[Git][ghc/ghc][wip/T18078] 3 commits: DmdAnal: Improve handling of precise exceptions
Simon Peyton Jones
gitlab at gitlab.haskell.org
Thu May 21 11:58:05 UTC 2020
Simon Peyton Jones pushed to branch wip/T18078 at Glasgow Haskell Compiler / GHC
Commits:
9bd20e83 by Sebastian Graf at 2020-05-15T10:42:09-04:00
DmdAnal: Improve handling of precise exceptions
This patch does two things: Fix possible unsoundness in what was called
the "IO hack" and implement part 2.1 of the "fixing precise exceptions"
plan in
https://gitlab.haskell.org/ghc/ghc/wikis/fixing-precise-exceptions,
which, in combination with !2956, supersedes !3014 and !2525.
**IO hack**
The "IO hack" (which is a fallback to preserve precise exceptions
semantics and thus soundness, rather than some smart thing that
increases precision) is called `exprMayThrowPreciseException` now.
I came up with two testcases exemplifying possible unsoundness (if
twisted enough) in the old approach:
- `T13380d`: Demonstrating unsoundness of the "IO hack" when resorting
to manual state token threading and direct use of primops.
More details below.
- `T13380e`: Demonstrating unsoundness of the "IO hack" when we have
Nested CPR. Not currently relevant, as we don't have Nested
CPR yet.
- `T13380f`: Demonstrating unsoundness of the "IO hack" for safe FFI
calls.
Basically, the IO hack assumed that precise exceptions can only be
thrown from a case scrutinee of type `(# State# RealWorld, _ #)`. I
couldn't come up with a program using the `IO` abstraction that violates
this assumption. But it's easy to do so via manual state token threading
and direct use of primops, see `T13380d`. Also similar code might be
generated by Nested CPR in the (hopefully not too) distant future, see
`T13380e`. Hence, we now have a more careful test in `forcesRealWorld`
that passes `T13380{d,e}` (and will hopefully be robust to Nested CPR).
**Precise exceptions**
In #13380 and #17676 we saw that we didn't preserve precise exception
semantics in demand analysis. We fixed that with minimal changes in
!2956, but that was terribly unprincipled.
That unprincipledness resulted in a loss of precision, which is tracked
by these new test cases:
- `T13380b`: Regression in dead code elimination, because !2956 was too
syntactic about `raiseIO#`
- `T13380c`: No need to apply the "IO hack" when the IO action may not
throw a precise exception (and the existing IO hack doesn't
detect that)
Fixing both issues in !3014 turned out to be too complicated and had
the potential to regress in the future. Hence we decided to only fix
`T13380b` and augment the `Divergence` lattice with a new middle-layer
element, `ExnOrDiv`, which means either `Diverges` (, throws an
imprecise exception) or throws a *precise* exception.
See the wiki page on Step 2.1 for more implementational details:
https://gitlab.haskell.org/ghc/ghc/wikis/fixing-precise-exceptions#dead-code-elimination-for-raiseio-with-isdeadenddiv-introducing-exnordiv-step-21
- - - - -
568d7279 by Ben Gamari at 2020-05-15T10:42:46-04:00
GHC.Cmm.Opt: Handle MO_XX_Conv
This MachOp was introduced by 2c959a1894311e59cd2fd469c1967491c1e488f3
but a wildcard match in cmmMachOpFoldM hid the fact that it wasn't
handled. Ideally we would eliminate the match but this appears to be a
larger task.
Fixes #18141.
- - - - -
502a0efa by Simon Peyton Jones at 2020-05-21T12:57:31+01:00
Implement cast worker/wrapper properly
The cast worker/wrapper transformation transforms
x = e |> co
into
y = e
x = y |> co
This is done by the simplifier, but we were being
careless about transferring IdInfo from x to y,
and about what to do if x is a NOINLNE function.
This resulted in a series of bugs:
#17673, #18093, #18078.
This patch fixes all that:
* Main change is in GHC.Core.Opt.Simplify, and
the new prepareBinding function, which does this
cast worker/wrapper transform.
See Note [Cast worker/wrappers].
* There is quite a bit of refactoring around
prepareRhs, makeTrivial etc. It's nicer now.
* Some wrappers from strictness and cast w/w, notably those for
a function with a NOINLINE, should inline very late. There
wasn't really a mechanism for that, which was an existing bug
really; so I invented a new finalPhase = Phase (-1). It's used
for all simplifier runs after the user-visible phase 2,1,0 have
run. (No new runs of the simplifier are introduced thereby.)
See new Note [Compiler phases] in GHC.Types.Basic;
the main changes are in GHC.Core.Opt.Driver
* Doing this made me trip over two places where the AnonArgFlag on a
FunTy was being lost so we could end up with (Num a -> ty)
rather than (Num a => ty)
- In coercionLKind/coercionRKind
- In contHoleType in the Simplifier
I fixed the former by defining mkFunctionType and using it in
coercionLKind/RKind.
I could have done the same for the latter, but the information
is almost to hand. So I fixed the latter by
- adding sc_hole_ty to ApplyToVal (like ApplyToTy),
- adding as_hole_ty to ValArg (like TyArg)
- adding sc_fun_ty to StrictArg
Turned out I could then remove ai_type from ArgInfo. This is
just moving the deck chairs around, but it worked out nicely.
See the new Note [AnonArgFlag] in GHC.Types.Var
* When looking at the 'arity decrease' thing (#18093) I discovered
that stable unfoldings had a much lower arity than the actual
optimised function. That's what led to the arity-decrease
message. Simple solution: eta-expand.
It's described in Note [Eta-expand stable unfoldings]
in GHC.Core.Opt.Simplify
I made a test case for #18078, and a very similar one for #17673.
The net effect of all this on nofib is very modest, but positive:
--------------------------------------------------------------------------------
Program Size Allocs Runtime Elapsed TotalMem
--------------------------------------------------------------------------------
anna -0.4% -0.1% -3.1% -3.1% 0.0%
fannkuch-redux -0.4% -0.3% -0.1% -0.1% 0.0%
maillist -0.4% -0.1% -7.8% -1.0% -14.3%
primetest -0.4% -15.6% -7.1% -6.6% 0.0%
--------------------------------------------------------------------------------
Min -0.9% -15.6% -13.3% -14.2% -14.3%
Max -0.3% 0.0% +12.1% +12.4% 0.0%
Geometric Mean -0.4% -0.2% -2.3% -2.2% -0.1%
- - - - -
30 changed files:
- compiler/GHC.hs
- compiler/GHC/Builtin/primops.txt.pp
- compiler/GHC/Cmm/Opt.hs
- compiler/GHC/Core/Arity.hs
- compiler/GHC/Core/Coercion.hs
- compiler/GHC/Core/Lint.hs
- compiler/GHC/Core/Opt/CallArity.hs
- compiler/GHC/Core/Opt/DmdAnal.hs
- compiler/GHC/Core/Opt/Driver.hs
- compiler/GHC/Core/Opt/FloatIn.hs
- compiler/GHC/Core/Opt/FloatOut.hs
- compiler/GHC/Core/Opt/LiberateCase.hs
- compiler/GHC/Core/Opt/SetLevels.hs
- compiler/GHC/Core/Opt/Simplify.hs
- compiler/GHC/Core/Opt/Simplify/Utils.hs
- compiler/GHC/Core/Opt/SpecConstr.hs
- compiler/GHC/Core/Opt/WorkWrap.hs
- compiler/GHC/Core/Opt/WorkWrap/Utils.hs
- compiler/GHC/Core/SimpleOpt.hs
- compiler/GHC/Core/Type.hs
- compiler/GHC/Core/Unfold.hs
- compiler/GHC/Core/Utils.hs
- compiler/GHC/Iface/Tidy.hs
- compiler/GHC/Types/Basic.hs
- compiler/GHC/Types/Demand.hs
- compiler/GHC/Types/ForeignCall.hs
- compiler/GHC/Types/Id.hs
- compiler/GHC/Types/Id/Make.hs
- compiler/GHC/Types/Var.hs
- + testsuite/tests/cmm/opt/T18141.hs
The diff was not included because it is too large.
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/c4eb188afb7b97dd96a116c491b1cb007b354bf8...502a0efae33f92b39bc522b4dc21729551cef5d0
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/c4eb188afb7b97dd96a116c491b1cb007b354bf8...502a0efae33f92b39bc522b4dc21729551cef5d0
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