[FFmpeg-devel] [PATCH] configure: disable the new optimizer in Visual Studio 2015 Update 3

[Kacper Michajlow]

2016-07-04 3:53 GMT+02:00 Kacper Michajlow <kasper93 at gmail.com>:
> 2016-07-03 23:39 GMT+02:00 Hendrik Leppkes <h.leppkes at gmail.com>:
>> On Tue, Jun 28, 2016 at 12:01 PM, Hendrik Leppkes <h.leppkes at gmail.com> wrote:
>>> On Tue, Jun 28, 2016 at 11:48 AM, Hendrik Leppkes <h.leppkes at gmail.com> wrote:
>>>> Visual Studio 2015 Update 3 introduced a new SSA optimizer, however
>>>> it unfortunately causes miscompilations. Until it is fixed, the new
>>>> optimizations are disabled and should be re-checked on subsequent
>>>> compiler releases.
>>>>
>>>> Fixes recent FATE failure of fate-lavf-pam on VS2015.
>>>
>>> On that note, i'm not exactly sure which code actually miscompiles.
>>> I though its pamenc, but the generated files are identical, then I
>>> though its the crc computing code (crcenc/adler32), but those seem
>>> fine as well and would likely screw up in more then one case if they
>>> would miscompile.
>>>
>>> So that leaves pnmdec, I suppose, but I didn't make much headway to
>>> poke around in that. If anyone has any handy hints how to find the
>>> code in question that might be helpful.
>>> Maybe the code is actually not-quite-right and might enjoy some straigtening.
>>>
>>
>> Applied the patch to disable the new optimizations in MSVC as I did
>> not spot anything obviously "bad" in pamdec - which does not mean
>> there isn't anything there, of course.
>>
>
> This is pretty bad miscompilation in new SSA optimizer. It assumes
> that variable declared in loop doesn't change the value even if there
> is assignment. I minimized testcese and reported the bug
> https://connect.microsoft.com/VisualStudio/feedback/details/2890170
>
> - Kacper

Sorry for bumping such old thread, but I just got information that the
bug has been fixed in MSVC.

It is fixed in hotfix for VS2015 - KB3207317
https://support.microsoft.com/en-us/kb/3207317 and of course in
upcoming VS2017.

I've confirmed that this hotfix fixes fate-lavf-pam test. The
following change will enable SSA Optimizer on newer compiler versions.

---
 configure | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/configure b/configure
index 398e843..cf82b3b 100755
--- a/configure
+++ b/configure
@@ -6317,9 +6317,9 @@ EOF
     check_func strtoll || add_cflags -Dstrtoll=_strtoi64
     check_func strtoull || add_cflags -Dstrtoull=_strtoui64
     # the new SSA optimzer in VS2015 U3 is mis-optimizing some parts
of the code
-    # this flag should be re-checked on newer compiler releases and put under a
-    # version check once its fixed
-    check_cflags -d2SSAOptimizer-
+    # Issue has been fixed in MSVC v19.00.24218.
+    check_cpp_condition windows.h "_MSC_FULL_VER >= 190024218" ||
+        check_cflags -d2SSAOptimizer-
 fi

 for pfx in "" host_; do
-- 
2.10.0.windows.1.325.ge6089c1

- Kacper