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Date: Mon, 28 Mar 2016 22:19:27 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] Fix atomic_arch.h for MIPS32 R6
On Mon, Mar 28, 2016 at 09:04:51AM -0400, Rich Felker wrote:
> On Mon, Mar 28, 2016 at 05:07:39AM +0000, Jaydeep Patil wrote:
> > >> >I was just saying it makes the code less cluttered to use them
> > >> >spuriously even though we don't need to:
> > >> >
> > >> > ".set push ; "
> > >> >#if __mips_isa_rev < 6
> > >> > ".set mips2 ; "
> > >> >#endif
> > >> > "ll %0, %1 ; .set pop"
> > >> >
> > >> >or similar.
> > >> >
> > >> >It's also not clear to me whether the "m" constraint is valid anymore
> > >> >for the R6 ll/sc instructions since they take a 9-bit offset now instead of a
> > >16-bit offset.
> > >> >The compiler could generate an address expression whose offset part
> > >> >does not fit in 9 bits. In that case we may need to #if the whole
> > >> >function (or at least the __asm__ statement) separately rather than just
> > >skipping the .set mips2....
> > >> >
> > >>
> > >> The "m" constrain is still valid here, as the offset will be 0 in this case..
> > >
> > >How can you assume the offset will be 0? It's the compiler's choice what to
> > >use. For instance, a_cas(&foo->bar, t, s) is likely to have an offset equal to
> > >offsetof(__typeof__(foo),bar). AFAIK this happens in practice with small
> > >offsets in mutex structures, etc. so the bug may be unlikely to be hit, but I
> > >think it's still an incorrect-constraint bug.
> >
> > Compiler generates appropriate LL/SC based on the offset.
> > Compiler adds the offset to the base register if it does not fit 9bits.
>
> The compiler has no way of knowing that the operand will be used with
> ll with the 9-bit offset restriction; as far as it knows, it will be
> used in a normal context where a 16-bit offset is valid. I don't have
> a toolchain that will target r6, but you can try the following program
> which produces an offset of 4096 for loading p[1024]:
>
> unsigned ll1k(volatile unsigned *p)
> {
> unsigned val;
> __asm__ __volatile__ ("ll %0, %1" : "=r"(val) : "m"(p[1024]) : "memory" );
> return val;
> }
>
> I would expect this to produce errors at assembly time on r6.
Indeed, the ZC constraint seems to have been added to address this;
see:
https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html
It's not available on old gcc versions (pre-mipsr6) so I think the
attached patch would be appropriate. Let me know if it works for you.
We still need to add r6 detection in configure and the dynamic linker
name in reloc.h, and fix the pthread_arch.h issue.
Rich
View attachment "mipsr6-atomic.diff" of type "text/plain" (1280 bytes)
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