Re: [PATCH] Fix up -fvar-tracking-uninit [PR126801]

On Thu, 20 Aug 2026, Jakub Jelinek wrote:

> Hi!
> 
> The r12-4397 change to get rid of AUTODETECT_VALUE unfortunately broke
> several things related to -fvar-tracking-uninit.
> Before that change, the option defaulted to 0, under some condition
> on darwin only set it to flag_var_tracking which at that point could have
> been whatever user specified or AUTODETECT_VALUE and finally in
> finish_options did:
>   /* If the user specifically requested variable tracking with tagging
>      uninitialized variables, we need to turn on variable tracking.
>      (We already determined above that variable tracking is feasible.)  */
>   if (flag_var_tracking_uninit == 1)
>     flag_var_tracking = 1;
> 
>   if (flag_var_tracking == AUTODETECT_VALUE)
>     flag_var_tracking = optimize >= 1;
> 
>   if (flag_var_tracking_uninit == AUTODETECT_VALUE)
>     flag_var_tracking_uninit = flag_var_tracking;
> i.e.
> 1) -fvar-tracking-uninit option specified on command line implied
>    -fvar-tracking
> 2) var-tracking was defaulted to 1 even for -O1 and above
> 3) in the darwin conditional case if flag_var_tracking_uninit was
>    defaulted to maybe on, it was set to flag_var_tracking
> 
> The r12-4397 change properly handled only 2), by adding
> default_options_table entry for OPT_fvar_tracking.
> 1) got lost in the patch, -fvar-tracking-uninit explicitly on command
>    line no longer implies -fvar-tracking
> 3) was probably assumed to be always initialized to AUTODETECT_VALUE
>    and so replaced with
>   /* One could use EnabledBy, but it would lead to a circular dependency.  */
>   if (!OPTION_SET_P (flag_var_tracking_uninit))
>      flag_var_tracking_uninit = flag_var_tracking;
> which effectively makes -fvar-tracking-uninit default to on whenever
> -fvar-tracking is on (explicitly or implicitly), unless overridden by user.
> 
> Also note that the value of flag_var_tracking_uninit is irrelevant if
> flag_var_tracking is off, the variable is only tested in various spots
> in the var-tracking pass guarded by flag_var_tracking.
> 
> Anyway, the following patch restores the 1) behavior, changes the
> darwin code to just set flag_var_tracking_uninit by default to 1 if
> the condition is met (worst case flag_var_tracking_uninit will be 1
> and flag_var_tracking will be 0, nothing will care), drops the bogus
> defaulting to -fvar-tracking-uninit on all other targets and finally fixes
> something that has been broken even before r12-4397, in my reading on
> darwin the condition would turn on flag_var_tracking_uninit even if
> user used explicit -fno-var-tracking-uninit.
> 
> Unfortunately the DW_OP_GNU_uninit support didn't come with a single
> testcase, neither on the gcc side nor on the gdb side and I haven't figured
> out easily a testcase which I could add to the testsuite for this (sure, I
> could from some *.o that contains it cvise reduce it).
> But one can actually see some DW_OP_GNU_uninit uses in the cc1plus binary,
> before the patch:
> readelf -wo cc1plus 2>&1 | grep DW_OP_GNU_uninit | wc -l
> 2228
> readelf -wo cc1plus 2>&1 | grep DW_OP_GNU_uninit.*DW_OP_GNU_uninit | wc -l
> 208
> after the patch:
> readelf -wo cc1plus 2>&1 | grep DW_OP_GNU_uninit | wc -l
> 0
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK.

Thanks,
Richard.

> 2026-08-20  Jakub Jelinek  <[email protected]>
> 
> 	PR debug/126801
> 	* opts.cc (finish_options): Don't set flag_var_tracking_uninit to
> 	flag_var_tracking by default.  Instead, set flag_var_tracking to
> 	flag_var_tracking_uninit if the latter was explicitly set.
> 	* config/darwin.cc (darwin_override_options): Only set
> 	flag_var_tracking_uninit if it wasn't explicitly set and set it to
> 	1 rather than flag_var_tracking.  Formatting fix.
> 
> --- gcc/opts.cc.jj	2026-08-18 08:47:22.044793076 +0200
> +++ gcc/opts.cc	2026-08-19 12:28:11.107209028 +0200
> @@ -1499,9 +1499,8 @@ finish_options (struct gcc_options *opts
>        opts->x_flag_var_tracking_assignments = 0;
>      }
>  
> -  /* One could use EnabledBy, but it would lead to a circular dependency.  */
> -  if (!opts_set->x_flag_var_tracking_uninit)
> -    opts->x_flag_var_tracking_uninit = opts->x_flag_var_tracking;
> +  if (opts_set->x_flag_var_tracking_uninit && opts->x_flag_var_tracking_uninit)
> +    opts->x_flag_var_tracking = 1;
>  
>    if (!opts_set->x_flag_var_tracking_assignments)
>      opts->x_flag_var_tracking_assignments
> --- gcc/config/darwin.cc.jj	2026-08-17 10:00:21.681856778 +0200
> +++ gcc/config/darwin.cc	2026-08-19 12:24:08.235382015 +0200
> @@ -3576,12 +3576,13 @@ darwin_override_options (void)
>        flag_asynchronous_unwind_tables = 0;
>      }
>  
> -  if (flag_var_tracking_uninit == 0
> +  if (!OPTION_SET_P (flag_var_tracking_uninit)
> +      && flag_var_tracking_uninit == 0
>        && generating_for_darwin_version >= 9
>        && (flag_gtoggle ? (debug_info_level == DINFO_LEVEL_NONE)
> -      : (debug_info_level >= DINFO_LEVEL_NORMAL))
> +	  : (debug_info_level >= DINFO_LEVEL_NORMAL))
>        && dwarf_debuginfo_p ())
> -    flag_var_tracking_uninit = flag_var_tracking;
> +    flag_var_tracking_uninit = 1;
>  
>    if (OPTION_SET_P (flag_pie) && flag_pie)
>      {
> 
> 	Jakub
> 
> 

-- 
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Jochen Jaser, Andrew McDonald, Abhinav Puri; (HRB 36809, AG Nuernberg)