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This is a harness to test the atomicity of certain operations, and to
make sure the compiler does not introduce data races in a
multi-threaded environment.
The basic premise is that we set up testcases such that the thing we
want test, say an atomic instruction which stores a double word is in
a function of its own. We then run this testcase within GDB,
controlled by a gdb script (simulate-thread.gdb). The gdb script will
break on the function to be tested, and then single step through every
machine instruction in the function. We set this up so GDB can make a
couple of inferior function calls before and after each of these
single step instructions for a couple of purposes:
1. One of the calls simulates another thread running in the
process which changes or access memory.
2. The other calls are used to verify that we always get the
expected behavior.
For example, in the case of an atomic store, anyone looking at the
memory associated with an atomic variable should never see any in
between states. If you have an atomic long long int, and it starts
with the value 0, and you write the value MAX_LONG_LONG, any other
thread looking at that variable should never see anything other than 0
or MAX_LONG_LONG. If you implement the atomic write as a sequence of
2 stores, it is possible for another thread to read the location after
the first store, but before the second one is complete. That thread
would then see an in-between state (one word would still be 0).
We simulate this in the testcase by having GDB step through the
program, instruction by instruction, and after each step, making an
inferior function call which looks at the value of the atomic variable
and verifies that it sees either 0 or MAX_LONG_LONG. If it sees any
other value, it fails the testcase.
This way, we are *sure* there is no in between state because we
effectively acted like an OS and switched to another thread after
every single instruction of the routine is executed and looked at the
results each time.
We use the same idea to test for data races to see if an illegal load
has been hoisted, or that two parallel bitfield writes don't overlap
in a data race.
Below is a skeleton of how a test should look like. For more details,
look at the tests themselves.
/* { dg-do link } */
/* { dg-options "-some-flags" } */
/* { dg-final { simulate-thread } } */
/* NOTE: Any failure must be indicated by displaying "FAIL:". */
#include "simulate-thread.h"
/* Called before each instruction, simulating another thread executing. */
void simulate_thread_other_threads()
/* Called after each instruction. Returns 1 if any inconsistency is
found, 0 otherwise. */
int simulate_thread_step_verify()
if (some_problem)
printf("FAIL: reason\n");
return 1;
return 0;
/* Called at the end of the program (simulate_thread_fini == 1). Verifies
the state of the program and returns 1 if any inconsistency is
found, 0 otherwise. */
int simulate_thread_final_verify()
if (some_problem)
printf("FAIL: reason\n");
return 1;
return 0;
/* The gdb script will break on simulate_thread_main(), so make sure
GCC does not inline it, thus making the break point fail. */
void simulate_thread_main()
/* Do stuff. */
int main()
/* Perform any setup code that will run outside of the testing
harness. Put code here that you do NOT want to be interrupted on
an instruction basis. E.g., setup code, and system library
calls. */
/* Do un-instrumented stuff. */
/* ... */
/* Start the instrumented show. */
/* Must be called at the end of the test. */
return 0;