From: Iain Sandoe <iain@sandoe.co.uk>
To: GCC Development <gcc@gcc.gnu.org>
Subject: An odd case with structure field alignment.
Date: Sun, 4 Sep 2022 14:32:56 +0100 [thread overview]
Message-ID: <88B0BAB2-A649-406E-B536-EE18431858F1@sandoe.co.uk> (raw)
Hi,
I am clearly missing something here … can someone point out where it is?
https://gcc.gnu.org/onlinedocs/gcc-3.3/gcc/Variable-Attributes.html#Variable%20Attributes
in the discussion of applying this to structure fields:
"The aligned attribute can only increase the alignment; but you can decrease it by specifying packed as well."
Consider:
struct odd {
int * __attribute__((aligned(2))) a;
char c;
};
I would expect, given reading of the information on the aligned attribute, that the under-alignment of a would be ignored (since there is no packed attribute on either the field or the struct).
However, on x86_64, powerpc64 linux and x86_64, powerpc Darwin, I see that the size of the struct is sizeof(pointer) + 2 and the alignment is 2.
OTOH:
struct OK {
int __attribute__((aligned(2))) a;
char c;
};
behaves as expected (the under-alignment is ignored, silently).
as does this…
struct maybe {
int *a __attribute__((aligned(2)));
char c;
};
* the type of the pointer does not seem to be relevant (i.e. AFAICT the behaviour is the same for char * etc.)
Is there some special rule about pointers that I have not found ?
[it’s making an ABI mismatch with clang, which treats the int * as expected from the documentation quoted above]
cheers
Iain
next reply other threads:[~2022-09-04 13:32 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-04 13:32 Iain Sandoe [this message]
2022-09-05 8:53 ` Richard Biener
2022-09-05 9:06 ` Iain Sandoe
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