Re: [patch] MIPS gas problems with gcc's explicit relocs

[Thiemo Seufer <ica2_ts at csv dot ica dot uni-stuttgart dot de>]

Maciej W. Rozycki wrote:
> Hello Richard,
> 
> > I just wanted to point out that using %lo(...) in "m" constraints
> > isn't in itself a new thing.  And that includes "m" constraints in
> > inline asms, since gcc doesn't treat them any differently from "m"
> > constraints in its own internal patterns.  For example, if you
> > select non-abicalls o32 or o64 code, I think it's been possible
> > for "m" to use %lo(...) for some time.  (Unless of course you
> > disabled it via -mno-split-addresses.)
> 
>  Well, it certainly is not forbidden, but it's never happened to me with 
> gcc 2.95.x.  I can't comment on anything between that and 3.4, though.

3.3 still leaves the argument alone.

> > If allowing %lo(...) in dla isn't acceptable (I've no opinion either
> > way really), then we'll just have to call:
> > 
> > > 	asm("dla\t%0,%1" : "=r" (result) : "m" (foo));
> > 
> > ill-formed.  You should only use "m" if the instruction can handle
> > every valid memory reference.
> 
>  Well, I think "la" and friends should accept any valid address that is
> accepted by memory transfer instructions (it's "load address" after all --
> why should it be selective on what "address" can be?).  I have no document
> providing a grammar for "la" and friends, but it's pretty well described
> in SGI's "MIPSpro Assembly Language Programmer's Guide;" for the purpose
> of references, mine is document #007-2418-004.

FWIW, I lost trust in SGI documentation some years ago.

>  It lists "la" together with "lw" and other memory load instructions as
> having "destination, address" operands (table 12, page 27).  So there's no
> differentiation between these two kinds of instructions.  Then the format 
> of "address" is specified as being one of (tables 7-8, pp 8-10):
> 
> - (base-register),
> 
> - expression,
> 
> - expression (base-register),
> 
> - index-register (base-register) (!),
> 
> - relocatable-symbol,
> 
> - relocatable-symbol +/- expression,
> 
> - relocatable-symbol +/- expression (index register).

This list seems to mention all possible variants, but not all always
valid ones.

> There's no explicit reference to %reloc operators here and they're only
> briefly mentioned elsewhere as a possibility to refer to relocations
> explicitly (section 4.9, page 24).

AFAIK the %reloc were hacked in the SGI toolchain later, and remained
scarcely documented.


Thiemo