Well, this is not just about branch prediction, but about "let the
compiler assume the condition is always true and let it eliminate code
that handles other cases".
I /think/ that in C with GCC assume() could be implemented (for release
mode, otherwise it's just like assert()) like
#define assume(cond) if(!(cond)) __builtin_unreachable()
I'm not sure what kind of optimizations GCC does based on "unreachable",
though.
However, something like expect() /might/ be a useful addition to the
language as well. Maybe as an annotation for if()/else?
Cheers,
Daniel
Am 04.08.2014 01:51, schrieb Mike Farnsworth:
This all seems to have a very simple solution, to use something like:
expect()
GCC for example has an intrinsic, __builtin_expect() that is used to
notify the compiler of a data constraint it can use in optimization for
branches. Why not make something like this a first-class citizen in D
(and even expand the concept to more than just branch prediction)?
That way you don't have to hijack the meaning of assert(), but
optimizations can be made based on the condition. __buitin_expect() in
gcc usually just figures the expected condition is fulfilled the vast
majority of the time, but it could be expanded to make a lack of
fulfillment trigger an exception (in non-release mode). And the
compiler is always free to optimize with the assumption the expectation
is met.
On Sunday, 3 August 2014 at 19:47:27 UTC, David Bregman wrote:
I am creating this thread because I believe the other ones [1,6] have
gotten too bogged down in minutiae and the big picture has gotten lost.
...
References:
[1]: http://forum.dlang.org/thread/lrbpvj$mih$1...@digitalmars.com
[2]: http://dlang.org/overview.html
[3]:
http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html
[4]: http://blog.regehr.org/archives/213
[5]: http://en.wikipedia.org/wiki/Heisenbug
[6]: http://forum.dlang.org/thread/jrxrmcmeksxwlyuit...@forum.dlang.org
