Re: [PATCH v4 20b/27] tcg: Vary the allocation size for TCGOp

[Philippe Mathieu-Daudé]

On 18/12/22 22:18, Philippe Mathieu-Daudé wrote:
> From: Richard Henderson <richard.henderson@linaro.org>
>
> We have been allocating a worst case number of arguments
> to support calls.  Instead, allow the size to vary.
> By default leave space for 4 args, to maximize reuse,
> but allow calls to increase the number of args to 32.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> [PMD: Split patch in two]
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>   accel/tcg/plugin-gen.c     | 10 ++++-----
>   include/exec/helper-head.h |  2 --
>   include/tcg/tcg.h          | 46 +++++++++++++-------------------------
>   tcg/tcg.c                  | 35 +++++++++++++++++++++--------
>   4 files changed, 47 insertions(+), 46 deletions(-)


> diff --git a/tcg/tcg.c b/tcg/tcg.c
> index 3f172cb1d6..ccbe947222 100644
> --- a/tcg/tcg.c
> +++ b/tcg/tcg.c
> @@ -1513,7 +1513,12 @@ void tcg_gen_callN(void *func, TCGTemp *ret, int nargs, 
> TCGTemp **args)
>           }
>       }
>   
> -    max_args = ARRAY_SIZE(op->args);
> +    /*
> +     * A Call op needs up to 4 + 2N parameters on 32-bit archs,
> +     * and up to 4 + N parameters on 64-bit archs
> +     * (N = number of input arguments + output arguments).
> +     */
> +    max_args = (64 / TCG_TARGET_REG_BITS) * nargs + 4;
>       op = tcg_emit_op(INDEX_op_call, max_args);
>   
>       pi = 0;
> @@ -2298,19 +2303,31 @@ void tcg_remove_ops_after(TCGOp *op)
>   static TCGOp *tcg_op_alloc(TCGOpcode opc, unsigned nargs)
>   {
>       TCGContext *s = tcg_ctx;
> -    TCGOp *op;
> +    TCGOp *op = NULL;
>   
> -    assert(nargs < ARRAY_SIZE(op->args));
> -    if (likely(QTAILQ_EMPTY(&s->free_ops))) {
> -        op = tcg_malloc(sizeof(TCGOp));
> -    } else {
> -        op = QTAILQ_FIRST(&s->free_ops);
> -        QTAILQ_REMOVE(&s->free_ops, op, link);
> +    if (unlikely(!QTAILQ_EMPTY(&s->free_ops))) {
> +        QTAILQ_FOREACH(op, &s->free_ops, link) {
> +            if (nargs <= op->nargs) {
> +                QTAILQ_REMOVE(&s->free_ops, op, link);
> +                nargs = op->nargs;
> +                goto found;
> +            }
> +        }
>       }
> +
> +    /* Most opcodes have 3 or 4 operands: reduce fragmentation. */
> +    nargs = MAX(4, nargs);
> +    op = tcg_malloc(sizeof(TCGOp) + sizeof(TCGArg) * nargs);
> +
> + found:
>       memset(op, 0, offsetof(TCGOp, link));
>       op->opc = opc;
> -    s->nb_ops++;
> +    op->nargs = nargs;

We can move this assignation before the 'found' label.

>   
> +    /* Check for bitfield overflow. */
> +    tcg_debug_assert(op->nargs == nargs);
> +
> +    s->nb_ops++;
>       return op;
>   }
>   

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>