[LLVMdev] [cfe-dev] Odd PPC inline asm constraint

[Hal Finkel]

On Tue, 01 May 2012 21:25:29 -0500
Peter Bergner <bergner at vnet.ibm.com> wrote:

> On Tue, 2012-05-01 at 19:58 -0500, Peter Bergner wrote:
> > On Tue, 2012-05-01 at 17:47 -0500, Hal Finkel wrote:
> > >  By default it should build for
> > >  whatever the current host is (no special flags required). To
> > >  specifically build for something else, use:
> > >  -ccc-host-triple powerpc64-unknown-linux-gnu
> > >  or
> > >  -ccc-host-triple powerpc-unknown-linux-gnu
> > 
> > So LLVM isn't biarch capable?  Meaning one LLVM compiler cannot
> > generate both 32-bit and 64-bit binaries?
> 
> Sorry for replying to my own message, but...
> 
> Oh, -ccc-host-triple is a compiler option and not a configure option.
> That does work, though it seems I have to link with gcc, since llvm
> still wants to link against the 64-bit crt*.o and libs.  Maybe it is
> easier to just have two separate builds.
> 
> That said, my simple dynamically linked hello world executed fine
> (ie, it was able to call into libc.so just fine), as well as an
> old C version of the SPEC97 tomcatv benchmark I have laying around.
> So it seems both 32-bit and 64-bit can call into shared libs.
> 
> Not to say I haven't seen some code gen warts (using -O3). :)
> 
> From hello.s:
> 
>     main:
>         mflr 0
>         stw 31, -4(1)
>         stw 0, 4(1)
>         stwu 1, -16(1)
>         lis 3, .Lstr at ha
>         mr 31, 1
>         la 3, .Lstr at l(3)
>         bl puts
>         li 3, 0
>         addi 1, 1, 16
>         lwz 0, 4(1)
>         lwz 31, -4(1)
>         mtlr 0
>         blr 
> 
> By the strict letter of the 32-bit ABI, the save and restore of
> r31 at a negative offset of r1 is verboten.  The ABI states the
> the stack space below the stack pointer is declared as volatile.
> I actually debugged a similar problem way back in my Blue Gene/L
> days, where gcc had a bug and was doing the same thing.  We ended
> up taking a signal between the restore of the stack pointer and
> the restore of the nonvolatile reg and the BGL compute node kernel
> trashed the stack below the stack pointer.

Just to confirm, this is an issue specific to the 32-bit ABI, correct?
gcc (4.4.6) seems to do the same thing for PPC64.

Thanks again,
Hal

> 
> The second wart is the dead copy to r31...which leads to the
> unnecessary save and restore of r31.
> 
> For tomcatv, we have to basically save/restore the entire set
> of non-volatile integer and fp registers.  Looking at how
> llvm does that shows:
> 
>         ...
>         lis 3, 56
>         ori 3, 3, 57680
>         stwx 16, 31, 3
>         lis 3, 56
>         ori 3, 3, 57684
>         stwx 17, 31, 3
>         lis 3, 56
>         ori 3, 3, 57688
>         stwx 18, 31, 3
>         lis 3, 56
>         ori 3, 3, 57692
>         stwx 19, 31, 3
>         lis 3, 56
>         ori 3, 3, 57696
>         stwx 20, 31, 3
>         lis 3, 56
>         ori 3, 3, 57700
>         stwx 21, 31, 3
>         [repeated over and over and ...]
> 
> Kind of ugly! :)  GCC on the other hand stashes away the old value of
> the stack pointer and then uses small negative offsets (legal at this
> point since we've already decremented the stack pointer) from that for
> all of its saves/restores:
> 
>         ...
>         lis 0,0xffc7
>         mr 12,1
>         ori 0,0,7728
>         stwux 1,1,0
>         mflr 0
>         stw 0,4(12)
>         stfd 14,-144(12)
>         stfd 15,-136(12)
>         stfd 16,-128(12)
>         stfd 17,-120(12)
>         stfd 18,-112(12)
>         ...
> For things that don't work, do you have a small example program
> that shows what's wrong?
> 
> Peter
> 
> 
> 
> 



-- 
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory