On Thu, Jan 26, 2017 at 05:00:44PM -0800, Andrew Pinski wrote: > On Thu, Jan 26, 2017 at 4:38 PM, Segher Boessenkool > <seg...@kernel.crashing.org> wrote: > > Scheduling should never move very expensive instructions to places they > > are executed more frequently. This patch fixes that, reducing the > > execution time of c-ray by over 40% (I tested on a BE Power7 system). > > > > Is there some existing way to test for "very expensive insn" or "very > > expensive insn we should not speculate"? Should there be a new hook? > > Is only disallowing (FP) SQRT and DIV a good solution? > > Seems like it should be checking the insn cost and compare that > against some parameter. That is possibly set by the target if needed.
But what is "insn cost"? Latency is no good at all -- we *want* insns with higher latency to be earlier. fsqrt is not pipelined, and that is what makes it so costly. (This isn't modeled in the scheduling description btw: that would make the automata sizes explode, the usual problem). Segher
