[fpc-devel] Difficulty in specifying record alignment... and more compiler optimisation shenanigans!
florian at freepascal.org
Sun Oct 27 09:13:01 CET 2019
Am 23.10.19 um 22:36 schrieb J. Gareth Moreton:
> So I did a bit of reading after finding the "mpx-linux64-abi.pdf"
> document. As I suspected, the System V ABI is like vectorcall when it
> comes to using the XMM registers... only the types __m128, __float128
> and __Decimal128 use the "SSEUP" class and hence use the entire
> register. The types are opaque, but both their size and alignment are
> 16 bytes, so I think anything that abides by those rules can be
> considered equivalent.
> If the complex type is unaligned, the two fields get their own XMM
> register. If aligned, they both go into %xmm0. At least that is what I
> gathered from reading the document - it's a little unclear sometimes.
I briefly tested with god bolt (https://godbolt.org/): records of two
double are passed in two xmm registers regardless of the alignment, two
floats (so single) are passed in one xmm register.
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