Re: Extend tree-call-cdce to calls whose result is used

[Michael Matz <matz at suse dot de>]

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.