I found some curiosity issues during finding optimal tool for compilation our program. Results of computations be different for different tools and different optimisation flags.
For investigating this issue I write simple code(full sources on github):
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1 2 3 4 5 |
int i; long double res; for(i=0;i<100000000;i++){ res += cos(i); } |
In this small test program I can’t reproduce different results for different flags. But 5 different results for so similar programs is enough to demonstrate discovered effect.
| Source code and tool | Result | Computation time, s |
| Assembler | 1.53436944477410652787 | 3.81 |
| gcc long | 1.53436944477462278158 | 4.4 |
| gcc double | 1.53436944477389380914 | 4.4 |
| icc long | 1.53436944477397574360 | 6.88 |
| icc double | 1.53436944477334602510 | 1.50 |
p.s. also I found that manually written assembly code 15% faster than code on C in spite of using “-O2” optimisation flag.
