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Improve busy loop stability #90

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Improve busy loop stability #90

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9e5707f
waste_cpu_cycles: Shield against compiler optimizations
douglas-raillard-arm Oct 8, 2019
884775a
waste_cpu_cycles: Use a more deterministic loop body
douglas-raillard-arm Oct 8, 2019
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22 changes: 17 additions & 5 deletions src/rt-app.c
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Original file line number Diff line number Diff line change
Expand Up @@ -201,20 +201,32 @@ static int create_thread(const thread_data_t *td, int index, int forked, int nfo
/*
* Function: to do some useless operation.
* TODO: improve the waste loop with more heavy functions
*
* This function must be protected against compiler optimizations, as it is
* somewhat pure and has not result.
*/
void waste_cpu_cycles(unsigned long long load_loops)
void __attribute__((noinline, optimize("O0"))) waste_cpu_cycles(unsigned long long load_loops)
{
double param, result;
double n;
unsigned long long i;

/*
* Adjust the number of loops to get calibration values close to what
* they used to be with the previous loop body.
*/
load_loops *= 10;

param = 0.95;
n = 4;
for (i = 0 ; i < load_loops ; i++) {
result = ldexp(param , (ldexp(param , ldexp(param , n))));
result = ldexp(param , (ldexp(param , ldexp(param , n))));
result = ldexp(param , (ldexp(param , ldexp(param , n))));
result = ldexp(param , (ldexp(param , ldexp(param , n))));
/*
* Add a "side-effect" to that function, so compilers will not
* try to remove the call to it altogether. See GCC
* documentation of "noinline" function attribute for details.
*/
asm("");
result = n/i;
}
return;
}
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