> -----Original Message-----
> From: Etienne Sandré-Chardonnal [mailto:[email protected]]
> Sent: Wednesday, March 27, 2013 3:43 PM
> To: [email protected]
> Subject: Re: [Mingw-w64-public] Performance changes when switching to
> 4.8.0
>
> I'm using the rubenvb with no "dw2" in the archive name, so I suppose I'm
> using SJLJ in 32bit and SEH in 64bit?
I guess so.
> Can there be a 30% speed difference just due to SJLJ, even if I do not use any
> exception handling in my code?
Yes, actually I had measured about 25% slow down e.g. in creator startup, and
for the test case in the bug report it's even 300%.
The jumps implementing SJLJ are automatically added to any code block that
_might_ throw, or propagate, an exception. So you don't have to have any try {}
catch {} blocks to get the slowdown, I understood.
> Other libs may use exceptions, but my app is
> running my code 99% of the time, as libs are only used for GUI and during
> initialization.
Well, it's enough if you're using e.g. QtCore, which is compiled with exception
handling enabled. Only if you do -fno-exceptions for all libs you should be on
the safe side, performance wise.
> What do I risk with dw2? I've read the main guidelines, but my understanding
> of exception handling and stack frame unwinding is too short to understand
> if this will impact my work.
There are problems (crashes) with dwarf as soon as you actually use exceptions,
and you want them to propagate to stacks not compiled with the same compiler.
Also Jonathan Liu reported issues with LoadLibrary/FreeLibrary, see
http://qt-project.org/wiki/MinGW-64-bit .
Anyway, we plan to switch to a dwarf toolchain for official Qt packages with
5.1 onwards.
Regards
Kai
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