[BetterC] Struct of size_ts and pointer decomposition before calling.

[9il via Digitalmars-d]

Hi,

Could we introduce special attribute for struct definitions to 
make function ABI look like it accepts all fields as separate 
data (recursively)?

Its application should be constrained because alignment and etc.

Reasons:

1. clean unified _and_ hight level ABI with existing C libraries 
in both export from D and import to D directions.

2. Speed. For example LAPACK routine for small size matrixes 
would be 5-10% slower if we implement them using ndslices API.

3. Code size. Many LAPACK routines only call a lot of BLAS 
functions and do not compute anything itself. If we use ndslices 
for GLAS ABI, then code size will grow.

Note that BLAS/GLAS routines are not and should not be inlined.

2 and 3 are not jokes, please see code examples below.

Thanks,
Ilya

ldc2 -betterC -output-s -O -release -boundscheck=off testcc.d
-----
pragma(inline, false)
ref back(size_t length, sizediff_t stride, double* ptr)
{
	return ptr[(length - 1) * stride];
}

auto foo(size_t length, sizediff_t stride, double* ptr)
{
	return back(length, stride, ptr);
}

///////////

struct Slice
{
	size_t length;
	sizediff_t stride;
	double* ptr;
}

pragma(inline, false)
ref back(Slice slice)
{
	with(slice)
	return ptr[(length - 1) * stride];
}


auto foo(Slice slice)
{
	return back(slice);
}

-----



-----
     // back 1 (desired)
	decq	%rdx
	imulq	%rsi, %rdx
	leaq	(%rdi,%rdx,8), %rax
	retq

     // foo 1 (desired)
	pushq	%rax
Lcfi0:
	callq	__D6testcc4backFNaNbNcNimlPdZd
	movsd	(%rax), %xmm0
	popq	%rax
	retq

///////////

       // back 1 (current)
	movq	8(%rsp), %rax
	decq	%rax
	imulq	16(%rsp), %rax
	shlq	$3, %rax
	addq	24(%rsp), %rax
	retq

       // foo 1 (current)
	subq	$24, %rsp
Lcfi1:
	movq	48(%rsp), %rax
	movq	%rax, 16(%rsp)
	movq	32(%rsp), %rax
	movq	40(%rsp), %rcx
	movq	%rcx, 8(%rsp)
	movq	%rax, (%rsp)
	callq	__D6testcc4backFNaNbNcNiS6testcc5SliceZd
	movsd	(%rax), %xmm0
	addq	$24, %rsp
	retq
-----