[DMD] Loop index incorrectly optimised out for -release -O
b2.temp at gmx.com
Sun Oct 10 02:41:55 UTC 2021
On Saturday, 9 October 2021 at 21:09:19 UTC, Teodor Dutu wrote:
> I discovered a bug in DMD whereby, during optimisation, the
> loop index variable is removed, despite being used afterwards.
> I reproduced the bug in [this
> I am unsure what to make of this, since the logs seem to
> contradict my experiment with LDC. Has anyone else encountered
> this? How did you proceed?
It is about optimization, it's about loop so a good candidate to
check is dmd.backend.gloop.d.
you have several optim functions there:
- `elimbasivs(ref loop l)`
- `elimfrivivs(ref loop l)`
- `elimopeqs(ref loop l)`
- `elimspec(ref loop l)`
enable the logging functions and recompile dmd.
It appears that the bug occurs in `elimbasivs`, for example if
you only enable [this
then dmd, during compilation of the test case, outputs
> Eliminating basic IV 'i'
this is confirmed because if you recompile DMD with an early
return added in `elimbasivs` then the program does not contain
Now to fix the bug, it's another piece of cake. At first glance
I'd say that there's a missing condition to `continue` (e.g skip
the optim) in the code before line 2966.
(I've used a [slightly modifed test
case](https://dpaste.org/sL4F) btw, 1. to bisect using digger
more easily, 2. to be sure that the message was not for another
loop as `i` is a common name)
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