On 06/08/14 19:09, David Faure wrote:
> I'm using helgrind quite a lot these days, and I love it.
>
> However I wonder if it doesn't give me false positives for the case of
> reading
> a value from a static object, which was set in the constructor.
>
> Given that gcc does indeed implement "threadsafe statics" as per C++11 (but
> even before C++11 came out), one can assume that gcc does "something like" a
> mutex around the creation of the object, and therefore that there is a
> "happens before" relation between the end of the constructor and the use of
> this object later on, right?
>
> In that case it would seem that helgrind needs to learn that, to avoid many
> false positives.
>
> Testcase attached.
>
> The assembly code says
> call __cxa_guard_acquire
> testl %eax, %eax
> je .L3
> .loc 1 16 0 discriminator 2
> movl $_ZZ11threadStartPvE9singleton, %edi
> call _ZN9SingletonC1Ev
> movl $_ZGVZ11threadStartPvE9singleton, %edi
> call __cxa_guard_release
> .L3:
>
> IIRC __cxa_guard_acquire/release is the protection around the static, but I'm
> not sure exactly what this means. Is there an actual happens-before relation
> here?
I think g++ will have to be modified to allow Helgrind and DRD to
recognize thread-safe statics on architectures that do not have relaxed
memory consistency. From the gcc source file gcc/cp/decl.c I derived
that on architectures without relaxed memory consistency that g++
protects static initialization as follows:
static <type> guard;
if (!guard.first_byte) {
if (__cxa_guard_acquire (&guard)) {
bool flag = false;
try {
// Do initialization.
flag = true; __cxa_guard_release (&guard);
// Register variable for destruction at end of program.
} catch {
if (!flag) __cxa_guard_abort (&guard);
}
}
If g++ would be modified such that the "if (!guard.first_byte)" test can
be skipped at run-time then it would become possible for Helgrind and
DRD to recognize static initialization by intercepting the
__cxa_guard_*() functions. However, I'm not sure which mechanism the gcc
maintainers will prefer for disabling that if-test at run-time.
Bart.
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