Integer constant folding in the presence of new primops

[Jan Stolarek]

Nicolas, I kinda like that explanation, because it relieves me of any responsibility for this 
problem :) Still, I have reasons to suspect that this might actually be my fault. Generated Core 
is slightly different - the generated worker function accepts parameters in different order - and 
I don't know why that happens. I also don't see why this would impact performance. Looks like I 
will need to become familiar with the profiling tools that you mentioned.

Janek

Dnia środa, 19 czerwca 2013, Nicolas Frisby napisał:
> I'm also seeing performance regressions in the shootout benchmarks that I
> can't identify in the asm. The new asm looks better but performs worse,
> with a ~15% slowdown.
>
> I fired up the performance counters in my CPU and the free Intel code for
> inspecting them showed that my CPU utilization took about a 10% hit, even
> while executing fewer total instructions.
>
>   1) Jan, perhaps we're seeing the same sort of behavior — the shootout
> benchmarks have extremely hot loops (hundreds of millions of iterations
> IIRC). I used ticky profiling too, and saw no suspicious changes in any
> counters.
>
>   2) Dear Low-level Gurus: How feasible is it that a ~15% slowdown in a
> program with a very hot loop is due to incidentally inhibiting some caching
> behavior (instr? data?)? Or perhaps effecting alignment? FTR my CPU is a
> Core i7-2620M, Sandy Bridge.
>
> Thanks all.
>
> On Wed, Jun 19, 2013 at 9:27 AM, Jan Stolarek <jan.stolarek at p.lodz.pl>wrote:
> > > If it's not sorted out, can you open a ticket, put in the relevant info
> >
> > (so
> >
> > > we don't need to look at the email trail), and we can tackle it when
> > > you get here.
> >
> > Currently there's a temporary workaround: I'm using new folding rules for
> > all primitive types,
> > except for Integer, in which case I left the old folding rules unchanged.
> > This of course should
> > be modified to make all rules uniform, but for now it at least passes
> > validation. I didn't fill
> > the ticket, because the bug does not exist yet :) It only manifests
> > itself in my patches, which
> > have not been applied yet. I'll add all the information from this
> > discussion to my github fork of
> > GHC and then move it to Trac once the bug makes it to HEAD.
> >
> > What worries me more about my patches is the performance regression in
> > kahan, because I see no
> > obvious differences in the generated assembly.
> >
> > Janek
> >
> > > Simon
> > >
> > > -----Original Message-----
> > > From: ghc-devs-bounces at haskell.org
> > > [mailto:ghc-devs-bounces at haskell.org]
> >
> > On
> >
> > > Behalf Of Jan Stolarek Sent: 20 May 2013 12:35
> > > To: Ian Lynagh
> > > Cc: ghc-devs at haskell.org
> > > Subject: Re: Integer constant folding in the presence of new primops
> > >
> > > > If you remove everything but the quotInteger test from
> > > > integerConstantFolding and compile with -ddump-rule-rewrites then
> > > > you'll see that the eqInteger rule fires before quotInteger. This is
> > > > presumably comparing against 0, as the definition of quot for Integer
> > > > (in GHC.Real) is
> > > >     _ `quot` 0 = divZeroError
> > > >     n `quot` d = n `quotInteger` d
> > >
> > > Yes, I noticed these two rules firing together - perhaps that's the
> > > explanation why. I created a small program for testing:
> > >
> > > main = print quotInt
> > > quotInt :: Integer
> > > quotInt = 100063 `quot` 156
> > >
> > > I noticed that when I define eqInteger wrapper to be NOINLINE, the call
> >
> > to
> >
> > > quot is translated to Core as:
> > >
> > > Main.quotInt =
> > >   GHC.Real.$fIntegralInteger_$cquot
> > >     (__integer 100063) (__integer 156)
> > >
> > > but when I change the wrapper to INLINE I get:
> > >
> > > Main.quotInt =
> > >   GHC.Real.$fNumRatio_$cquot             <-------- NumRatio instead of
> > > IntegralInteger (__integer 100063) (__integer 156)
> > >
> > > All rule firing happens later (I used -ddump-simpl-iterations
> > > -ddump-rule-firings), except that for $fNumRatio_$cquot the quot rules
> > > don't fire.
> > >
> > > > Do you also still have eqInteger wired in? It sounds like you might
> > > > have given them both the same unique?
> > >
> > > No, they didn't have the same unique. I modified the existing rules to
> >
> > work
> >
> > > on the new primops and ignore their wrappers. At the moment I reverted
> > > these changes so that I can make progress and leave this problem for
> >
> > later.
> >
> > > Janek
> > >
> > > _______________________________________________
> > > ghc-devs mailing list
> > > ghc-devs at haskell.org
> > > http://www.haskell.org/mailman/listinfo/ghc-devs
> >
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