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Re: [patch] MIPS gas problems with gcc's explicit relocs
On Sat, 29 May 2004, Thiemo Seufer wrote:
> > > > asm("dla\t%0,%1" : "=r" (result) : "m" (foo));
> > > [snip]
> > > > lui $2,%highest(foo)
> > > > daddiu $2,$2,%higher(foo)
> > > > dsll $2,$2,16
> > > > daddiu $2,$2,%hi(foo)
> > > > dsll $2,$2,16
> > > > #APP
> > > > dla $2,%lo(foo)($2)
> > > > #NO_APP
> > >
> > > IMHO this is broken in the compiler. It should either only provide the
> > > sympol for %1, [...]
> >
> > This is what gcc 2.95 did and it was inferior -- for each %1 reference, a
> > full address load was performed. A better alternative (and the only one
> > if I'd use the "R" constraint; 2.95 got it wrong, though) would be
> > finishing the address load sequence before the inline asm and substituting
> > "($2)" for %1. It would waste an instruction if %1 was used for loads and
> > stores only, though.
>
> I can't follow you here. As long as gcc is supposed to leave inline asm
> alone it shouldn't attempt to do (parts of) the expansion itself. The
> resulting asm should be something like
>
> #APP
> dla $2,foo
> #NO_APP
>
> and leave the interpretation up to the assembler.
Well, "%lo(foo)($2)" is a correct address of the C variable "foo" here.
> > I like the current behavior of gcc for being optimal.
>
> Creating broken code can hardly be called optimal.
Please justify. Note e.g. "dla $4,0x100($2)" is currently OK, but you
consider "dla $4,%lo(foo)($2)" bad. Why?
> > Anyway, I think gas should be consistent with its interpretation of
> > addresses for load/store and for load address instructions, i.e. it should
> > be possible to split:
> >
> > <op> <reg>,<addr>
> >
> > into:
> >
> > la <tmp>,<addr>
> > <op> <reg>,(<tmp>)
> >
> > for any valid <addr>.
>
> This would be broken syntax.
>
> <op> <reg>,<addr>
>
> loads an absolute address in a register, while
Nope, it loads the content pointed to by addr, of which the register part
happens to be $zero.
> <op> <reg>,<addr>(<reg2>)
>
> loads the _content_ pointed to by reg2, with addr as offset.
>
> <op> <reg>,(<reg2>)
>
> merely means an offset of zero, and is invalid for (d)la.
But "la <reg>,0(<reg2>)" works.
Maciej
--
+ Maciej W. Rozycki, Technical University of Gdansk, Poland +
+--------------------------------------------------------------+
+ e-mail: macro@ds2.pg.gda.pl, PGP key available +