[fpc-devel] Re: freepascal support for debian armhf

[Wookey]

+++ peter green [2011-09-06 02:05 +0100]:
> First a bit of background, on arm ebi gcc has three options for
> "-mfloat-abi"
> 
> soft: do all floating point in software, ignore the fpu type selection
> softfp: use the selected fpu type but use the same calling
> conventions as "soft" (that is parameters are passed using integer
> registers). This gives binary compatibility with soft float code but
> in the case of vfp it comes at a substantial performance cost as all
> parameters must be moved between fpu and integer registers which
> causes pipeline stalls
> hard: use the selected fpu type and use fpu type specific calling
> conventions.
> 
> Debian is in the process of introducing a new port called "armhf"
> which uses -mfloat-abi=hard with VFP (which seems to be become the
> dominant FPU now).
> 
> Afaict from quick tests freepascal does support vfp but based on my
> tests always uses the "soft" calling convention. So it won't link
> correctly with C code that uses floating point parameters on debian
> armhf.
> 
> 1: can someone who knows the fpc arm code confirm the above statement?
> 2: does anyone have any idea what it would take to add support for
> the eabi vfp calling convention and for building fpc binaries that
> use it by default (just as armel FPC binaries use eabi by default)

I have no idea about the internals of pascal, but you have understood
the variants correctly, and the reference doc for this stuff is here
(I believe that's a freely accessible doc): 
http://infocenter.arm.com/help/topic/com.arm.doc.ihi0042d/IHI0042D_aapcs.pdf

section 5 tells you about the calling conventions, and section 6 about
the normal variants (note the bit about it applying only to
non-variadic functions - does pascal support variadic functions?)

The things to bear in mind when considering the change from armel to
armhf are as follows:

1) The calling convention change: you can use FP registers instead of
integer registers

2) The extra functionality available: a VFP unit with 31 single
precision or 16 double precision registers. Note that newer hardware
will have twice as many registers, but that doesn't affect the
calling convention. In debian armhf (and Ubuntu armhf, and hopefully
everyone else shipping arm binary distros) we've picked the VFP3d16
hardware variant as a baseline to build for. Obviously it is useful to
people who build their own code if the compiler supports the d32
hardware variants too, but that won't be used in distro-built binaries. 

3) Some other things change that might catch you out:
a) Thumb interworking needs to be supported, which has requirements on state
between function calls (any function can be thumb or not-thumb).
b) The SWP instruction is gone. It may fault, it may get fixed up by
the kernel (slowly), but it shouldn't be used anymore (it's not
compatible with mutli-core designs, Load/store exclusive should be
used instead).  

Hopefully that's helpful to anyone looking at this. 

Wookey
-- 
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