John Mastro wrote:
> the only line in the diff between the two resulting
> files that mentions main_thread is that the output from the source with
> GCALIGNED says:
> 
>      .lcomm main_thread,592,8
> 
> Whereas the assembly output from the source without GCALIGNED says:
> 
>      .lcomm main_thread,592,32

Thanks, this helped me see the problem. It turns out that 'struct foo 
__attribute__ ((aligned (8)))' does not work with GCC, and that one must put the 
attribute somewhere else. This appears to be a bug in GCC, and I reported the 
GCC bug here:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82914

The workaround is easy: say 'struct __attribute__ ((aligned (8))) foo' instead. 
I installed the attached patch into the emacs-26 branch and merged it into 
master. Please give it a try.