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

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?

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