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Bug 1758

Summary:	ei_declare_aligned_stack_constructed_variable ignores alignment requirement of custom types
Product:	Eigen	Reporter:	Christoph Hertzberg <chtz>
Component:	Core - general	Assignee:	Nobody <eigen.nobody>
Status:	CONFIRMED ---
Severity:	Crash	CC:	chtz, gael.guennebaud, jacob.benoit.1
Priority:	Lowest	Keywords:	test-needed
Version:	3.3 (current stable)
Hardware:	All
OS:	All
Whiteboard:
Bug Depends on:
Bug Blocks:	1608

Description Christoph Hertzberg 2019-10-14 12:31:58 UTC

As found on this SO post:
https://stackoverflow.com/questions/58366939/eigen3-boost-mpfr-segfault-when-alignment-is-disabled

Eigen fails to work with MPFR if compiled with `-DEIGEN_MAX_ALIGN_BYTES=0`.
The reason appears to be that the specific type used in the example requires 16 byte alignment, which however is ignored by `ei_declare_aligned_stack_constructed_variable`.

Similar issues likely arise for custom types which require more than the standard alignment.

Comment 1 Gael Guennebaud 2019-10-14 20:24:59 UTC

Do we re

Comment 2 Gael Guennebaud 2019-10-14 20:28:38 UTC

Do we really want to fix that one? I mean compiling with `-DEIGEN_MAX_ALIGN_BYTES=0` while using a scalar type requiring over-alignment at the same time does not looks right to me. I'm not against a fix, but I see it as low priority.

Comment 3 Christoph Hertzberg 2019-10-15 09:48:47 UTC

Ok, good point. We could statically assert, if someone tries to use an over-aligned type. But that requires some code to make sure that PODs don't break (e.g., alignof(double)==8 on many platforms).

--> Changed to blocking 3.x

Also, we don't really have a good mechanism to individually set the alignment of types (and I don't see good solutions without breaking the API ...)

Comment 4 Nobody 2019-12-04 18:50:07 UTC

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