Re: mips address+symbol issue.

[Richard Sandiford <rsandifo at redhat dot com>]

cgd@broadcom.com writes:
> I can think of several possible solutions here:
>
>         (1) don't allow GCC to emit "dla sym+offset" for
>         -mno-explicit-relocs, or maybe just for -mno-explicit-relocs
>         and the ABIs which stuff 64-bit pointers into 32-bit object
>         files.
>
>         (2) make binutils emit long code sequences for sym + offset when
>         assembling objects for an ABI which stuffs 64-bit pointers
>         into 32-bit object files.
>
>         (3) punt.  (maybe even xfail the test for some MIPS targets w/
>         -mno-explicit-relocs.)
>
> It seems to me that (2) is *probably* the right fix, but others'
> opinions would be appreciated.  I think I'm OK with (3), too, but it
> might bite others down the road...

I guess the arguments in favour of (2) are that:

  - It would work for older gccs.  Some people refuse to use anything
    newer than 2.95, for understandable reasons.

  - It's counter-intuitive for "dla $2,a-2000000000" not to load the
    address "a-2000000000"

But I agree with Ian (and you?) that pessimising the common case would
be a bad idea.  Another downside is that it would make the macro longer
than it traditionally has been, which might lead to gcc (of whatever
vintage) generating out-of-range branches.

  (I guess that's only a minor problem though, since the MIPS port
  has traditionally been poor at predicting whether a branch will
  be out of range.)

As you say, this doesn't happen for -mexplicit-relocs, but (as you
probably guessed) that's not because we're being smart.  It's just
that we're not optimising this case particularly well.  Explicit-reloc
code could in theory contain %his and %los with same sort of big offsets
that gas uses.

The patch to implement (1) would be:

@@ -918,8 +918,7 @@ mips_symbolic_constant_p (rtx x, enum mi
   switch (*symbol_type)
     {
     case SYMBOL_GENERAL:
-      /* %hi() and %lo() can handle anything.  */
-      return true;
+      return Pmode == SImode || ABI_HAS_64BIT_SYMBOLS;
 
     case SYMBOL_SMALL_DATA:
       /* Make sure that the offset refers to something within the

...which should fix both the explicit-reloc and macro cases.  We could
do better by allowing offsets that refer to something in the underlying
object: I'll try to come up with a patch soon.

I'm reluctant to punt since the code is valid.

On a related note: what do folks think about the current default ABI for
mipsisa64-elf (i.e. the LP64 version of EABI64)?  Is it really the best
choice?  The use of 32-bit object files kind-of limits what you can with
64-bit addresses, and if you only need 32-bit addresses, you get much
better performance from the -mlong32 version.

I think we can choose whatever ABI we like for 3.4 since 3.3 used
the infamous, now-defunct "MEABI".

Richard