Re: [Gold] Question about the incorrect variable alignment if the alignment is greater than the page size

[Cary Coutant <ccoutant at google dot com>]

>> In the test, I recommend that rather than using
>> large_symbol_alignment.sh you just write your main function to verify
>> that the symbols are correctly aligned.  And put a space between the
>> right parenthesis and "int".
>
> Done. I had to make artificial volatile variables because gcc always tried to
> delete the true "if" branch as a dead code (in O0 as well), eventhough
> I check in gdb
> that the condition is true:
>
> 40|   if ((reinterpret_cast<uintptr_t>(&aligned_16k_var) & 16383) != 0
> 41|       || (reinterpret_cast<uintptr_t>(&aligned_8k_var) & 8191) != 0
> 42|       || (reinterpret_cast<uintptr_t>(&aligned_4k_var) & 4095) != 0)
> 43|       return 1;
> 44+>  return 0;
>
> (gdb) p (reinterpret_cast<uintptr_t>(&aligned_16k_var) & 16383) != 0
> $2 = true
>
> Gcc probably assumes that if the variable been declared with attribute
> aligned then it will
> certanly have that alignment...

I'd suggest using a noinline function instead, like this:

bool
is_aligned(const int& var, int align) __attribute__((noinline));

bool
is_aligned(const int& var, int align)
{
  return (reinterpret_cast<uintptr_t>(&var) & (align - 1)) == 0;
}

...
  if (!is_aligned(aligned_16k_var, 16384)
      || !is_aligned(aligned_8k_var, 8192)
      || !is_aligned(aligned_4k_var, 4096))
      return 1;
...

With that, you don't need to force -O0 in the Makefile (and you didn't
have to force -g anyway), so you can simplify Makefile.am:

check_PROGRAMS += large_symbol_alignment
large_symbol_alignment_SOURCES = large_symbol_alignment.cc
large_symbol_alignment_DEPENDENCIES = gcctestdir/ld
large_symbol_alignment_LDFLAGS = -Bgcctestdir/

Also, I suggest reordering the variable declarations like this:

__attribute__ ((aligned(8192)))  int aligned_8k_var;
__attribute__ ((aligned(4096)))  int aligned_4k_var;
__attribute__ ((aligned(16384))) int aligned_16k_var;

so that there will be less chance of an accidental alignment that
makes the test pass even when the linker is broken.

+int
+main (int argc __attribute__ ((unused)),
+      char** argv __attribute__ ((unused)))

Either "main()" or "main(int, char**)" (no space before the paren) is
all you need here.

For future reference, when sending a patch, you shouldn't include the
diffs for ChangeLog or for generated files like Makefile.in. Just put
your ChangeLog entry at the top of the patch.

-cary