[Mac:Xcode] Mysterious pointer-alignment requirement #97693

New Issue

Loren Osborn · 2022-04-29T00:07:02+02:00

Loren Osborn commented

2022-04-29 00:07:02 +02:00

System Information
Operating system: macOS-12.3.1-x86_64-i386-64bit 64 Bits
Graphics card: AMD Radeon Pro 5500M OpenGL Engine ATI Technologies Inc. 4.1 ATI-4.8.15

Blender Version
Broken: version: 3.2.0 Alpha, branch: origin/master, commit date: 2022-04-28 04:03, hash: 198a763944
Worked: (newest version of Blender that worked as expected)

Short description of error
This issue was detected by Xcode's "Undefined Behavior Sanitizer"
For some reason Xcode is complaining that pointers to blender::deg::Node objects and it's 4 derived classes must be 16 byte aligned (on a system with 8 byte pointers). Keep in mind, it seems it is requiring the pointers to be aligned... not the objects they are pointing to. It also seems to only affect these 4 classes.

I am pretty stumped on this second issue, so I am request additional help

Exact steps for others to reproduce the error

Open blender project in Xcode as per "Build as an Xcode project" tutorial
Edit the "schema" via Product->Schema->Edit Schema..
Under the "Run" side panel, select the "Diagnostic" page
Under "Runtime Sanitization" (first section) check "Undefined Behavior Sanitizer" (last item in section)
Build and run blender
After blender launches, go into the "Issue Navigator" sidebar (5th icon at the top of the left sidebar) and select "Runtime"
You will see two types of alignment issues reported (that I am submitting as separate bugs)

(This issue) for some reason I am not yet able to determine yet, the system expects blender::deg::Node* (and its 4 derived classes) to be aligned to 16 byte boundaries.
(#97691 / D14798) within the RNA function dispatch mechanism, ParameterList items are being written and read without to memory alignment.

This issues is present at Blender startup without needing to load any .blend file.

I am including a stack dump of the issue log with stack traces. I hope it will help diagnose the issue.

alignment_issue_dump_traces.txt

**System Information** Operating system: macOS-12.3.1-x86_64-i386-64bit 64 Bits Graphics card: AMD Radeon Pro 5500M OpenGL Engine ATI Technologies Inc. 4.1 ATI-4.8.15 **Blender Version** Broken: version: 3.2.0 Alpha, branch: origin/master, commit date: 2022-04-28 04:03, hash: 198a763944 Worked: (newest version of Blender that worked as expected) **Short description of error** This issue was detected by Xcode's "Undefined Behavior Sanitizer" For some reason Xcode is complaining that pointers to `blender::deg::Node` objects and it's 4 derived classes must be 16 byte aligned (on a system with 8 byte pointers). Keep in mind, it seems it is requiring the **pointers** to be aligned... not the objects they are pointing to. It also seems to only affect these 4 classes. I am pretty stumped on this second issue, so I am request additional help **Exact steps for others to reproduce the error** 1. Open blender project in Xcode as per "Build as an Xcode project" tutorial 2. Edit the "schema" via Product->Schema->Edit Schema.. 3. Under the "Run" side panel, select the "Diagnostic" page 4. Under "Runtime Sanitization" (first section) check "Undefined Behavior Sanitizer" (last item in section) 5. Build and run blender 6. After blender launches, go into the "Issue Navigator" sidebar (5th icon at the top of the left sidebar) and select "Runtime" 7. You will see two types of alignment issues reported (that I am submitting as separate bugs) * (This issue) for some reason I am not yet able to determine yet, the system expects `blender::deg::Node*` (and its 4 derived classes) to be aligned to 16 byte boundaries. * (#97691 / [D14798](https://archive.blender.org/developer/D14798)) within the RNA function dispatch mechanism, ParameterList items are being written and read without to memory alignment. This issues is present at Blender startup without needing to load any .blend file. I am including a stack dump of the issue log with stack traces. I hope it will help diagnose the issue. [alignment_issue_dump_traces.txt](https://archive.blender.org/developer/F13036070/alignment_issue_dump_traces.txt)

Loren Osborn commented

2022-04-29 00:07:02 +02:00

Added subscriber: @linux_dr

Ray molenkamp commented

2022-04-29 01:56:15 +02:00

Added subscriber: @LazyDodo

Ray molenkamp commented

2022-04-29 01:56:15 +02:00

Changed status from 'Needs Triage' to: 'Archived'

Ray molenkamp closed this issue

2022-04-29 01:56:15 +02:00

Ray molenkamp commented

2022-04-29 01:56:15 +02:00

for developer questions please use devtalk.blender.org, i left you some pointers on chat how to improve your questions.

Brecht Van Lommel commented

2022-04-29 02:25:26 +02:00

Added subscriber: @brecht

Brecht Van Lommel commented

2022-04-29 02:25:26 +02:00

For reference, this is most likely due to the use of MEM_CXX_CLASS_ALLOC_FUNCS in the depsgraph code.

The operator new overloading there does not seem to take into account alignment.

For reference, this is most likely due to the use of `MEM_CXX_CLASS_ALLOC_FUNCS` in the depsgraph code. The `operator new` overloading there does not seem to take into account alignment.

Loren Osborn commented

2022-04-29 15:10:20 +02:00

@brecht, I will look into this. I didn’t look for a custom operator new that might not respect memory alignments bigger than maxalign_t. That said, my first attempt to resolve this was adding alignas(sizeof(void*)*2) to all 5 affected classes, which did not resolve the issue.

Because of this, I [perhaps wrongly] presumed that the issue was the alignment of the pointers, rather than the objects pointed to. The pointers are mostly allocated on the stack so would not be affected by operator new; so I wrote a special AlignedRawPointer<> class, based on AlignedStorage<>, and replaced all blender::deg::Node*s (and derived pointers) with this custom pointer type, but haven’t gotten it to compile properly yet.

I will investigate if operator new properly handles 16 byte alignments, and resolve that, before I waste any more time on my AlignedRawPointer<> implementation.

I did a quick search for MEM_CXX_CLASS_ALLOC_FUNCS and did not find much relevant. Thank you for the tip; I will investigate it more thoroughly. Thank you for the suggestion.

If you have any other suggestions, I’d appreciate them.

@brecht, I will look into this. I didn’t look for a custom `operator new` that might not respect memory alignments bigger than `maxalign_t`. That said, my first attempt to resolve this was adding `alignas(sizeof(void*)*2)` to all 5 affected classes, which did not resolve the issue. Because of this, I [perhaps wrongly] presumed that the issue was the alignment of the **pointers**, rather than the objects pointed to. The pointers are mostly allocated on the **stack** so would not be affected by `operator new`; so I wrote a special `AlignedRawPointer<>` class, based on `AlignedStorage<>`, and replaced all `blender::deg::Node*`s (and derived pointers) with this custom pointer type, but haven’t gotten it to compile properly yet. I will investigate if `operator new` properly handles 16 byte alignments, and resolve that, before I waste any more time on my `AlignedRawPointer<>` implementation. I did a quick search for `MEM_CXX_CLASS_ALLOC_FUNCS` and did not find much relevant. Thank you for the tip; I will investigate it more thoroughly. Thank you for the suggestion. If you have any other suggestions, I’d appreciate them.

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[Mac:Xcode] Mysterious pointer-alignment requirement #97693