Hi, On Sat, 7 Nov 2015, Richard Sandiford wrote:
> For -fmath-errno, builtins.c currently expands calls to sqrt to: > > y = sqrt_optab (x); > if (y != y) > [ sqrt (x); or errno = EDOM; ] > > - the call to sqrt is protected by the result of the optab rather > than the input. It would be better to check !(x >= 0), like > tree-call-cdce.c does. It depends. With fast-math (and hence without NaNs) you can trivially optimize away a (y != y) test. You can't do so with !(x>=0) at all. > - the branch isn't exposed at the gimple level and so gets little > high-level optimisation. > > - we do this for log too, but for log a zero input produces > -inf rather than a NaN, and sets errno to ERANGE rather than EDOM. > > This patch moves the code to tree-call-cdce.c instead, This somehow feels wrong. Dead-code elimination doesn't have anything to do with the transformation you want, it rather is rewriting all feasible calls into something else, like fold_builtins does. Also cdce currently doesn't seem to do any checks on the fast-math flags, so I wonder if some of the conditions that you now also insert for calls whose results are used stay until final code. > Previously the pass was only enabled by default at -O2 or above, but the > old builtins.c code was enabled at -O. The patch therefore enables the > pass at -O as well. The pass is somewhat expensive in that it removes dominator info and schedules a full ssa update. The transformation is trivial enough that dominators and SSA form can be updated on the fly, I think without that it's not feasible for -O. But as said I think this transformation should better be moved into builtin folding (or other call folding), at which point also the fast-math flags can be checked. The infrastructure routines of tree-call-cdce can be used there of course. If so moved the cdce pass would be subsumed by that btw. (because the dead call result will be trivially exposed), and that would be a good thing. Ciao, Michael.
