fix Issue 20855 - stack overflow when compiling large file

[WalterBright]

No description provided.

[dlang-bot]

Thanks for your pull request, @WalterBright!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20855	minor	stack overflow when compiling large file

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#12409"

[Geod24]

Since the code is so trivial, I think it'd be better to just write a script generating it.
Also this doesn't really fix the issue, just adds a hard-coded limit which works on your system.

[WalterBright]

Yes, switching the test case to be generative is better; done.

The fundamental problem here is the backend is recursive in nature. It's difficult to figure out just where to put the checks without engaging in whack-a-mole. The check I put in is conservative, it needs to go double that before it stack overflows. You're doing something wrong anyway with 10,000 statements in a function.

I also put the check in the backend rather than the frontend, as other compilers have their own means of dealing with the problem.

[ibuclaw]

Isn't the issue that your doing something quadratic when traversing trees in the dmd backend? Why can't it be fixed to not be quadratic? (i.e: isn't this just issue 6401)

[WalterBright]

Quadratic doesn't cause stack overflow. Recursive depth does. Trying different statement counts in the attached test clearly indicate a stack overflow in optelem(), and optelem() is recursive.

There are many quadratic algorithms in the optimizer, which is why it is slow.

[WalterBright]

Don't approve yet. I'm going to try out another idea.

[WalterBright]

I completely changed the approach. Instead of connecting ExpStatements with OPcomma, it creates a new block for each ExpStatement. It resolves the stack overflow by changing the data structure from recursive to iterative. But it does not resolve the quadratic behavior in the optimizer. I cut the iterations in the test case from 100,000 to around 8,000 so it does not bog down the test suite.

There are already separate bugzilla issues for the quadratic behavior, so I won't concern this PR with them.

[WalterBright]

The test failures have something to do with -preview=fieldwise. Will investigate more tomorrow.

[WalterBright]

This PR uncovered an unrelated optimizer bug. I disabled the problem code, and will fix the optimizer in a followup PR.

[WalterBright]

Ubuntu test fails with:

CI agent stopped responding!

[WalterBright]

Windows_VisualD_LDC x86-mscoff failed

"D:\a\1\s\generated\Windows\RelWithAsserts\Win32\dmd.exe" -conf= -g -m32mscoff -O -w -preview=dip1000 -I../druntime\import -unittest -version=StdUnittest -version=CoreUnittest -c  -ofunittest2a.obj std\array.d  std\functional.d  std\path.d  std\outbuffer.d  std\utf.d

core.exception.AssertError@..\dmd\backend\cgcod.d(1045): Assertion failure

I have no idea why this should fail only when built with LDC?

[WalterBright]

buildkite AuburnSounds fails with:

core.exception.AssertError@source/inteli/xmmintrin.d(1154): unittest failure

Who is in charge of AuburnSounds?

[thewilsonator]

Guillaume Piolat @p0nce

[WalterBright]

@p0nce please come help with #12409

Implemented using a mixin.

[WalterBright]

@Geod24 This is ready to go now, but pull #12444 first. (This incorporates the changes from 12444 so it'll pass the test suite.)

The test suite failures are the usual heisenbugs which are plaguing the test suites.

[WalterBright]

@Geod24 @RazvanN7 this is ready to go.

[Geod24]

This looks like it's going to come back to haunt you down the road 🤷

[WalterBright]

Yay!

[MoonlightSentinel]

Maybe related to these changes?
https://issues.dlang.org/show_bug.cgi?id=22098

[CyberShadow]

This pull request introduced a regression:
https://issues.dlang.org/show_bug.cgi?id=22175

[FeepingCreature]

It's still regressed on master. Has been since 2.097.

[ibuclaw]

This PR introduced a regression https://issues.dlang.org/show_bug.cgi?id=23084