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digitalmars.D - [DMD] Loop index incorrectly optimised out for -release -O

reply Teodor Dutu <teodor.dutu gmail.com> writes:
Hi,

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 
issue](https://issues.dlang.org/show_bug.cgi?id=22372).

Using run.dlang.io, I tried to find a regression, but all 
supported DMD versions (2.060 and newer) are printing the same 
incorrect output (`Exception: i = 0; n = 1`). My experiment can 
be found [here](https://run.dlang.io/is/uYfTzl). This seems to be 
a backend issue, as both LDC and LDC-beta are working fine.

However, when analysing the CI outputs from [the 
PR](https://github.com/dlang/dmd/pull/13116) which made me come 
across this bug, I noticed that all 3 of DMD, LDC and GDC are 
failing the same test, which I've also been able to reproduce 
myself:
- LDC: 
https://cirrus-ci.com/task/6291197929979904?logs=test_druntime#L1449
- DMD: 
https://cirrus-ci.com/task/5728247976558592?logs=test_druntime#L1390
- GDC: 
https://cirrus-ci.com/task/4883823046426624?logs=test_druntime#L1411

The failure of this test is caused by the issue I mentioned above 
and the code with which I reproduced the bug is based on it.

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?

Thanks,
Teodor
Oct 09 2021
next sibling parent reply Iain Buclaw <ibuclaw gdcproject.org> writes:
On Saturday, 9 October 2021 at 21:09:19 UTC, Teodor Dutu wrote:

 However, when analysing the CI outputs from [the 
 PR](https://github.com/dlang/dmd/pull/13116) which made me come 
 across this bug, I noticed that all 3 of DMD, LDC and GDC are 
 failing the same test, which I've also been able to reproduce 
 myself:
 - LDC: 
 https://cirrus-ci.com/task/6291197929979904?logs=test_druntime#L1449
 - DMD: 
 https://cirrus-ci.com/task/5728247976558592?logs=test_druntime#L1390
 - GDC: 
 https://cirrus-ci.com/task/4883823046426624?logs=test_druntime#L1411


Just to clarify, those are all DMD.  The difference in those 
pipelines is who built DMD.
Oct 09 2021
parent Teodor Dutu <teodor.dutu gmail.com> writes:
On Saturday, 9 October 2021 at 22:05:15 UTC, Iain Buclaw wrote:

 On Saturday, 9 October 2021 at 21:09:19 UTC, Teodor Dutu wrote:

 However, when analysing the CI outputs from [the 
 PR](https://github.com/dlang/dmd/pull/13116) which made me 
 come across this bug, I noticed that all 3 of DMD, LDC and GDC 
 are failing the same test, which I've also been able to 
 reproduce myself:
 - LDC: 
 https://cirrus-ci.com/task/6291197929979904?logs=test_druntime#L1449
 - DMD: 
 https://cirrus-ci.com/task/5728247976558592?logs=test_druntime#L1390
 - GDC: 
 https://cirrus-ci.com/task/4883823046426624?logs=test_druntime#L1411


 Just to clarify, those are all DMD.  The difference in those 
 pipelines is who built DMD.


Oh I see. Thanks for pointing it out.
Oct 10 2021
prev sibling parent reply Basile.B <b2.temp gmx.com> writes:
On Saturday, 9 October 2021 at 21:09:19 UTC, Teodor Dutu wrote:

 Hi,

 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 
 issue](https://issues.dlang.org/show_bug.cgi?id=22372).

 [...]

 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 
printf](https://github.com/dlang/dmd/blob/feab6d82364c8f2c4344e71292f2619c78da480f/src/dmd/backend/gloop.d#L2966)

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 
the bug.

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)
Oct 09 2021
next sibling parent Imperatorn <johan_forsberg_86 hotmail.com> writes:
On Sunday, 10 October 2021 at 02:41:55 UTC, Basile.B wrote:

 On Saturday, 9 October 2021 at 21:09:19 UTC, Teodor Dutu wrote:

[...]


 It is about optimization, it's about loop so a good candidate 
 to check is dmd.backend.gloop.d.

 [...]


Yup, I was also dissecting gloop last night
Oct 10 2021
prev sibling parent Walter Bright <newshound2 digitalmars.com> writes:
On 10/9/2021 7:41 PM, Basile.B wrote:

 [...]


Thanks for doing the detective work in finding where the problem is. Made it 
much quicker for me to fix it.
Oct 15 2021