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Re: [patch] MIPS gas problems with gcc's explicit relocs
On Fri, 28 May 2004, Thiemo Seufer wrote:
> Maciej W. Rozycki wrote:
> [snip]
> > 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 like the current behavior of gcc for being optimal.
> [...] or handle the dla expansion correctly, preferably with
> better optimization for superscalar processors (that is, a
> lui - lui - daddiu - daddiu - dsll32 - daddu sequence).
Hmm, do you suggest gcc should interpret the inline asm anyhow?
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>.
Maciej
--
+ Maciej W. Rozycki, Technical University of Gdansk, Poland +
+--------------------------------------------------------------+
+ e-mail: macro@ds2.pg.gda.pl, PGP key available +