[Issue 19128] New: argument to alloca may be too large

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19128

          Issue ID: 19128
           Summary: argument to alloca may be too large
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: belka at caraus.de

In rt.arrayassign._d_arraysetassign alloca is called if the buffer should be
allocated is larger than 16 bytes:

void[16] buf = void;
void[] tmp;
if (element_size > buf.sizeof)
{
    tmp = alloca(element_size)[0 .. element_size];
}
else
    tmp = buf[];

It is dangerous since alloca unavailable to allocate causes undefined
behaviour, so the alloca man page states:

The alloca() function returns a pointer to the beginning of the allocated
space.  If the allocation causes stack overflow, program behavior is undefined.

See related discussion:
https://github.com/D-Programming-GDC/GDC/pull/699

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