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unpredictable.c
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unpredictable.c
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#include <stdlib.h>
void nop(void*, int);
#define N 20000
#define ITERS 10000
static int *m_s1, *m_s2, *m_s3, *m_dst;
/* This loop body contains a condition which is hard to predict well. It's
* possible to eliminate the control flow by using conditional move
* instructions, but then the value of z must also be unconditionally computed,
* effectively in speculation. Determining that this is worthwhile is difficult
* with only static analysis. With PMU feedback that confirming that the branch
* is very poorly predicted, the compiler will attempt more aggressive
* transformations and risk more speculation in order to eliminate the branch.
* flow.
*/
void unpredictable(int *dst, int *s1, int *s2, int *s3) {
#pragma novector
#pragma nounroll
for (int i = 0; i < N; i++) {
int *p;
if(s1[i] > 8000) {
p = &s2[i] ;
int z = i * i * i * i * i * i * i;
nop(p, z);
} else {
p = &s3[i];
nop(p, 3);
}
dst[i] = *p;
}
}
void init(void) {
m_s1 = malloc(sizeof(int)*N);
m_s2 = malloc(sizeof(int)*N);
m_s3 = malloc(sizeof(int)*N);
m_dst = malloc(sizeof(int)*N);
for (int i = 0; i < N; i++) {
m_s1[i] = (i*i*i*i*i*i*i) % N;
m_s2[i] = 0;
m_s3[i] = 1;
}
}
int main(void) {
init();
for(int i=0; i<ITERS; ++i)
#pragma noinline
unpredictable(m_dst, m_s1, m_s2, m_s3);
return 0;
}