eliminate quadratic behavior in updaterd()

[WalterBright]

This precomputes the vector needed for updaterd(), whereas before it was computed on the fly. It did that to minimize memory consumption, but that is much less of factor these days. So we're trading memory consumption for speed.

In some pathological cases, i.e. https://issues.dlang.org/show_bug.cgi?id=7157, this change doubles the speed of the optimization pass. The time taken is still quadratic, though. accumaecpx() remains quadratic.

[MartinNowak]

Auto-merge toggled on

[andralex]

cool beans

[WalterBright]

It also seems to produce about a 5% speed improvement in compiling ordinary code.

[wilzbach]

It also seems to produce about a 5% speed improvement in compiling ordinary code.

Would be interesting to see whether @CyberShadow's Is D slim yet is able to detect this as well, but it currently throws an error at me :/

[MartinNowak]

Hardly any visible effect on the test suite btw.

[JackStouffer]

@MartinNowak probably because as a work around, people moved away from functions which slowed down DMD a lot.

[WalterBright]

The test suite is designed to consist of small functions. The quadratic behavior does not move to the fore unless the function is quite large. It's usually considered bad form to write gigantic functions, so it was rarely a problem.