-ffreestanding option

[Zheng Luo (Vic)]

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.