Re: [PATCH] DATA_ALIGNMENT vs. DATA_ABI_ALIGNMENT (PR target/56564)

[Richard Henderson <rth@redhat.com>]

On 06/07/2013 02:14 PM, Jakub Jelinek wrote:
>> > When the linker merges common blocks, it chooses both maximum size and maximum
>> > alignment.  Thus for any common block for which we can prove the block must
>> > reside in the module (any executable, or hidden common in shared object), we
>> > can go ahead and use the increased alignment.
> But consider say:
> one TU:
> struct S { char buf[15]; } s __attribute__((aligned (32)));
> 
> another TU:
> char c = 7;
> struct S { char buf[15]; } s = { { 1, 2 } };
> char d = 8;
> int main () { return 0; }
> 
> (the aligned(32) is there just to simulate the DATA_ALIGNMENT optimization
> increase).  Linker warns about this (thus the question is if we want to
> increase the alignment for optimization on commons at all) and doesn't align
> it.
> 

Oh, right.  I hadn't considered commons unifying with non-common variables,
and the failure of commoning in that case.  I'd mostly been thinking of
uninitialized Fortran-like common blocks, where it is more common for the
blocks to have nothing in common but the name.



r~