lib32 followup (visibility issue and/or toolchain bug?)

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Hello,

Preempting some future work I'm expecting to arrive, I had a go at using
__builtin_*() in obj32.

This is formed of 2 patches on top of this series:
https://xenbits.xen.org/gitweb/?p=people/andrewcoop/xen.git;a=shortlog;h=refs/heads/xen-lib32

Patch 1 introduces lib32 beside obj32, with strlen() being the first
broken-out function, and patch 2 swaps to __builtin_strlen().

Both compile, but the difference that patch 2 introduces was unexpected.

With just lib32, and taking strsubcmp() as an example, we get:

00000000 <strsubcmp>:
   0:        83 ec 0c                 sub    $0xc,%esp
   3:        89 5c 24 04              mov    %ebx,0x4(%esp)
   7:        89 74 24 08              mov    %esi,0x8(%esp)
   b:        89 c6                    mov    %eax,%esi
   d:        89 d3                    mov    %edx,%ebx
   f:        89 d0                    mov    %edx,%eax
  11:    /-- e8 fc ff ff ff           call   12 <strsubcmp+0x12>
            12: R_386_PC32    strlen
  16:        89 c1                    mov    %eax,%ecx
  18:        89 da                    mov    %ebx,%edx
  1a:        89 f0                    mov    %esi,%eax
  1c:    /-- e8 fc ff ff ff           call   1d <strsubcmp+0x1d>
            1d: R_386_PC32    .text.strncmp
  21:        8b 5c 24 04              mov    0x4(%esp),%ebx
  25:        8b 74 24 08              mov    0x8(%esp),%esi
  29:        83 c4 0c                 add    $0xc,%esp
  2c:        c3                       ret

which all seems fine.  We get a plain PC32 relocation to strlen (which
is now in the separate library).

However, with patch 2 in place (simply swapping the plain extern for
__builtin_strlen(), we now get:

00000000 <strsubcmp>:
   0:        83 ec 0c                 sub    $0xc,%esp
   3:        89 1c 24                 mov    %ebx,(%esp)
   6:        89 74 24 04              mov    %esi,0x4(%esp)
   a:        89 7c 24 08              mov    %edi,0x8(%esp)
   e:    /-- e8 fc ff ff ff           call   f <strsubcmp+0xf>
            f: R_386_PC32    __x86.get_pc_thunk.bx
  13:        81 c3 02 00 00 00        add    $0x2,%ebx
            15: R_386_GOTPC    _GLOBAL_OFFSET_TABLE_
  19:        89 c7                    mov    %eax,%edi
  1b:        89 d6                    mov    %edx,%esi
  1d:        89 d0                    mov    %edx,%eax
  1f:    /-- e8 fc ff ff ff           call   20 <strsubcmp+0x20>
            20: R_386_PLT32    strlen
  24:        89 c1                    mov    %eax,%ecx
  26:        89 f2                    mov    %esi,%edx
  28:        89 f8                    mov    %edi,%eax
  2a:    /-- e8 fc ff ff ff           call   2b <strsubcmp+0x2b>
            2b: R_386_PC32    .text.strncmp
  2f:        8b 1c 24                 mov    (%esp),%ebx
  32:        8b 74 24 04              mov    0x4(%esp),%esi
  36:        8b 7c 24 08              mov    0x8(%esp),%edi
  3a:        83 c4 0c                 add    $0xc,%esp
  3d:        c3                       ret


The builtin hasn't managed to optimise away the call to strlen (that's
fine).  But, we've ended up spilling %ebx to the stack, calculating the
location of the GOT and not using it.

So, as it stands, trying to use __builtin_strlen() results in worse code
generation.  One thing I noticed was that we're not passing
-fvisibility=hidden into CFLAGS_x86_32, but fixing that doesn't help
either.  We do have the pragma from compiler.h, so I'm out of visibility
ideas.

Anything else I've missed?

~Andrew