alloca

[Red_prig]

Suddenly, the stack allocator I wrote, is it normal or not?

Code: Pascal  [Select][+][-]

1. function alloca(size:qword):Pointer; sysv_abi_default; assembler; nostackframe;
2. asm
3.  movqq       %rsp,%rax
4.  subq        %rdi,%rax
5.  lea     -8(%rax),%rax
6.  andq        $-32,%rax
7.  movqq     (%rsp),%rdi
8.  movqq       %rax,%rsp
9.  lea    -32(%rsp),%rsp
10.  jmp    %rdi
11. end;  
12.  

[Bart]

Well, that depends on your definition of "normal".

Bart

[Zvoni]

Well, that depends on your definition of "normal".

Bart

"Normal" is way overrated, IMO....

[Bart]

"Normal" is way overrated, IMO....

I hope so, because I'm not.

Bart

[Red_prig]

Normal in the sense that it seems to work as intended .... I posted this only because I could not find anything serious on this topic for Pascal

[Warfley]

Yeah alloca is something that is missing from freepascal.
The problem with custom written assembly is that it of course is highly target CPU dependent, so a general compiler intrinsic or something similar would be much nicer

[Thaddy]

I also have the suspicion that the parameter must be a multiple of 8.... As is the code does not work.

[Red_prig]

I didn’t understand you about the requirement to align by 8 bytes ... given that in the code both the size of the allocated data and the final stack address are aligned by 32 bytes (this is even unnecessary, but I had to)

[Thaddy]

Yeah alloca is something that is missing from freepascal.
The problem with custom written assembly is that it of course is highly target CPU dependent, so a general compiler intrinsic or something similar would be much nicer

I have alloca for win64. I use it in my stack allocated classes. Code is on this forum.
I am a bit amazed that on linux64  __builtin_alloca does not work, at least I can't get it to work. It should work on any linux.


BTW the function by OP does not work at all.

[Red_prig]

Hmm, it probably only works in simple functions (may be calling convention depened?)... but that's enough for me, for variable-sized local data inside a function.

[Red_prig]

If I'm not mistaken, then __builtin_alloca works in a similar way, only for use in the current function, or in child functions called by the same function

[Blaazen]

But your alloca is Linux-only since it passes parameter in rdi, isn't it?

[Red_prig]

I've specified a sysv_abi_default directive which makes it use parameter passing like in lunix, even under windows. Actually, I wrote and used it only under Windows 

[PascalDragon]

I am a bit amazed that on linux64  __builtin_alloca does not work, at least I can't get it to work. It should work on any linux.

__builtin_alloca is provided by GCC or Clang and not the operating system.

[Thaddy]

Yes, I know, I rephrase it: Why is it is not accessible through {$linklib c}  on my system? (Debian 64 under wsl1 and Debian 64 on  wsl2)