Re: bootstrap-debug-lean + flags in producer vs compare

[Alexandre Oliva <aoliva@redhat.com>]

On Oct 30, 2017, Jeff Law <law@redhat.com> wrote:

> I'd support either or both.  Your call.

Thanks.  I think I'll go with both, then.

Ok to install as follows? (edited the comments and added a ChangeLog)


Use compare-debug for bootstrap-debug-lean to ignore != DW_AT_producer

Unlike bootstrap-debug, bootstrap-debug-lean used to pass compare using
the traditional compare command, because it compiled both stage2 and
stage3 with options that used to generate identical output
(-fcompare-debug= in stage2 vs -fcompare-debug in stage3).

Since we started adding relevant command-line flags to DW_AT_producer,
this is no longer the case, and stages 2 and 3 object files that differ
in nothing but the DW_AT_producer strings.


-fcompare-debug is short for -fcompare-debug=-gtoggle, so stage3
compiles twice, once with the normal options, once with toggled -g, to
then compare the temporary final dumps.  When enabled, both compilations
get from the driver an additional -frandom-seed flag (if none is given
explicitly).

-fcompare-debug= is short for -fno-compare-debug, disabling the second
compilation.


The difference between the DW_AT_producer lines are the different
-fcompare-debug flags, and the presence of the -frandom-seed flag in the
stage3 compilation.

It is easy and sensible enough to filter the -fcompare-debug flags out
of the DW_AT_producer string.  This option should never affect the
compilation output, it just determines whether or not to perform an
additional compilation that should produce the same executable output.

However, dropping -frandom-seed is not quite correct, in that it might
have other consequences on the compilation.  So, it makes little sense
to make the effort to drop it when it's implicit; for those comparing
compiler output differences, it might even hint at what causes, and
could fix, difficult to explain differences, namely, explicitly
supplying -frandom-seed options.

They don't seem to matter for compiler bootstraps, though, at least
for now, so we can safely refrain from issuing -frandom-seed (or maybe
we already issue it where needed :-), and then, for
bootstrap-debug-lean, use the compare-debug script, that strips out
debug information before comparing the object files.


for  config/ChangeLog

	* bootstrap-debug-lean.mk (do-compare): Use the
	contrib/compare-debug script.

for  gcc/ChangeLog

	* dwarf2out.c (gen_producer_string): Discard
	OPT_fcompare_debug.

---
 config/bootstrap-debug-lean.mk |    1 +
 gcc/dwarf2out.c                |    1 +
 2 files changed, 2 insertions(+)

diff --git a/config/bootstrap-debug-lean.mk b/config/bootstrap-debug-lean.mk
index e215280b09f3..5f2db80687fa 100644
--- a/config/bootstrap-debug-lean.mk
+++ b/config/bootstrap-debug-lean.mk
@@ -9,3 +9,4 @@
 
 STAGE2_CFLAGS += -fcompare-debug=
 STAGE3_CFLAGS += -fcompare-debug
+do-compare = $(SHELL) $(srcdir)/contrib/compare-debug $$f1 $$f2
diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index b8f4e4888f1b..01d90808fd3b 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -23403,6 +23403,7 @@ gen_producer_string (void)
       case OPT_fltrans_output_list_:
       case OPT_fresolution_:
       case OPT_fdebug_prefix_map_:
+      case OPT_fcompare_debug:
 	/* Ignore these.  */
 	continue;
       default:


-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist|Red Hat Brasil GNU Toolchain Engineer