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Re: New 64 bit Cygwin DLL


2013/3/20 Corinna Vinschen schrieb:
> On Mar 19 12:29, Christopher Faylor wrote:
>> On Tue, Mar 19, 2013 at 05:24:07PM +0100, Corinna Vinschen wrote:
>> >On Mar 19 12:08, Christopher Faylor wrote:
>> >> On Tue, Mar 19, 2013 at 10:46:22AM +0100, Corinna Vinschen wrote:
>> >> >Hi guys,
>> >> >
>> >> >
>> >> >at long last, it looks like we found the real bug which was the reason
>> >> >for the random crashes.
>> >> >
>> >> >There's a function sigdelayed, written in assembler, which is called
>> >> >when a thread got a signal.  Due to the way the function is called,
>> >> >it turned out that it was missing two crucial features:
>> >> >
>> >> >- It can be called with any stack alignment, but on x86_64 it's important
>> >> >  that the stack is always 16 byte aligned when calling functions.  So
>> >> >  sigdelayed had to make sure to align the stack before trundling along.
>> >>
>> >> >- sigdelayed only saved and restored the CPU registers which are
>> >> >  callee-saved in the Microsoft ABI, plus the registers used for the
>> >> >  return value of a function.  Given how sigdelayed is called, this
>> >> >  was insufficient.  The original, interrupted function needs the CPU
>> >> >  in its original state when sigdelayed returns to it, so sigdelayed
>> >> >  has to save and restore *all* registers.
>> >>
>> >> That's not really true for the 32-bit version.  eax and ebx aren't
>> >> normally saved around function calls but they are for sigdelayed.
>> >
>> >Yes, I saw that.  And I thought it's necessary only for 32 bit due to
>> >the different calling conventions, so I only saved the caller-saved and
>> >return value registers on x86_64.  So, in fact, this is entirely my own
>> >fault.
>> >
>> >> It doesn't currently save floating point and debugging registers though.
>> >
>> >There may be situations in which it might be helpful to save and restore
>> >the FP regs on 32 bit as well, though.  For instance, printf is a SIGFE
>> >function dealing with FP values.
>>
>> Right.
>
> While we're at it, does i686 use the 387 FP regs or the xmms regs for FP?
> I'm pretty fuzzy in this math stuff.  I know that x86_64 uses the xmms
> regs, but that's it...

x86 uses by default x87, and x64 always SSE.  There are switches to
change use of FPU-instructions and mode, but in general I wouldn't
recomment to use them.

Kai


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