maria-developers team mailing list archive

[Kristian Nielsen <knielsen@xxxxxxxxxxxxxxx>]

Michael Widenius <monty@xxxxxxxxxxxx> writes:

> knielsen> === modified file 'configure.in'
> knielsen> --- a/configure.in	2009-04-23 13:06:16 +0000
> knielsen> +++ b/configure.in	2009-06-10 09:13:53 +0000

> knielsen> @@ -1739,28 +1763,9 @@ case "$with_atomic_ops" in
> knielsen>                    [Use pthread rwlocks for atomic ops]) ;;
> knielsen>    "smp") ;;
> knielsen>    "")
> knielsen> -    AC_CACHE_CHECK([whether the compiler provides atomic builtins],
> knielsen> -                   [mysql_cv_gcc_atomic_builtins], [AC_TRY_RUN([
> knielsen> -      int main()
> knielsen> -      {
> knielsen> -        int foo= -10; int bar= 10;
> knielsen> -        if (!__sync_fetch_and_add(&foo, bar) || foo)
> knielsen> -          return -1;
> knielsen> -        bar= __sync_lock_test_and_set(&foo, bar);
> knielsen> -        if (bar || foo != 10)
> knielsen> -          return -1;
> knielsen> -        bar= __sync_val_compare_and_swap(&bar, foo, 15);
> knielsen> -        if (bar)
> knielsen> -          return -1;
> knielsen> -        return 0;
> knielsen> -      }
> knielsen> -    ], [mysql_cv_gcc_atomic_builtins=yes_but_disabled],
> knielsen> -       [mysql_cv_gcc_atomic_builtins=no],
> knielsen> -       [mysql_cv_gcc_atomic_builtins=no])])
> knielsen> -
> knielsen> -    if test "x$mysql_cv_gcc_atomic_builtins" = xyes; then
> knielsen> -      AC_DEFINE(HAVE_GCC_ATOMIC_BUILTINS, 1,
> knielsen> -                [Define to 1 if compiler provides atomic builtins.])
> knielsen> +    if test "x$mysql_cv_gcc_atomic_builtins" = xyes_but_disabled; then
> knielsen> +      AC_DEFINE([MY_ATOMIC_MODE_GCC_BUILTINS], [1],
> knielsen> +                [Use GCC atomic builtins for atomic ops])

> Under which circumstances could the last 'if' be true?  The new code
> don't seem to be able to set mysql_cv_gcc_atomic_builtins to
> yes_but_disabled which makes the last if always false and we never
> define MY_ATOMIC_MODE_GCC_BUILTINS as 1.

You are right that the if can never be true.

However, this is unchanged behaviour from before the patch, check the line with
[mysql_cv_gcc_atomic_builtins=yes_but_disabled] above.

Serg changed the "yes" to "yes_but_disabled" some time ago in the Maria
tree. This is a hack to disable the use of GCC intrinsics in my_atomic. Serg
told me that the reason is that there were failures in the unit tests when
using GCC intrinsics.

I did try to run unit tests with this hack removed, but I was not able to
repeat any failures. I do think we should remove the hack and instead fix the
real issue, if it is still there. However, I wanted to keep this separate from
the XtraDB merge, which is quite complicated enough without introducing
additional unrelated issues.

 - Kristian.