Re: [Qemu-devel] [PATCH v6 18/50] tcg: Reserve temporary index 0

[Emilio G. Cota]

On Mon, Oct 16, 2017 at 10:25:37 -0700, Richard Henderson wrote:
> Since we cast indicies to pointers, reserving 0 allows

s/indicies/indices/

> us to use NULL for unused/dummy instead of (T *)-1.
> 
> Signed-off-by: Richard Henderson <address@hidden>
> ---
(snip)
> @@ -737,7 +737,7 @@ extern bool parallel_cpus;
>  static inline size_t temp_idx(TCGTemp *ts)
>  {
>      ptrdiff_t n = ts - tcg_ctx.temps;
> -    tcg_debug_assert(n >= 0 && n < tcg_ctx.nb_temps);
> +    tcg_debug_assert(n > 0 && n < tcg_ctx.nb_temps);
>      return n;
>  }
>  
> diff --git a/tcg/tcg.c b/tcg/tcg.c
> index 129aecca60..7cf39f7067 100644
> --- a/tcg/tcg.c
> +++ b/tcg/tcg.c
> @@ -333,7 +333,10 @@ void tcg_context_init(TCGContext *s)
>      int *sorted_args;
>  
>      memset(s, 0, sizeof(*s));
> -    s->nb_globals = 0;
> +    /* Reserve temp index 0 so that, with the funny casting that we do,
> +       the first one doesn't look like NULL.  */
> +    s->nb_globals = 1;
> +    s->nb_temps = 1;
>  
>      /* Count total number of arguments and allocate the corresponding
>         space */

I like this change, although operating on the 0th element makes
me uneasy.

For instance, I managed to trigger the above assert by manually calling 
dump_regs
(it is otherwise called before aborting, which is why you missed it.)
This fixes it:

diff --git a/tcg/tcg.c b/tcg/tcg.c
index 7cf39f7..49176e0 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -2034,7 +2034,7 @@ static void dump_regs(TCGContext *s)
     int i;
     char buf[64];
 
-    for(i = 0; i < s->nb_temps; i++) {
+    for (i = 1; i < s->nb_temps; i++) {
         ts = &s->temps[i];
         printf("  %10s: ", tcg_get_arg_str_ptr(s, buf, sizeof(buf), ts));
         switch(ts->val_type) {

Is it worth changing other [0, nb_temps/globals) loops to start from 1?

Thanks,

                E.