RFR: 8329257: AIX: Switch HOTSPOT_TOOLCHAIN_TYPE from xlc to gcc [v3]

[Joachim Kern]

On Tue, 16 Apr 2024 09:15:19 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> That was kind of where the discussion started, and which Kim did not like. If I read him correctly, his suggestion was instead to place:
>> 
>> #if defined(_AIX)
>> #include <alloca.h>
>> #endif
>> 
>> in the files where `alloca` is needed on AIX.
>
> (If some of these files happen to be files which are not compiled on Windows, I assume it will not hurt to drop the ifdef guard, but then again, it can certainly be kept as well for consistency.)

@magicus @TheShermanTanker @TheRealMDoerr @kimbarrett 
Let me summarize the choices we have and ask for your vote.
Julian dislikes the `-Dalloca'(size)'=__builtin_alloca'(size)'` in `flags-cflags.m4` I introduced to get rid of 

#if defined(_AIX)
#include <alloca.h>
#endif

in `globalDefinitions_gcc.hpp`. 

We have three possible solutions

1. Reintroduce

#if defined(_AIX)
#include <alloca.h>
#endif

in `globalDefinitions_gcc.hpp`. 

2. Unconditionally introduce only `#include <alloca.h>` in `globalDefinitions_gcc.hpp`. This should work for all platforms using this header including the unofficial Windows/gcc Port, although only AIX needs it. 

3. Add 

#if defined(_AIX)
#include <alloca.h>
#endif

to the sources using alloca(). These are
/hotspot/share/runtime/os.cpp
/hotspot/share/runtime/javaThread.cpp
/hotspot/share/utilities/vmError.cpp
Here we need the AIX condition, because otherwise the classic Windows Build (NTAMD64) fails.

I will implement the solution with the most likes and having no dislike.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18536#discussion_r1568650313