Re: New 64 bit Cygwin DLL

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

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.

> I thought I mentioned when you asked what could be causing these
> random problems that it was likely to be related to signals.  I
> guess I just thought it though since I can't find the email which
> I thought I sent suggesting it.  Sorry.

No worries.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
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