Re: [PATCH] Avoid generating unused labels in genmatch.

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Mar 16, 2022 at 1:31 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This patch is the second of two changes to genmatch that don't affect
> the executable code, but reduce the amount of debugging information
> generated in stage3 of a build, but adhering more closely to GNU style
> guidelines.
>
> This patch avoids generating "next_after_fail1:;" label statements
> in genmatch, if this label is unused/never referenced as the target
> of a goto.  Because jumps to these labels are always forwards, we
> know at the point the label is emitted whether it is used or not.
> Because a debugger may set a breakpoint these labels, this increase
> the size of g{imple,eneric}-match.o in a stage3 build.  To save a
> little extra space, I also shorten the label to "Lfail1:;" and only
> increment the counter when necessary.  This reduces the size of
> gimple-match.o by 58K on x86_64-pc-linux-gnu.
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check with no new failures.  Ok for mainline?

I wonder if we could make this nicer by abstracting sth like

class fail_label
{
   fail_label (unsigned num);
   emit_goto () { fprintf (..., "goto L%u", num); used = true; }
   emit_label () { fprintf ("L%u:;\n", num); }
  unsigned num;
  bool used;
};

or some better abstraction capturing all of the global state.

>
>
> 2022-03-16  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         * gcc/genmatch.cc (fail_label_used): New global variable.
>         (expr::gen_transform): Set fail_label_used whenever a goto
>         FAIL_LABEL is generated.
>         (dt_simplify::gen_1): Clear fail_label_used when generating
>         a new (provisional) fail_label.  Set fail_label used whenever
>         a goto fail_label is generated.  Avoid emitting fail_label:
>         if fail_label_used is false, instead decrement fail_label_cnt.
>
>
> Thanks in advance,
> Roger
> --
>