Re: Fwd: Bug#1050663: dar option --on-fly-isolate creates catalogue with broken slice_layout

On Wed, Aug 30, 2023 at 03:24:12PM -0500, John Goerzen wrote:
> On Wed, Aug 30 2023, Denis Corbin wrote:
> 
> >> Summary
> >> =======
> >> g++ 10 works fine with optimization.
> >> g++ 11 or newer work only without optimization.
> >> Hope this helps.
> >
> > Thanks for confirming this is a gcc issue. I don't know what should be the next
> > step, if someone fills a bug report to gcc Debian maintainer?
> 
> Well, to be sure, we have a couple of possibilities:
> 
> 1) This is a gcc bug,
> 
> or 2) There is something in dar (or, for that matter, bzip2 or some
> other library) that is relying on some sort of C/C++ undefined behavior
> (UB) that the optimizer is taking in a different direction.  Other
> possibilities could include race conditions, etc.

To find the root cause why the optimizer does something harmfull in our
case is way over my head.

But...
I have found a workaround for dar. The appended patch works for me with
dar v2.7.11 and g++ v13.2.0, v12.3.0 and v11.4.0. Means, files generated
with OnFlyIsolation are readable again with default optimizations
activated.
It seems the optimizer does not like the used ternary operator.

I tested the patch with g++ v10.4.0 and 9.3.0, too without problems but
these versions are working with or without this patch anyway.

Hope this helps.

Tom

diff -rupN DAR_a/src/libdar/slice_layout.cpp DAR_b/src/libdar/slice_layout.cpp
--- DAR_a/src/libdar/slice_layout.cpp	2023-09-02 09:08:49.657051708 +0200
+++ DAR_b/src/libdar/slice_layout.cpp	2023-09-02 09:11:39.240669572 +0200
@@ -54,7 +54,11 @@ namespace libdar
 
     void slice_layout::write(generic_file & f) const
     {
-	char tmp = older_sar_than_v8 ? OLDER_THAN_V8 : V8;
+	char tmp = V8;
+	if(older_sar_than_v8)
+	{
+		tmp = OLDER_THAN_V8;
+	}
 	first_size.dump(f);
 	other_size.dump(f);
 	first_slice_header.dump(f);