Implement null dereference check #22040

rikkimax · 2025-10-28T09:16:42Z

Approved work: https://forum.dlang.org/post/[email protected]

Currently turned on to see what the CI does.

Will request Walter to review once I'm happy with CI.

Heavily inspired by OpenD with thanks to @adamdruppe.

EDIT: Special thanks to @limepoutine for helping to get both function pointers and delegate function pointers to be null checked!

dlang-bot · 2025-10-28T09:16:45Z

Thanks for your pull request and interest in making D better, @rikkimax! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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Bugzilla references

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Testing this PR locally

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dub run digger -- build "master + dmd#22040"

rikkimax · 2025-10-28T10:23:51Z

Fun fact, lazy parameters can have a null object. Oh well we just can't check for null on the object of a delegate prior to a call.

nordlow · 2025-10-28T13:56:57Z

So a NullPointerError is unrecoverable, right?

rikkimax · 2025-10-28T13:57:31Z

So I take it a NullPointerError unrecoverable, right?

It is an Error. Something has to die in response, like all Error's.

nordlow · 2025-10-28T14:00:20Z

Is the plan to have -check=nullderef=on be on my default or optionally in debug mode or something else?

adamdruppe · 2025-10-28T14:00:42Z

There's no technical reason why you can't recover from a NullPointerError in general, though you may choose not to as it can indicate you don't understand program state.

rikkimax · 2025-10-28T14:01:16Z

Will using -check=nullderef=on affect performance?

Possibly.

I intend to wire up the fast DFA engine to elide the checks. But can't do it until what does exist is proven to work.

adamdruppe · 2025-10-28T14:03:05Z

The performance impact is very small. The cpu's branch predictor handles it well and ldc's code generator is decent at removing dead checks and arranging the code blocks in a cache-friendly manner. (dmd's isn't so good but ldc is known to optimize better than dmd in many ways)

nordlow · 2025-10-29T09:54:34Z

So a single unittest in libdparse at

https://buildkite.com/dlang/dmd/builds/43787/steps/canvas?sid=019a2b1c-9d02-4993-b9df-dfb58a18783d

fails buildkite/dmd.

The libdparse/test/fail_files/killer2.d contains

unittest
{
version (53056)[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[
union {}
Klse
gosh ÿ� 'A+ SPL : tape];
		}
	}
}

. Could the CI failure be caused by the compiler existing with error code other than what the tests expects? I also wonder how these seemingly random test cases where deduced...

rikkimax · 2025-10-29T10:35:11Z

I'm aware, but the problem will be in the compiled libdparse, and without compiling it locally I won't be figuring out which emitted hook did it.

As far as I know this could be related to XMM intrinsics, and since those are more of a known problem they're up first.

rikkimax · 2025-12-09T18:25:17Z

I just spent a good 8 hours debugging automem, thinking it wasn't supposed to fail.

https://github.com/atilaneves/automem/blob/master/source/automem/ref_counted.d#L181

I wasn't even looking in the right library, thought it was Phobos's one lol.

rikkimax · 2025-12-09T19:00:46Z

For the record, here is a known good implementation for delegate function pointer check.
2930e3d#diff-54719c55eec4a6cfe3b7b7f4b5289d0b5a0b9d41e8589a023bba4d68921f8be0

The reason I'm not using that is because I think it can be improved quite significantly.

limepoutine · 2025-12-10T03:19:04Z

I just spent a good 8 hours debugging automem, thinking it wasn't supposed to fail.

https://github.com/atilaneves/automem/blob/master/source/automem/ref_counted.d#L181

I wasn't even looking in the right library, thought it was Phobos's one lol.

The test tests exactly that AssertError, i.e. that thrown in _get().

rikkimax · 2025-12-10T06:02:06Z

I just spent a good 8 hours debugging automem, thinking it wasn't supposed to fail.
https://github.com/atilaneves/automem/blob/master/source/automem/ref_counted.d#L181
I wasn't even looking in the right library, thought it was Phobos's one lol.

The test tests exactly that AssertError, i.e. that thrown in _get().

Let's just say I was barely seeing water at 3am, that's how deep I was into CI failures problems I was trying to fix!

However, on the plus side, I like the solution a lot better than we came up with previously.

limepoutine · 2025-12-10T06:10:53Z

This is much cleaner than the previous one. Thank you for the work.

rikkimax · 2025-12-10T08:24:25Z

CI has done the correct, thing.

Anything else you need @WalterBright before I turn it off and take it out of draft?

WalterBright · 2025-12-11T08:25:40Z

changelog/dmd.nullderefcheck.dd

@@ -0,0 +1,8 @@
+On null dereference check is now available


An optional check for a null dereference is added.

WalterBright · 2025-12-11T08:26:32Z

changelog/dmd.nullderefcheck.dd

@@ -0,0 +1,8 @@
+On null dereference check is now available
+
+A new check is implemented for when a null dereference occurs.


I don't understand that sentence. Perhaps: "code is inserted to check a reference for null before it is dereferenced."

WalterBright · 2025-12-11T08:27:25Z

changelog/dmd.nullderefcheck.dd

+This can be enabled by using ``-check=nullderef=on``
+What happens may be customized by the ``-checkaction`` switch and by setting a new handler in ``core.exception``.
+
+Due to issues in dmd's backend, not all pointer dereferences are guaranteed to get a check.


What does that mean?

Disabling of intrinsics and their arguments.

I don't particularly want to explain this in a changelog as it won't impact everyone, but it does need acknowledgment that dmd specifically can't check all pointer dereferences.

I can disable the intrinsic check in a follow-up PR for you to look into.

WalterBright · 2025-12-11T08:27:46Z

changelog/dmd.nullderefcheck.dd

+A new check is implemented for when a null dereference occurs.
+
+This can be enabled by using ``-check=nullderef=on``
+What happens may be customized by the ``-checkaction`` switch and by setting a new handler in ``core.exception``.


No mention of what the default behavior is.

Could add a brief discussion of why one would want this feature.

WalterBright · 2025-12-11T08:28:54Z

compiler/src/dmd/astenums.d

+/// What should happen when a dereference on null happens
+enum CHECKACTIONDEREF : ubyte
+{
+    D,             /// call D hook function


What is the "D hook function"?

WalterBright · 2025-12-11T08:29:41Z

compiler/src/dmd/astenums.d

+{
+    D,             /// call D hook function
+    Assert,        /// lower to an assert
+    Ignore         /// ignore


What happens if one turns on checking and then ignores it? What is the purpose of that?

Dead code, removed.

rikkimax · 2025-12-11T13:46:38Z

I've turned this off now, will need confirmation that @WalterBright doesn't want any more changes.

rikkimax force-pushed the nullderef-barrier branch 5 times, most recently from 1ca48e7 to c0d2c45 Compare October 28, 2025 10:23

rikkimax force-pushed the nullderef-barrier branch 13 times, most recently from 86f1e3b to 05957b2 Compare October 28, 2025 13:46

rikkimax force-pushed the nullderef-barrier branch from 05957b2 to 7f74a82 Compare October 28, 2025 13:57

rikkimax force-pushed the nullderef-barrier branch from 7f74a82 to 0686734 Compare October 29, 2025 10:43

rikkimax force-pushed the nullderef-barrier branch 5 times, most recently from 2930e3d to f241687 Compare December 9, 2025 17:52

rikkimax force-pushed the nullderef-barrier branch 2 times, most recently from 32c1384 to 5ee989a Compare December 9, 2025 18:59

rikkimax force-pushed the nullderef-barrier branch 3 times, most recently from eb4a825 to 57ed5c1 Compare December 9, 2025 19:48

rikkimax force-pushed the nullderef-barrier branch from 57ed5c1 to 47116b7 Compare December 10, 2025 06:00

rikkimax force-pushed the nullderef-barrier branch from 47116b7 to 8db71ae Compare December 10, 2025 06:22

WalterBright reviewed Dec 11, 2025

View reviewed changes

rikkimax force-pushed the nullderef-barrier branch from 8db71ae to 7e31fd7 Compare December 11, 2025 08:45

Implement null dereference check

773864e

rikkimax force-pushed the nullderef-barrier branch from 7e31fd7 to 773864e Compare December 11, 2025 13:44

rikkimax marked this pull request as ready for review December 11, 2025 13:46

rikkimax requested a review from ibuclaw as a code owner December 11, 2025 13:46

		@@ -0,0 +1,8 @@
		On null dereference check is now available

		A new check is implemented for when a null dereference occurs.

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Implement null dereference check #22040

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