[OpenMP][AIX] Set worker stack size to 2 x KMP_DEFAULT_STKSIZE if system stack size is too big (#81996)

This patch sets the stack size of worker threads to `2 x
KMP_DEFAULT_STKSIZE` (2 x 4MB) for AIX if the system stack size is too
big. Also defines maximum stack size for 32-bit AIX.

Author: xingxue-ibm

openmp/runtime/src/kmp.h

@@ -1181,7 +1181,11 @@ extern void __kmp_init_target_task();
 #define KMP_MIN_STKSIZE ((size_t)(32 * 1024))
 #endif
 
+#if KMP_OS_AIX && KMP_ARCH_PPC
+#define KMP_MAX_STKSIZE 0x10000000 /* 256Mb max size on 32-bit AIX */
+#else
 #define KMP_MAX_STKSIZE (~((size_t)1 << ((sizeof(size_t) * (1 << 3)) - 1)))
+#endif
 
 #if KMP_ARCH_X86
 #define KMP_DEFAULT_STKSIZE ((size_t)(2 * 1024 * 1024))

openmp/runtime/src/kmp_settings.cpp

@@ -255,8 +255,13 @@ static void __kmp_stg_parse_bool(char const *name, char const *value,
 // placed here in order to use __kmp_round4k static function
 void __kmp_check_stksize(size_t *val) {
   // if system stack size is too big then limit the size for worker threads
+#if KMP_OS_AIX
+  if (*val > KMP_DEFAULT_STKSIZE * 2) // Use 2 times, 16 is too large for AIX.
+    *val = KMP_DEFAULT_STKSIZE * 2;
+#else
   if (*val > KMP_DEFAULT_STKSIZE * 16) // just a heuristics...
     *val = KMP_DEFAULT_STKSIZE * 16;
+#endif
   if (*val < __kmp_sys_min_stksize)
     *val = __kmp_sys_min_stksize;
   if (*val > KMP_MAX_STKSIZE)