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Postings from September 2012
On Sat, Sep 01, 2012 at 09:05:10PM +0100, Dave Mitchell wrote:
> >
> > OK, thanks. I don't know any x86_64 assembler, nor do I know its alignment
> > constrains or whatnot, so I'm not able to work out from this what went
> > wrong. And whether it's us doing something dodgy or them.
>
> See http://www.damtp.cam.ac.uk/cosmos/private/documentation/vtune/users_guide/mergedProjects/analyzer_ec/mergedProjects/reference_olh/instruct32_hh/vc181.htm
>
> (fond from google search).
>
> movaps basically copies a 16-byte check to/from one of FPU registers; the
> data must be 16-byte aligned. From ther above, sv_any is only 8-byte
> alignegned
>
> On my Linux system, the line
> StructCopy(SvANY(sv), xpvmg, XPVMG);
> is expanded to
> (*((XPVMG*)(xpvmg)) = *((XPVMG*)((sv)->sv_any)));
>
> So, struct xpvgv's are being allocated on 8-byte boundaries within the arena,
> yet the compiler assumes the structure is 16-byte aligned. After that I get
> confused.
>
> PS the structure (xpvmg and xpvgv) expand as
>
> ptr
> ptr/STRLEN
> STRLEN/I32
> ptr/IV/UV
> NV/ptr/{U32,U32}/{I32,U8}
>
> where the / show different elements of a union
The code is built with usemorebits, hence uselongdouble, so the NV
element would force 16-byte alignment.
So the compiler is correct to assume 16-byte alignment (assuming I
remember the x64 ABI correctly).
Tony