let app invariant failure, HALP Re: how to write a ghc primop that acts on an unevaluated argument?

Thu Nov 27 17:03:22 UTC 2014

are you quite sure you did –dverbose-core2core –ddump-occur-anal?   I see none of that output in the file.

Regardless of  the flags you set for the primop, I’m surprised that the invariant is invalidated.  Should not happen.

As well as the revised output, could you enclose a snippet of primops.txt.pp for prefetchValue1,2,3#

S

From: Carter Schonwald [mailto:carter.schonwald at gmail.com]
Sent: 25 November 2014 06:47
To: Simon Peyton Jones
Cc: Edward Kmett; ghc-devs at haskell.org; Joachim Breitner
Subject: Re: let app invariant failure, HALP Re: how to write a ghc primop that acts on an unevaluated argument?

ok attached is the log of
./inplace/bin/ghc-stage2 codetester.hs  -O2 -dcore-lint  -fforce-recomp -ddump-simpl -ddump-to-file -dverbose-core2core -ddump-occur-anal -ddump-inlinings  > output ^&1

the relevant snippet is bellow.

It looks like the Float-Out transformation is what tripping it.

That said, reading through what the semantics are implied by has_side_effects = True, the prefetch* family of operations ARE side effectful with respect the fact that they DO modify the CPU cache in a way that changes what memory locations are in various levels of CPU cache, so despite seeming read-like, they are modifying the state of the system, just in a way that is ONLY visible wrt the performance characteristics/runtime  of the resulting program.

specifically, Duplication, Floating out, and Specultive evaluation all change *when* the prefetch runs and changes the CPU state, and Discard means this "mutation" of the CPU cache state would not happen.

if prefetch was pure semantically (ie equiv to a -> ()  ) , they'd just be no op, they're exclusively a tool for performance engineering, and accordingly when and where they appear in code matters!

i'm more than happy to write out a more detailed version of this explanation somewhere if you like.

-Carter

""""

*** Core Linted result of Simplifier:
*** Float out(FOS {Lam = Just 0, Consts = True, OverSatApps = True}):
*** Core Linted result of Float out(FOS {Lam = Just 0, Consts = True, OverSatApps = True}):
*** Core Lint errors : in result of Float out(FOS {Lam = Just 0,
                                                   Consts = True,
                                                   OverSatApps = True}) ***
<no location info>: Warning:
    In the expression: prefetchValue2#
                         @ [Char]
                         @ RealWorld
                         lvl_s3Oi
                         (prefetchValue1#
                            @ Int
                            @ RealWorld
                            lvl_s3Oj
                            (prefetchAddr3# @ RealWorld ds1_a3zg 0 ds_a3zf))
    This argument does not satisfy the let/app invariant:
      prefetchValue1#
        @ Int
        @ RealWorld
        lvl_s3Oj

"""

On Mon, Nov 24, 2014 at 10:43 PM, Carter Schonwald <carter.schonwald at gmail.com<mailto:carter.schonwald at gmail.com>> wrote:
huh, apparently i was mixing up '-' and some other similar dash character, time to let my rebuild of ghc go through then try gain :)

On Mon, Nov 24, 2014 at 10:37 PM, Carter Schonwald <carter.schonwald at gmail.com<mailto:carter.schonwald at gmail.com>> wrote:
when i run
./inplace/bin/ghc-stage2 codetester.hs  -O2 -dcore-lint -S  -fforce-recomp -ddump-simpl -ddump-to-file –dverbose-core2core –ddump-occur-anal –ddump-inlinings
i get
target ‘–dverbose-core2core’ is not a module name or a source file

what am I doing wrong in this CLI invocation?

On Mon, Nov 24, 2014 at 10:23 AM, Simon Peyton Jones <simonpj at microsoft.com<mailto:simonpj at microsoft.com>> wrote:
Carter

That smells wrong to me.  These flags have a very carefully defined meaning; see
            Note [PrimOp can_fail and has_side_effects]
in PrimOp.lhs

If you say it has side effects when it doesn’t, you’ll confuse your successor reading the code in five years time.

Better to find out what is going on and why.  Might you do that? What transformation invalidates the let/app invariant?  Make a small test case, use –dverbose-core2core –ddump-occur-anal –ddump-inlinings.  I would far rather that than install a land-mine in the code.

Simon

From: Carter Schonwald [mailto:carter.schonwald at gmail.com<mailto:carter.schonwald at gmail.com>]
Sent: 24 November 2014 00:54
To: Edward Kmett
Cc: ghc-devs at haskell.org<mailto:ghc-devs at haskell.org>; Simon Peyton Jones; Joachim Breitner
Subject: Re: let app invariant failure, HALP Re: how to write a ghc primop that acts on an unevaluated argument?

woot, solved it, at least in a way thats OK for now.

if I mark the prefetchValue operations as has_side_effects=True, the core lint failure goes away! I'm not sure if thats the right semantics in the long term, but it does give me a simple way to make sure it works safely for 7.10

pardon all the noise
-Carter

On Sun, Nov 23, 2014 at 4:26 PM, Carter Schonwald <carter.schonwald at gmail.com<mailto:carter.schonwald at gmail.com>> wrote:
ok, i'm getting a let/app invariant failure when i build my test case with O1 or O2 but not without

http://lpaste.net/114881

any help would be appreciated on how to address that

On Sun, Nov 23, 2014 at 4:13 PM, Carter Schonwald <carter.schonwald at gmail.com<mailto:carter.schonwald at gmail.com>> wrote:
yup, i have that!

    wrapFetch prefetchValue0# (error "this shouldn't get evaluated")

in the test suite!

in contrast
    wrapFetch prefetchValue0# $! (error "this shouldn't get evaluated") does explode

shall I add a "should fail" test with the latter? (it doesn't seem worthwhile)

On Sun, Nov 23, 2014 at 3:53 PM, Edward Kmett <ekmett at gmail.com<mailto:ekmett at gmail.com>> wrote:
Maybe test for laziness in the argument by just putting something in that goes boom when forced, e.g. 'undefined'?

On Sun, Nov 23, 2014 at 2:04 PM, Carter Schonwald <carter.schonwald at gmail.com<mailto:carter.schonwald at gmail.com>> wrote:
Hey All,
as part of trying to get some fixups for how prefetch works into 7.10,
i'm adding a "prefetchValue" primop that prefetchs the memory location of a lifted heap value

namely

several operations of the following form

primop PrefetchValueOp1 "prefetchValue1#" GenPrimOp
   a -> State# s -> State# s
   with strictness  = { \ _arity -> mkClosedStrictSig [botDmd, topDmd] topRes }

I'd like some feedback on the strictness information design by someone who's familiar with how that piece of GHC. the idea being that prefetchValue is lazy in its polymorphic argument (it doesn't force it, it just does a prefetch on the heap location, which may or may not be evaluated).

https://phabricator.haskell.org/D350

is the code in question. And i *believe* i'm testing for being lazy in that argument correctly.

thoughts?

many thanks!
-Carter

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