https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88437
Bug ID: 88437
Summary: Excessive struct alignment on x86_64
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: jwerner at chromium dot org
Target Milestone: ---
Compile the following test file for the x86_64 target:
struct {
void *a;
void *b;
} mystruct __attribute__((__section__(".data.mystruct"));
Then run objdump -x on the output file. The .data.mystruct section will have an
alignment of 2**4, even though the largest member of the struct only requires
an alignment of 2**3. GCC always seems to align structures to the whole struct
size, at least to a maximum of 2**5.
This behavior seems questionable at best, and it is inconsistent from all other
architectures (I tested i686 and aarch64, both of which always only align to
the largest member). I assume it may be intended as a cache line
optimization(?), but it affects code size very negatively, so it should at
least be up to the user and not hardcoded by architecture. I couldn't find any
flag in the documentation to disable this. It even occurs with -Os even though
that's meant to strongly favor size over other optimizations.
