options: Clarify 'Init' option property usage for streaming optimization (was: [PATCH] options, lto: Optimize streaming of optimization nodes)

Hi!

On 2020-11-18T10:36:35+0100, Jakub Jelinek via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> Honza mentioned that especially for the new param machinery, most of
> streamed values are probably going to be the default values.  Perhaps
> somehow we could stream them more effectively.
>
> This patch implements it and brings further savings, the size
> goes down from 574 bytes to 273 bytes, i.e. less than half.

Neat idea about the XOR-encoding!

> Not trying to handle enums because the code doesn't know if (enum ...) 10
> is even valid, similarly non-parameters because those really generally
> don't have large initializers, and params without Init (those are 0
> initialized and thus don't need to be handled).

Given this only looks at 'Init', I understand this may actually
mis-optimize in the case that there is an 'Init' present, but that value
is then "by default" overridden via 'LANG_HOOKS_INIT_OPTIONS_STRUCT', or
'TARGET_OPTION_OVERRIDE', for example.  (I'm however not claiming that's
an actual problem to be worried about -- just for my understanding.)

Either way:

>       * optc-save-gen.awk: Initialize var_opt_init.  In
>       cl_optimization_stream_out for params with default values larger than
>       10, xor the default value with the actual parameter value.  In
>       cl_optimization_stream_in repeat the above xor.

> --- gcc/optc-save-gen.awk.jj  2020-09-14 10:51:54.493740942 +0200
> +++ gcc/optc-save-gen.awk     2020-09-14 11:39:39.441602594 +0200
> @@ -1186,6 +1186,7 @@ for (i = 0; i < n_opts; i++) {
>               var_opt_val_type[n_opt_val] = otype;
>               var_opt_val[n_opt_val] = "x_" name;
>               var_opt_hash[n_opt_val] = flag_set_p("Optimization", flags[i]);
> +             var_opt_init[n_opt_val] = opt_args("Init", flags[i]);
>               n_opt_val++;
>       }
>  }

Reading through the options handling, I was confused why 'Init' needs to
be handled in 'gcc/optc-save-gen.awk'.  To help the next person -- like,
me in a few weeks ;-) -- wondering about this, OK to push the attached
"options: Clarify 'Init' option property usage for streaming
optimization"?


Grüße
 Thomas


> @@ -1257,10 +1258,21 @@ for (i = 0; i < n_opt_val; i++) {
>       otype = var_opt_val_type[i];
>       if (otype ~ "^const char \\**$")
>               print "  bp_pack_string (ob, bp, ptr->" name", true);";
> -     else if (otype ~ "^unsigned")
> -             print "  bp_pack_var_len_unsigned (bp, ptr->" name");";
> -     else
> -             print "  bp_pack_var_len_int (bp, ptr->" name");";
> +     else {
> +             if (otype ~ "^unsigned") {
> +                     sgn = "unsigned";
> +             } else {
> +                     sgn = "int";
> +             }
> +             if (name ~ "^x_param" && !(otype ~ "^enum ") && var_opt_init[i]) {
> +                     print "  if (" var_opt_init[i] " > (" var_opt_val_type[i] ") 10)";
> +                     print "    bp_pack_var_len_" sgn " (bp, ptr->" name" ^ " var_opt_init[i] ");";
> +                     print "  else";
> +                     print "    bp_pack_var_len_" sgn " (bp, ptr->" name");";
> +             } else {
> +                     print "  bp_pack_var_len_" sgn " (bp, ptr->" name");";
> +             }
> +     }
>  }
>  print "  for (size_t i = 0; i < sizeof (ptr->explicit_mask) / sizeof (ptr->explicit_mask[0]); i++)";
>  print "    bp_pack_value (bp, ptr->explicit_mask[i], 64);";
> @@ -1281,10 +1293,18 @@ for (i = 0; i < n_opt_val; i++) {
>               print "  if (ptr->" name")";
>               print "    ptr->" name" = xstrdup (ptr->" name");";
>       }
> -     else if (otype ~ "^unsigned")
> -             print "  ptr->" name" = (" var_opt_val_type[i] ") bp_unpack_var_len_unsigned (bp);";
> -     else
> -             print "  ptr->" name" = (" var_opt_val_type[i] ") bp_unpack_var_len_int (bp);";
> +     else {
> +             if (otype ~ "^unsigned") {
> +                     sgn = "unsigned";
> +             } else {
> +                     sgn = "int";
> +             }
> +             print "  ptr->" name" = (" var_opt_val_type[i] ") bp_unpack_var_len_" sgn " (bp);";
> +             if (name ~ "^x_param" && !(otype ~ "^enum ") && var_opt_init[i]) {
> +                     print "  if (" var_opt_init[i] " > (" var_opt_val_type[i] ") 10)";
> +                     print "    ptr->" name" ^= " var_opt_init[i] ";";
> +             }
> +     }
>  }
>  print "  for (size_t i = 0; i < sizeof (ptr->explicit_mask) / sizeof (ptr->explicit_mask[0]); i++)";
>  print "    ptr->explicit_mask[i] = bp_unpack_value (bp, 64);";


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