On Wednesday 11. December 2013 10.12.22 Knoll Lars wrote: > On 11/12/13 10:03, "Simon Hausmann" <simon.hausm...@digia.com> wrote: > >On Wednesday 11. December 2013 09.36.46 Simon Hausmann wrote: > >[...] > > > >> > If we apply the change I mentioned in the other email, we change > >> > >>cases 2 > >> > >> > and 3: > >> > > >> > 2bis) distro builds Qt once with -no-sse2 flags > >> > > >> > => all libs compiled without SSE2 > >> > => QtQml generates interpreted code only > >> > > >> > 3bis) distro builds Qt once with -no-sse2 and then some libs with > >> > >>-config > >> > >> > sse2 => most libs compiled without SSE2, a few with SSE2 code > >> > > >> > => the non-SSE2 QtQml will generate interpreted code, the SSE2 one > >> > >>will > >> > >> > generate SSE2 code and will be used only in machines less than 14 > >> > >>years > >> > >> > old > >> > >> Yep, let's do that. It's relatively painless to support this kind of > >> > >>build > >> > >> configuration in QtQml at least. > > > >To puts words where my mouth is, here's a proposed fix for QtQml: > > https://codereview.qt-project.org/#change,73710 > > Is this the right fix for 5.2? I'm a bit afraid that this would cause > Linux distributions to install the sse disabled QtQml by default, leading > to pretty bad performance.
Good question... > The other option for 5.2 would be to do runtime detection of SSE2 and > enable/disable the JIT according to that. I think the main issue with run-time detection is that we need -msse2 -mfpmath=sse in order to get reliable math results (no 80-bit temps) that match with our test expectations, when using the interpreter or non-inline math functions. We could of course do that only with CONFIG(developer-build) or so, but it's a bit scary then that the interpreter in release builds would use a different precision than during development, and it might cause confusion in bug reports. > PS: Actually I think QtQml in 5.0 and 5.1 was also requiring SSE (because > of V8). >From a quick glance at the V8 sources and a bit of google searching, it appears that V8's ia32 code generator supports SSE2 through run-time checks and falls back to run-time function calls otherwise. For example: https://codereview.chromium.org/7324026/patch/1/2 Simon _______________________________________________ Development mailing list Development@qt-project.org http://lists.qt-project.org/mailman/listinfo/development
