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Re: [wwwdocs] Describe behavior of -flifetime-dse in class constructors
- From: Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>
- To: Martin LiÅka <mliska at suse dot cz>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Cc: jason at redhat dot com, Jan Hubicka <hubicka at ucw dot cz>
- Date: Tue, 16 Feb 2016 14:59:10 +0000
- Subject: Re: [wwwdocs] Describe behavior of -flifetime-dse in class constructors
- Authentication-results: sourceware.org; auth=none
- References: <56C33857 dot 7080007 at suse dot cz>
On 16/02/16 14:55, Martin LiÅka wrote:
Hello.
As I finally hunted issue in Firefox that was responsible for start-up segfault, I would like
to describe a new behavior of the compiler that emits clobbers to class constructors (w/ -flifetime-dse).
As also Richi spotted quite similar issue in openjade package, I think it worth for mentioning in porting:
Ok?
Thanks,
Martin
<p>
-Finally, the type and mangling of flexible array members has changed
+Furthermore, the type and mangling of flexible array members has changed
from previous releases. While in GCC 5 and prior the type of a flexible
array member is an array of zero elements (a GCC extension), in GCC 6 it
is that of an array of an unspecified bound (i.e., <tt>T[]</tt> as opposed
@@ -324,6 +324,50 @@
<tt>-fabi-version</tt> or <tt>-Wabi</tt> option to disable or warn about.
</p>
+<p>
+Finally, the C++ compiler (with enabled <code>-flifetime-dse</code>)
+has been more aggressive in dead-store elimination in situations where
+a memory store to a location precedes a constructor to the
+memory location. Described situation can be commonly found in programs
+which zero a memory that is eventually passed to a placement new operator:
Minor nit, but I notice we have a different "Finally" point now ;).
Perhaps we shouldn't bother saying "finally" as we keep finding new entries for this list.
Thanks,
Kyrill