Re: [PATCH v1 16/19] target/arm: Relax ordered/atomic alignment checks for LSE2

[Richard Henderson]

On 2/23/23 06:49, Peter Maydell wrote:
> On Thu, 16 Feb 2023 at 03:09, Richard Henderson
> <richard.henderson@linaro.org> wrote:
> > FEAT_LSE2 only requires that atomic operations not cross a
> > 16-byte boundary.  Ordered operations may be completely
> > unaligned if SCTLR.nAA is set.
> >
> > Because this alignment check is so special, do it by hand.
> > Make sure not to keep TCG temps live across the branch.
> >
> > Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>
>
> > +static void check_lse2_align(DisasContext *s, int rn, int imm,
> > +                             bool is_write, MemOp mop)
> > +{
> > +    TCGv_i32 tmp;
> > +    TCGv_i64 addr;
> > +    TCGLabel *over_label;
> > +    MMUAccessType type;
> > +    int mmu_idx;
> > +
> > +    tmp = tcg_temp_new_i32();
> > +    tcg_gen_extrl_i64_i32(tmp, cpu_reg_sp(s, rn));
> > +    tcg_gen_addi_i32(tmp, tmp, imm & 15);
> > +    tcg_gen_andi_i32(tmp, tmp, 15);
> > +    tcg_gen_addi_i32(tmp, tmp, memop_size(mop));
> > +
> > +    over_label = gen_new_label();
> > +    tcg_gen_brcond_i32(TCG_COND_LEU, tmp, tcg_constant_i32(16), over_label);
>
> This brcond ends the basic block and destroys the content
> of TCG temporaries, which is bad because some of the
> callsites have set some of those up before calling this
> function (eg gen_compare_and_swap() has called cpu_reg()
> which might have created and initialized a temporary
> for xZR).

xzr uses tcg_constant_i64(), which has no lifetime issues.

> Using a brcond anywhere other than directly in a top level
> function where you can see it and work around this awkward
> property seems rather fragile.
>
> (Ideally there would be a variant of brcond that didn't
> trash temporaries, because almost all the time that is
> an annoying hazard rather than a useful property.)

I've cc'd you on a patch set that fixes all the temporary lifetime stuff.

v1: 
https://patchew.org/QEMU/20230130205935.1157347-1-richard.henderson@linaro.org/
v2: 
20230222232715.15034-1-richard.henderson@linaro.org/">https://patchew.org/QEMU/20230222232715.15034-1-richard.henderson@linaro.org/


r~