[Bug debug/126801] [13/14/15/16/17 Regression] fvar-tracking-uninit is on by accident via r12-4397

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126801

--- Comment #5 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <[email protected]>:

https://gcc.gnu.org/g:7e784194fe49e45c8fb78168064a9b2627493f09

commit r17-3469-g7e784194fe49e45c8fb78168064a9b2627493f09
Author: Jakub Jelinek <[email protected]>
Date:   Thu Aug 20 12:35:41 2026 +0200

    Fix up -fvar-tracking-uninit [PR126801]

    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

    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.

    Reviewed-by: Richard Biener <[email protected]>