Hello all,
I've recently been working on a port of the mir.random Mersenne
Twister implementation to Phobos, and I've run into a bit of
weirdness with CTFE.
Specifically, I use some CTFE functionality to generate a default
starting state for the generator:
https://github.com/WebDrake/mersenne-twister-range/blob/dda0bb006ee4633ae4fa227f9bad1453ba0e9885/src/mersenne_twister_range.d#L126
https://github.com/WebDrake/mersenne-twister-range/blob/dda0bb006ee4633ae4fa227f9bad1453ba0e9885/src/mersenne_twister_range.d#L166
However, it turns out that while this works while compiling with
the latest ldc2 beta (v1.1.0-beta6, which uses the dmd 2.071.2
frontend), it doesn't when compiled with the latest dmd master or
with dmd 2.072.1 (as installed from the deb package downloaded
from dlang.org).
Specifically, if we look at the unittests where the generator is
initialized with implicit construction, versus where the default
seed is explicitly supplied:
https://github.com/WebDrake/mersenne-twister-range/blob/dda0bb006ee4633ae4fa227f9bad1453ba0e9885/src/mersenne_twister_range.d#L399-L407
... different results emerge when compiled with dmd; it turns out
that in the implicit-construction case, only the `state.z`
variable is set (the rest are all zeroes), whereas with the
explicit seeding, all elements of the `state` variable are set
correctly.
When ldc2 is used, this doesn't happen and in both cases the
`state` variable is set correctly.
Can anyone advise what could be going wrong here? This looks
like a nasty CTFE bug to me :-(
Thanks & best wishes,
-- Joe
