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Re: optimization/6162: gcc 3.0.4: certain i386 asm reloader ice
- From: Michael Matz <matz at suse dot de>
- To: Kevin Ryde <user42 at zip dot com dot au>
- Cc: Nathanael Nerode <neroden at twcny dot rr dot com>, <gcc-gnats at gcc dot gnu dot org>,<gcc-bugs at gcc dot gnu dot org>, <gcc at gcc dot gnu dot org>,Torbjörn Granlund <tege at swox dot com>
- Date: Wed, 30 Oct 2002 23:38:45 +0100 (CET)
- Subject: Re: optimization/6162: gcc 3.0.4: certain i386 asm reloader ice
Hi,
On Thu, 31 Oct 2002, Kevin Ryde wrote:
> > Well, to find it is quite easy. reload is inherently incapable of
> > handling more than one commutative operand, but there are two of them in
> > the asm. If you are unlucky both of them would need to be swapped to make
> > the insn valid, and this is what breaks reload, as it only can swap the
> > last pair. I don't know why this constraint isn't documented, but anyway,
> > that's the reason.
>
> The stuff in question is from longlong.h actually (shared by GMP and
> GCC). All the add_ssaaaa's (or it looks like all) have two
> commutatives.
Wow. Indeed. Some of them even have such funny things like:
__asm__ ("{a%I4|add%I4c} %1,%3,%4\n\t{ame|addme} %0,%2" \
: "=r" ((USItype) (sh)), \
"=&r" ((USItype) (sl)) \
: "%r" ((USItype) (ah)), \
"%r" ((USItype) (al)), \
"rI" ((USItype) (bl)));
which obviously is broken, or
__asm__ ("addu.co %1,%r4,%r5\n\taddu.ci %0,%r2,%r3" \
: "=r" ((USItype) (sh)), \
"=&r" ((USItype) (sl)) \
: "%rJ" ((USItype) (ah)), \
"rJ" ((USItype) (bh)), \
"%rJ" ((USItype) (al)), \
"rJ" ((USItype) (bl)))
which is useless (they have the same constraints, so commutativity doesn't
matter at all). Some of them probably are just lucky, like:
__asm__ ("add.f %1, %4, %5\n\tadc %0, %2, %3" \
: "=r" ((USItype) (sh)), \
"=&r" ((USItype) (sl)) \
: "%r" ((USItype) (ah)), \
"rIJ" ((USItype) (bh)), \
"%r" ((USItype) (al)), \
"rIJ" ((USItype) (bl)))
If called with only variables, or constants already in the right operands,
the swapping also doesn't take place. But if then also operand matching
comes into play (like some asms in longlong and also your example) the
chance of getting unlucky is even higher. Anyway given that longlong.h
uses it I again looked into reload, and no, it simply can't handle
correctly more than one pair which needs swapping. It keeps track of only
one operand which is commutative (the 'commutative' variable), and it only
tries each alternative twice at most (once normally and once with the last
pair swapped). Therefore I think longlong.h is broken, but I wonder why
this never popped up.
Ciao,
Michael.