Re: [PATCH] handle sprintf(d, "%s", ...) in gimple-ssa-sprintf.c

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

On Tue, May 2, 2017 at 4:41 PM, Martin Sebor <msebor@gmail.com> wrote:
>>>> FWIW, my fix for bug 79062 is only partial (it gets the pass
>>>> to run but the warnings are still not issued).  I don't quite
>>>> understand what prevents the warning flag(s) from getting set
>>>> when -flto is used.  This seems to be a bigger problem than
>>>> just the sprintf pass not doing something just right.
>>>
>>>
>>> I've never dug deeply in the LTO stuff, but I believe we stream the
>>> compiler
>>> flags, so it could be something there.
>>
>>
>> We do.
>>
>>> Alternately you might be running into a case where in LTO mode we
>>> recreate
>>> base types.  Look for a type equality tester that goes beyond just
>>> testing
>>> pointer equality.
>>>
>>> ie, in LTO I think we'll create a type based on the streamed data, but I
>>> also think we'll create various basic types.  Thus in LTO mode pointer
>>> equality may not be sufficient.
>>
>>
>> We make sure that for most basic types we end up re-using them where
>> possible.
>> char_type_node is an example where that generally doesn't work because
>> it's
>> value depends on a command-line flag.
>
>
> That answers the first part of the question of why the sprintf
> pass wouldn't run (or do anything) with -flto.   With it fixed
> (as in fold-const.c or tree-ssa-strlen.c as you suggested in
> bug 79602) it runs and the optimization does its job, but no
> warnings are issued.  The wan_foo_flags for warnings that are
> enabled implicitly (e.g., by -Wall or -Wextra on the command
> line) are clear.  There seem to be dependencies between warnings
> in c.opt that ignore LTO (as a language), but even with those
> corrected (i.e., with LTO added as a language to -Wformat and
> -Wall) the flags are still clear when LTO runs.  Does that ring
> any bells for you?

You can look at the lto_opts section (it's just a string) and see
that we seem to fail to pass through -Wall (or any warning option
I tried).  This is because

      /* Also drop all options that are handled by the driver as well,
         which includes things like -o and -v or -fhelp for example.
         We do not need those.  The only exception is -foffload option, if we
         write it in offload_lto section.  Also drop all diagnostic options.  */
      if ((cl_options[option->opt_index].flags & (CL_DRIVER|CL_WARNING))
          && (!lto_stream_offload_p || option->opt_index != OPT_foffload_))
        continue;

which means you have to explicitely enable diagnostics you want at
link time at the moment.

If you want to change that you have to do some changes to lto-wrapper.c
as for example only pass through warning options that are set on all
input files (warning options are not kept per function).

Richard.

>
> Thanks
> Martin