regression for compressed oops that require shift and add

[Deneau, Tom]

Doug --

I will try to get some more details on this.

-- Tom

> -----Original Message-----
> From: Doug Simon [mailto:doug.simon at oracle.com]
> Sent: Monday, June 02, 2014 3:30 AM
> To: Deneau, Tom
> Cc: graal-dev at openjdk.java.net
> Subject: Re: regression for compressed oops that require shift and add
> 
> The code appears to be correct for the AMD64 backend (i.e.
> UncompressPointer.emitCode()). But the HSAIL backend code looks
> plausibly correct as well. Do you know where the wrong code path is
> taken in the HSAIL backend while emitting code?
> 
> -Doug
> 
> On May 30, 2014, at 10:30 PM, Deneau, Tom <tom.deneau at amd.com> wrote:
> 
> > All --
> >
> > I have noticed a regression when I update to the latest default tip.
> > This is in the area of compressed pointers.  We typically run with
> different heap sizes which
> > force different compression schemes.  The failure is in the scheme
> where the 32-bit compressed pointer
> > must be multiplied by 8 and then add a base,  we get this for example
> with a max heap size of 31g.
> >
> > The problem is that the shift and add must only be applied if the
> original compressed oop is not 0.
> >
> > Previous codegen (good):
> > 	ld_global_s32 $s1, [$d1 + 16];
> > 	cvt_u64_u32 $d1, $s1;
> > 	cmp_eq_b1_u64 $c0, $d1, 0x0;
> > 	mad_u64 $d1, $d1, 8, 0x7fb447fff000;     // $d1 = $d1 * 8 + base
> > 	cmov_b64 $d1, $c0, 0x0, $d1;             // conditionally make
> $d1 0 if it was originally 0
> >
> > Current codegen (bad);
> > 	ld_global_s32 $s1, [$d1 + 16];
> > 	cvt_u64_u32 $d1, $s1;
> > 	mad_u64 $d1, $d1, 8, 0x7f0137fff000;
> >
> > The other compression schemes seem to work ok.
> > I confess I had not merged with default for a few weeks so my previous
> working version was based on default 8d0242a07f7e.  But I'm not sure
> where the regression happened in between there.
> >
> > I guess you would not notice this in your gate unless you ran with a
> heap size big enough to cause this kind of compression.
> >
> > -- Tom
> >