On 05/03/2017 04:20 AM, Richard Biener wrote:
On Tue, May 2, 2017 at 4:41 PM, Martin Sebor <mse...@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).
Great, thanks for the pointer! I might tackle that at some point.
Jeff, given that we now understand why the warnings are suppressed
(i.e., the root cause of the rest of bug 79062), are you okay with
treating that independently of this PR (bug 77671) and committing
this patch (including the two-line fix to enable the sprintf return
value optimization with LTO)?
Martin
