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| /freebsd-10.2-release/lib/msun/ld128/ | ||
| H A D | e_rem_pio2l.h | diff 176466 Fri Feb 22 17:26:24 MST 2008 bde Remove the "quick check no cancellation" optimization for 9pi/2 < |x| < 32pi/2 since it is only a small or negative optimation and it gets in the way of further optimizations. It did one more branch to avoid some integer operations and to use a different dependency on previous results. The branches are fairly predictable so they are usually not a problem, so whether this is a good optimization depends mainly on the timing for the previous results, which is very machine-dependent. On amd64 (A64), this "optimization" is a pessimization of about 1 cycle or 1%; on ia64, it is an optimization of about 2 cycles or 1%; on i386 (A64), it is an optimization of about 5 cycles or 4%; on i386 (Celeron P2) it is an optimization of about 4 cycles or 3% for cos but a pessimization of about 5 cycles for sin and 1 cycle for tan. I think the new i386 (A64) slowness is due to an pipeline stall due to an avoidable load-store mismatch (so the old timing was better), and the i386 (Celeron) variance is due to its branch predictor not being too good. |
| /freebsd-10.2-release/lib/msun/ld80/ | ||
| H A D | e_rem_pio2l.h | diff 176466 Fri Feb 22 17:26:24 MST 2008 bde Remove the "quick check no cancellation" optimization for 9pi/2 < |x| < 32pi/2 since it is only a small or negative optimation and it gets in the way of further optimizations. It did one more branch to avoid some integer operations and to use a different dependency on previous results. The branches are fairly predictable so they are usually not a problem, so whether this is a good optimization depends mainly on the timing for the previous results, which is very machine-dependent. On amd64 (A64), this "optimization" is a pessimization of about 1 cycle or 1%; on ia64, it is an optimization of about 2 cycles or 1%; on i386 (A64), it is an optimization of about 5 cycles or 4%; on i386 (Celeron P2) it is an optimization of about 4 cycles or 3% for cos but a pessimization of about 5 cycles for sin and 1 cycle for tan. I think the new i386 (A64) slowness is due to an pipeline stall due to an avoidable load-store mismatch (so the old timing was better), and the i386 (Celeron) variance is due to its branch predictor not being too good. |
| /freebsd-10.2-release/lib/msun/src/ | ||
| H A D | e_rem_pio2.c | diff 176466 Fri Feb 22 17:26:24 MST 2008 bde Remove the "quick check no cancellation" optimization for 9pi/2 < |x| < 32pi/2 since it is only a small or negative optimation and it gets in the way of further optimizations. It did one more branch to avoid some integer operations and to use a different dependency on previous results. The branches are fairly predictable so they are usually not a problem, so whether this is a good optimization depends mainly on the timing for the previous results, which is very machine-dependent. On amd64 (A64), this "optimization" is a pessimization of about 1 cycle or 1%; on ia64, it is an optimization of about 2 cycles or 1%; on i386 (A64), it is an optimization of about 5 cycles or 4%; on i386 (Celeron P2) it is an optimization of about 4 cycles or 3% for cos but a pessimization of about 5 cycles for sin and 1 cycle for tan. I think the new i386 (A64) slowness is due to an pipeline stall due to an avoidable load-store mismatch (so the old timing was better), and the i386 (Celeron) variance is due to its branch predictor not being too good. |
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