[Issue 19159] New: `alloca` does not work in -betterC

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19159

          Issue ID: 19159
           Summary: `alloca` does not work in -betterC
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: slavo5150 at yahoo.com

extern(C) void main()
{
    import core.stdc.stdlib : alloca;

    enum length = 20;
    auto msg = (cast(char*)alloca(length))[0 .. length];
}

https://run.dlang.io/is/dHAqPo

$ dmd -betterC main.d
error: undefined reference to '__alloca'
collect2: error: ld returned 1 exit status
Error: linker exited with status 1

Works fine with LDC, however: https://run.dlang.io/is/m45jG1

Also, `alloca` should not be in `core.stdc.stdlib`; it's not a standard C
function.

It may also be better to allocate static arrays on the stack when their length
is supplied at runtime.  See https://issues.dlang.org/show_bug.cgi?id=18788. 
That would make `alloca` and `__alloca` just an implementation detail that
users wouldn't even need to use directly.

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