Re: [Qemu-devel] [PATCH v3 20/20] tcg-arm: Convert to CONFIG_QEMU_LDST_OPTIMIZATION

[Peter Maydell <peter.maydell@linaro.org>]

On 28 March 2013 15:33, Richard Henderson <rth@twiddle.net> wrote:
> +static inline void tcg_out_nop(TCGContext *s)
> +{
> +    if (use_armv7_instructions) {
> +        /* Architected nop introduced in v6k.  */
> +        /* ??? This is an MSR (imm) 0,0,0 insn.  Anyone know if this
> +           also Just So Happened to do nothing on pre-v6k so that we
> +           don't need to conditionalize it?  */

Well, it corresponds to a "do nothing" kind of MSR (because
all the field_mask bits are zero) but I bet it's more expensive
than mov r0, r0 on at least some pre-v6k cores.

> +        tcg_out32(s, 0xe320f000);
> +    } else {
> +        /* Prior to that the assembler uses mov r0, r0.  Unlike the nop
> +           above, this is guaranteed to consume execution resources.  */

Guaranteed by who? Catching this case in the decoder and treating it
exactly like NOP is a perfectly legal implementation.
(For that matter there's nothing restricting an implementation of
the architectural NOP from tying up every execution resource on
the core for 500 cycles.)

> +        tcg_out_dat_reg(s, COND_AL, ARITH_MOV, 0, 0, 0, SHIFT_IMM_LSL(0));
> +    }

-- PMM