alloca is slow and dangerous

[Johan]

On Sunday, 3 January 2021 at 19:15:05 UTC, welkam wrote:
> On Friday, 1 January 2021 at 17:55:33 UTC, Steven Schveighoffer 
> wrote:
>> D has alloca. It's in core.std.stdlib
>>
>> https://dlang.org/phobos/core_stdc_stdlib.html#.alloca
>>
> Does it work on all compilers and all platforms?

Why wouldn't it?

In C/C++/D/... language land, there is no such thing as "_the_ 
stack". Yeah, the _compiler_ may decide to use the special 
instructions/register of the CPU that address "the stack", but 
there is no guarantee it will; nor is there a guarantee that the 
binary executable with CPU stack instructions will actually use 
the stack and not just dynamically allocate things. There are 
CPUs that do not have such special instructions, and there are 
platforms that do not use a "stack" in the common interpretation 
of the word to execute a D program.
Examples: Webassembly, an executable running with ASan's 
FakeStack enabled, a binary running in an emulator, ...

LDC emits the same LLVM IR "alloca" instruction for local 
variables (`int i;`) as for `alloca` function calls. Simplified: 
if you can write `int i;` for your platform, 
`core.stdc.stdlib.alloca` also works. ;)

-Johan