C backend: struct size fixes (#858) #110
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… differences A structure which decreases its alignment (using FIELD = n) is annotated with both the "packed" and the "alignment(n)" attributes. That leads to a problem if such a structure is nested inside another structure that uses a greater alignment than the child structure. In that case the field representing the nested structure is aligned with the same properties as the nested structure. So the alignment has to be increased explicitly if the nested structure is packed with a smaller alignment than the parent structure. That is accomplished by adding the "alignment(n)" attribute to those fields.
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I changed the test to work on systems with 4-byte QWord-alignment (like Linux) rather than 8-byte alignment. |
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I tested the old test case (with long+longint) from SourceForge ticket #858, and it indeed causes the error with -gen gcc for linux-x86_64, win32 and win64 (not for linux-x86). But the new test case (with byte+short) seems to work fine even with fbc 1.05.0. We should probably keep both as regression tests. ABI differences between platforms can be handled with #if statements (e.g. different #asserts for each target) in the test case. |
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I tried on win64 only and it seems that's the only platform where the modified test works. |
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I also tried on win32 and ubuntu-x86_64 and could reproduce the issue with the new test code as well. Are you sure that you cannot reproduce it on these platforms with the latest changes? Maybe I should rather create a log-test instead of a unit-test? |
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Hmm, my mistake, I tested with just the type declarations, without the Dim statement using the type, so the struct didn't get emitted by -gen gcc. After fixing that everything works (fails) as expected. |
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The issue addressed by this PR, https://sourceforge.net/p/fbc/bugs/858/, can be closed, I think. |
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Fixes #858: C backend: fix internal structure size mismatch due to wrong padding when nesting packed structures
The alignment of nested structures which are packed with a smaller alignment than the natural or specified alignment of the parent structure has to be specified explicitly, otherwise the field will be packed too.