declare variables in switch-case, so move
> > > declarations to the beginning of a function. Otherwise we may get such
> > > build errors:

...

> > >   static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool extra_pass)
> > >   {
> > > -     const bool is32 = BPF_CLASS(insn->code) == BPF_ALU ||
> > > -                       BPF_CLASS(insn->code) == BPF_JMP32;
> > > +     u8 t0 = -1;
> > Here "t0" seems to be a versatile temp value, while the "t1" below is
> > the actual GPR $t1. What about renaming "t0" to something like "tmp" to
> > reduce confusion? I believe due to things like "t0 = LOONGARCH_GPR_ZERO"
> > the "t0" is 100% not an actual mapping to $t0.
> I rename t7 to t0 because there is no t3-t6, t7 looks very strange.
> But from emit_cond_jmp() the 3rd and 4th parameters have no difference
> so I suppose t0 is just OK, then whether rename it to tmp depends on
> Tiezhu's opinion.

Use "tmp" seems better due to it is a temp value.

> > > +     u64 func_addr;
> > > +     bool func_addr_fixed;
> > > +     int i = insn - ctx->prog->insnsi;
> > > +     int ret, jmp_offset;
> > >       const u8 code = insn->code;
> > >       const u8 cond = BPF_OP(code);
> > >       const u8 t1 = LOONGARCH_GPR_T1;
> > > @@ -400,8 +402,8 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
> > >       const u8 dst = regmap[insn->dst_reg];
> > >       const s16 off = insn->off;
> > >       const s32 imm = insn->imm;
> > > -     int jmp_offset;
> > > -     int i = insn - ctx->prog->insnsi;
> > > +     const u64 imm64 = (u64)(insn + 1)->imm << 32 | (u32)insn->imm;
> > > +     const bool is32 = BPF_CLASS(insn->code) == BPF_ALU || BPF_CLASS(insn->code) == BPF_JMP32;
> > Please consider reducing diff damage and not touching parts not directly
> > affected by this change. For example this "is32" declaration and
> > initialization was moved although not related to this change.

It looks reasonable, one change per patch is better.

> I think defining variables from simple to complex and grouping them
> can make life easier. :)

No strong opinion on this, I am OK either way.

Thanks,
Tiezhu