18-Nov-2013 11:37, Jonathan M Davis пишет:
On Monday, November 18, 2013 08:22:37 qznc wrote:
On Sunday, 17 November 2013 at 21:00:16 UTC, Jonathan M Davis
wrote:
will definitely result in multiple calls to pure_func. It's not
that it's
impossible for the compiler to do it - it's perfectly possible.
But doing so
would require at least basic code flow analysis, and dmd almost
never does any
kind of code flow analysis.
I remember Walter being against flow analysis in the frontend.
This is middleend optimization stuff, though. Does dmd really use
no data flow analysis anywhere?
The backend, of course:
https://github.com/D-Programming-Language/dmd/blob/master/src/backend/gflow.c
It has very, very limited flow analysis that it uses in constructors to avoid
multiple calls to super as well as guaranteeing that nothing gets initialized
multiple times in a const or immutable constructor. It may also does some very
minimal flow analysis when dealing with value range propagation (
http://www.drdobbs.com/tools/value-range-propagation/229300211 ). It may also
do some minimal checks for missing return statements. But it general, no, it
doesn't do flow analysis, and when it does, it's very minimal. Walter does not
consider it to be worth the complexity that it requires.
I think it was about doing it in the frontend.
And while in this particular case, it would certainly be much nicer for
repeated calls to a pure function to be optimized within an entire function
instead of just within an expression, I don't think that it actually ends up
mattering much in practice, simply because it's generally rare to make the
exact same call multiple times within a function. And if you do, it's trivial
to save the result in a variable to be reused.
--
Dmitry Olshansky
