-ffreestanding option

[Iain Buclaw]

On 25 July 2018 at 12:32, Zheng Luo (Vic) via D.gnu <d.gnu at puremagic.com> wrote:
> On Wednesday, 25 July 2018 at 10:05:50 UTC, Mike Franklin wrote:
>>
>> On Wednesday, 25 July 2018 at 08:37:28 UTC, Zheng (Vic) Luo wrote:
>>
>>> Current implementation of compilers assumes the existence of some symbols
>>> from libc, which leads to an infinite loop if we want to implement
>>> primitives like "memset" with our own code because the compiler will
>>> optimize consecutive set with "memset". This suggests that we cannot write a
>>> freestanding program without supports from compiler. With "-betterC" flag,
>>> dmd/gdc/ldc also come into this issue[5], which also applies to C/C++[1] and
>>> rust [2][3][4].
>>
>>
>> GDC doesn't seem to be affected.  See https://explore.dgnu.org/g/ZJVjAu
>> i.e. no recursive calls to `memset`, but I don't know if I just got lucky
>> with my implementation.
>>
>>> It would be better to provide a standard flag like "-ffreestanding" (or
>>> -fno-builtin?) to disable such optimizations to facilitate freestanding
>>> programming instead of forcing the developers to hack around different
>>> compiler implementations, so I was wondering is there any progress on this
>>> problem?
>>
>>
>> According to https://wiki.dlang.org/Using_GDC, `-fno-builtin` is already
>> there.
>>
>> From my experience I haven't yet found a need for `-ffreestanding`, as GDC
>> always seems to do the right thing for me.  It does generate calls for
>> `memset`, `memcmp`, etc..., but as long as I provide my own implementation
>> with the correct symbol name as it expects (i.e. `memset` with no name
>> mangling, a.k.a `extern(C) void* memset(void*, int, size_t)`) it seems to
>> work fine.
>>
>> Mike
>
>
> The real problem is that current D toolchain makes no promise about the
> generation of calls to libc: When are they called/Which subset is called/How
> to disable the calls? What if users embed this memset snippet in their own
> function? Instead of forcing developers to avoid memset-like access pattern
> in a freestanding environment and increasing their mental burden, a
> universal flags to disable these the generation of these calls will probably
> be a better choice.

And even if you avoid having such calls generated by the front-end,
the gcc optimizer may decide to add calls to memset if the right pass
is enabled - i.e: -ftree-loop-distribute-patterns.

I think in this case, the work of providing your own implementation is
so trivial in comparison to the hoops and red lines needed to go
without, that it's just not worth it.

Iain.