IBM XL C/C++ for Transactional Memory for AIX

Following is the source code for the complete program foo.c:

#include <omp.h>

extern int usleep(useconds_t ) __attribute__((tm_func_notrans));

int incre(int sum) __attribute__((tm_func)) ;
int incre(int sum)
{
 usleep(1);
 return sum+1;
}

int main() {
 int sum = 0;

 printf("Num of OMP threads: %d\n", omp_get_max_threads());

#pragma omp parallel for
 for(int i=0; i <100; i++) {
 #pragma tm atomic default(trans)
 {
 sum = incre(sum);
 }
 }

 printf("sum = %d\n", sum);
}

Steps for compiling and executing the source program foo.c are as follows:

% stmxlc –qsmp=omp foo.c -lstm
% setenv OMP_NUM_THREADS 4
% ./a.out
[mycomputer] Num of OMP threads: 4
[mycomputer] sum = 100

• This release (Version 0.9) does not support non-default installation (NDI). It requires that IBM XL C (or C/C++) for Enterprise Edition for AIX be installed in its default location.
• C++ exceptions-handling occurances in a transaction are not supported. An exception may exit the transaction without properly committing or aborting a transaction.
• In this release, the goto statement in the tm atomic region is supported only in the C++ compiler, not the C compiler.
• It is, in general, unsafe to call libraries inside a transaction.
• Transactional memory might not work properly with other synchronization methods (such as locks).
• Software transactional memory might incur a high overhead for instrumented memory references.
• The STM run-time environment supports weak atomicity.

We will make every effort to reply to the postings, but we cannot guarantee that every posting will receive a reply. No updates or fixes to the compilers will be provided during the alpha phase, but all reported bugs will be considered for resolution in future releases.

Functions annotated by tm_func require the compiler to instrument its memory references, whereas functions annotated by tm_func_notrans do not.

The programmer guarantees that functions annotated with the attribute tm_func_notrans are safe to call from a transactional memory region without the compiler's instrumenting their memory references.

tm_trans_read() and tm_trans_write() are used to directly instrument references in #pragma tm atomic default(notrans) regions or tm_func_notrans functions.

tm_notrans_read() and tm_notrans_write() are used to tell the compiler not to instrument a memory reference in #pragma tm atomic default(trans) regions or tm_func functions.

• -libstm.a is the standard STM library: % stmxlc foo.c -lstm
• -libstmstat.a is the debug version of the STM library for collecting transaction run-time statistics: % stmxlc foo.c -lstmstat

Following is foo-stat.c:

#include <omp.h>

int incre(int sum) __attribute__((tm_func)) ;
int incre(int sum)
{
 sleep(1);
 return sum+1;
}

int main() {
 int sum = 0;

 printf("Num of OMP threads: %d\n", omp_get_max_threads());

#pragma omp parallel for
 for(int i=0; i <100; i++) {
 #pragma tm atomic default(trans)
 {
 sum = incre(sum);
 }
 }

 stm_print_stats();
 printf("sum = %d\n", sum);
}

% stmxlc –qsmp=omp foo-stat.c -lstmstat
% setenv OMP_NUM_THREADS 4
% ./a.out
[mycomputer] Num of OMP threads: 4
[mycomputer] sum = 100

The wording of the warning message might differ between the XL C and XL C/C++ compilers:

• Compiled with XL C compiler: "The function foo is called from a transactional context but its prototype is not annotated with attribute tm_func or tm_func_notrans."
• Compiled with XL C/C++ compiler: "The function 'void foo(int)' should be annotated with tm_func or tm_func_notrans function attribute."

The warning message is as follows:

• Compiled with XL C compiler: "A call through a function pointer in a transactional context may invoke a function that is not annotated with attribute tm_func or tm_func_notrans."
• Compiled with XL C/C++ compiler: "User should make sure the function that the function pointer 'fpt' points to should be annotated with transactional memory function attribute."

• ld: 0711-317 ERROR: Undefined Symbol: .stm_desc
• ld: 0711-317 ERROR: Undefined symbol: .stm_begin
• ld: 0711-317 ERROR: Undefined symbol: .stm_end

When using TM intrinsics to access pointer types in C, type casting to (void**) or (void*) is typically needed. For example:

char* p[10];
char* val;
...
tm_trans_write((void**) &p[2], (void*) val);