https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89357
Bug ID: 89357
Summary: alignas for automatic variables with alignment greater
than 16 fails
Product: gcc
Version: 8.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: kretz at kde dot org
Target Milestone: ---
Target: aarch64-*-*, arm-*-*
Test case (cf. https://godbolt.org/z/ubJge4):
void g(int &);
auto f0() {
__attribute__((aligned(128))) int x;
g(x);
}
auto f1() {
alignas(128) int x;
g(x);
}
In f0, x is aligned on 128 Bytes, in f1 it is aligned on 16 Bytes. GCC emits
the warning "requested alignment 128 is larger than 16 [-Wattributes]". The
warning is not helpful as it only states an obvious fact (128 > 16). In any
case, it is unclear why the alignment is rejected in the first place.
http://eel.is/c++draft/dcl.align#2.2 says "[...] the implementation does not
support that alignment in the context of the declaration, the program is
ill-formed". Thus, compiling with alignment of 16 is non-conforming. If the
alignment is unsupported this needs to be a error.
My preferred solution would be to just align x in f1 to 128 Bytes. Why should
alignas and the aligned attribute behave differently?
On x86, power, mips, and msp430 (tested on Compiler Explorer) f0 and f1 are
equivalent.
