[Bug middle-end/41001] alloca broken for -fno-builtin

["joseph at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #4 from joseph at codesourcery dot com  2009-08-07 22:36 -------
Subject: Re:  alloca broken for -fno-builtin

On Fri, 7 Aug 2009, ktietz at gcc dot gnu dot org wrote:

> Well, if so. It makes no sense that -fno-builtins tries to call a function
> which isn't present. But for other compilers, alloca can be invoked as
> function, too. The compiler builtin part in all cases is, to be aware that
> stack frame changes (hot-region, call save-area, and co are adjusted after
> calling it).

-fno-builtin means more or less exactly that the compiler *should not* 
assume anything special about a function from its name (unless the name 
starts __builtin or some similar reserved-namespace cases such as __sync) 
- that is, you declare alloca and because of -fno-builtin the compiler 
assumes this is your own function with no special semantics whatever.  By 
-fno-builtin you are declaring that alloca, and all other normally 
built-in functions, are normal functions with no special semantics the 
compiler needs to know about.  So of course it generates a call like it 
would to any other random function you might have defined in another 
translation unit.

If you want to use alloca with its traditional memory allocation semantics 
with -fno-builtin, you'll need to use a malloc-based emulation such as 
that in libiberty, not something that uses the stack.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41001