On Thu, 18 Apr 2024 04:26:21 GMT, Kim Barrett <kbarr...@openjdk.org> wrote:
>> I opened https://bugs.openjdk.org/browse/JDK-8330539 so we don't lose track >> of this, but we can keep the discussion/voting here. > > For the impatient, I suggest adopting mechanism 2, i.e. unconditionally > include <alloc.h> in globalDefinitions_gcc.hpp. > > We can't include <alloca.h> in shared code, and there is a use in shared code > (in the relatively recently added JavaThread::pretouch_stack). > > When I questioned whether we needed to include <alloca.h> at all, I referred > to a Linux man page I'd found on the internet (the same page mdoerr linked > to), which says (in part) > > "By default, modern compilers automatically translate all uses of alloca() > into the built-in ..." > > Apparently I should have kept digging, because it seems that page is > old/incorrect. A seemingly more recent Linux man page describes a different > way of handling it that is closer to what we're seeing, but still not quite > correct. > > glibc's <stdlib.h> includes <alloca.h> if __USE_MISC is defined. > One of the ways __USE_MISC can become defined is if _GNU_SOURCE is defined, > and we define that for both gcc and clang toolchains. > > We include <stdlib.h> in globalDefinitions_gcc.hpp. So when building with gcc, > globalDefinitions.hpp implicitly includes <alloca.h>. > > The glibc definition of alloca is > > #ifdef __GNUC__ > # define alloca(size) __builtin_alloca (size) > #endif /* GCC. */ > > So that explains why we don't need any explicit include of <alloca.h> when > building with gcc. I expect there's something similar going on with Visual > Studio and Xcode/clang. But apparently not with Open XLC clang. On AIX `stdlib.h` also would define `alloca`, if `__STRICT_ANSI__` wouldn't be set. 780 #if !defined(__xlC__) || defined(__ibmxl__) || defined(__cplusplus) 781 #if defined(__IBMCPP__) && !defined(__ibmxl__) 782 extern "builtin" char *__alloca (size_t); 783 # define alloca __alloca 784 #elif defined(__GNUC__) && !defined(__STRICT_ANSI__) 785 #undef alloca 786 #define alloca(size) __builtin_alloca (size) 787 #endif A small plain Testprogramm not using all of the flags we used in jdk build, does not set `__STRICT_ANSI__` and then `alloca` is defined correct. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/18536#discussion_r1583360569
