comp: Fix alignment of bitfields in some edge cases. #3247
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In order to properly compute the size of the bitfield for overaligned fields, we need to know the offset in the struct, not just in the allocation unit. E.g., something like: char; // Byte 1 char: 8; // Byte 2 short: 16; // Bytes 3 and 4 Should align differently than: char: 8; // Byte 1 short: 16; // Bytes 3 and 4 If we can't find the start offset in the struct, we fall back to our previous behavior (this can happen with C++ templates for example). We remove some code related to `is_ms_struct`, which was effectively dead. If someone cares about it they can resurrect it. We also don't align per-bitfield unit, but in order to mostly preserve behavior, we keep inserting a field at the beginning of the struct rather than using repr(align). Otherwise we hit rust-lang#2179 in cases where we did not use to. We might want to extend that approach to repr(align) stuff, in order to try to fix rust-lang#2179 more often, but for now do the minimal change. Fixes rust-lang#1377 Fixes rust-lang#3105 Fixes rust-lang#743 Fixes rust-lang#981 Fixes rust-lang#1314 Co-authored-by: qinghon <[email protected]>
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In order to properly compute the size of the bitfield for overaligned fields, we need to know the offset in the struct, not just in the allocation unit.
E.g., something like:
char; // Byte 1
char: 8; // Byte 2
short: 16; // Bytes 3 and 4
Should align differently than:
char: 8; // Byte 1
short: 16; // Bytes 3 and 4
If we can't find the start offset in the struct, we fall back to our previous behavior (this can happen with C++ templates for example).
We remove some code related to
is_ms_struct, which was effectively dead. If someone cares about it they can resurrect it.We also don't align per-bitfield unit, but in order to mostly preserve behavior, we keep inserting a field at the beginning of the struct rather than using repr(align). Otherwise we hit #2179 in cases where we did not use to.
We might want to extend that approach to repr(align) stuff, in order to try to fix #2179 more often, but for now do the minimal change.
Fixes #1377
Fixes #3105
Fixes #743
Fixes #981
Fixes #1314