I've been playing around with using D with no runtime on Linux, but recently I was thinking it would be nice to have an alloca implementation. I was thinking I could just bump the stack pointer (with alignment considerations) but from what I understand compilers sometimes generate code that references variables relative to RSP instead of RBP? I've seen people saying that a proper alloca can't be implemented without help from the compiler...

I took a peek in druntime and found rt.alloca which has __alloca implemented with inline asm. I tried throwing that in my project and calling it but it segfaults on rep movsq. The comments in the code suggest it is trying to copy temps on the stack but I seem to get a really large garbage RCX, I don't fully follow what is going on yet.

Is there any way I can get a working alloca without using druntime, c runtime, etc?

On Sunday, 30 April 2017 at 05:07:31 UTC, 岩倉 澪 wrote:
> I've been playing around with using D with no runtime on Linux, but recently I was thinking it would be nice to have an alloca implementation. I was thinking I could just bump the stack pointer (with alignment considerations) but from what I understand compilers sometimes generate code that references variables relative to RSP instead of RBP? I've seen people saying that a proper alloca can't be implemented without help from the compiler...
>
> I took a peek in druntime and found rt.alloca which has __alloca implemented with inline asm. I tried throwing that in my project and calling it but it segfaults on rep movsq. The comments in the code suggest it is trying to copy temps on the stack but I seem to get a really large garbage RCX, I don't fully follow what is going on yet.
>
> Is there any way I can get a working alloca without using druntime, c runtime, etc?

As a follow-up, here is a simple example of what I mean:

first, let's create a main.d, we'll define our own entry point and make a call to alloca in main:

extern (C):
void _start()
{
    asm nothrow @nogc
    {
        naked;
        xor RBP, RBP;
        pop RDI;
        mov RSI, RSP;
        and RSP, -16;
        call main;
        mov RDI, RAX;
        mov RAX, 60;
        syscall;
        ret;
    }
}
pragma(startaddress, _start);
int main(int argc, char** argv)
{
    import rt.alloca;
    void* a = __alloca(42);
    return 0;
}

Next, let's make an rt directory and copy the source of druntime's rt.alloca into rt/alloca.d

Now let's compile these:

dmd -betterC -debuglib= -defaultlib= -boundscheck=off -vgc -vtls -c -gc main.d rt/alloca.d

Great, now we need to strip symbols out to make this work, like so:

objcopy -R '.data.*[0-9]TypeInfo_*' -R '.[cd]tors.*' -R .eh_frame -R minfo -R .group.d_dso -R .data.d_dso_rec -R .text.d_dso_init -R .dtors.d_dso_dtor -R .ctors.d_dso_ctor -N __start_minfo -N __stop_minfo main.o
objcopy -R '.data.*[0-9]TypeInfo_*' -R '.[cd]tors.*' -R .eh_frame -R minfo -R .group.d_dso -R .data.d_dso_rec -R .text.d_dso_init -R .dtors.d_dso_dtor -R .ctors.d_dso_ctor -N __start_minfo -N __stop_minfo alloca.o

With that out of the way, we are ready to link:

ld main.o alloca.o

And when we try to run ./a.out we get a segfault.

What I want is a way to allocate on the stack (size of allocation not necessarily known at compile-time) and for the compiler to be aware that it can't generate code that refers to variables on the stack relative to rsp, or anything else that might break the naive implementation of alloca as simply bumping rsp with inline asm. Apparently this "magic" __alloca can't be used outside of the compiler, or is there a way to make it work?

You can try ldc and llvm intrinsics
http://llvm.org/docs/LangRef.html#alloca-instruction
http://llvm.org/docs/LangRef.html#llvm-stacksave-intrinsic

On Thursday, 4 May 2017 at 12:50:02 UTC, Kagamin wrote:
> You can try ldc and llvm intrinsics
> http://llvm.org/docs/LangRef.html#alloca-instruction
> http://llvm.org/docs/LangRef.html#llvm-stacksave-intrinsic

Ah, yep!

pragma(LDC_alloca) void* alloca(size_t);

This appears to work with ldc. It would be nice if there was a way to do this with dmd/other compilers as well though. If it were up to me I'd have alloca defined by the language standard and every compiler would have to provide an implementation like this. At the very least I'd like to have an alloca that works with dmd, as I want to do debug builds with dmd and release builds with ldc.

On Thursday, 4 May 2017 at 14:54:58 UTC, 岩倉 澪 wrote:
> On Thursday, 4 May 2017 at 12:50:02 UTC, Kagamin wrote:
>> You can try ldc and llvm intrinsics
>> http://llvm.org/docs/LangRef.html#alloca-instruction
>> http://llvm.org/docs/LangRef.html#llvm-stacksave-intrinsic
>
> Ah, yep!
>
> pragma(LDC_alloca) void* alloca(size_t);
>
> This appears to work with ldc. It would be nice if there was a way to do this with dmd/other compilers as well though. If it were up to me I'd have alloca defined by the language standard and every compiler would have to provide an implementation like this. At the very least I'd like to have an alloca that works with dmd, as I want to do debug builds with dmd and release builds with ldc.

embedded platform?

On Wednesday, 10 May 2017 at 20:25:45 UTC, aberba wrote:
> On Thursday, 4 May 2017 at 14:54:58 UTC, 岩倉 澪 wrote:
>> On Thursday, 4 May 2017 at 12:50:02 UTC, Kagamin wrote:
>>> You can try ldc and llvm intrinsics
>>> http://llvm.org/docs/LangRef.html#alloca-instruction
>>> http://llvm.org/docs/LangRef.html#llvm-stacksave-intrinsic
>>
>> Ah, yep!
>>
>> pragma(LDC_alloca) void* alloca(size_t);
>>
>> This appears to work with ldc. It would be nice if there was a way to do this with dmd/other compilers as well though. If it were up to me I'd have alloca defined by the language standard and every compiler would have to provide an implementation like this. At the very least I'd like to have an alloca that works with dmd, as I want to do debug builds with dmd and release builds with ldc.
>
> embedded platform?

An embedded platform would be a good use-case for this, but I'm just trying to do this on Linux x86_64 personally. It's a fun experiment to see how far I can push D to give me low-level control without dependencies