Behavior of touch#

[Carter Schonwald]

https://github.com/ghc/ghc/blob/8c10b67ba049477cc9ed23e61f5bd119e1cefc29/compiler/cmm/CmmMachOp.hs#L556

and
https://github.com/ghc/ghc/blob/8c10b67ba049477cc9ed23e61f5bd119e1cefc29/compiler/nativeGen/X86/CodeGen.hs#L1731

spell it out a bit more

so touch is preserved through the CMM level, and then gets erased when
doing final code gen.
Its meant to ensure on heap pointers remain reachable

On Tue, Dec 16, 2014 at 1:43 AM, Carter Schonwald <
carter.schonwald at gmail.com> wrote:
>
> the point of touch is to prevent premature GC, it actually gets erased at
> the CMM level i believe.
> That is, it only makes sense to apply touch to lifted types on the heap!
>
> On Mon, Dec 15, 2014 at 4:21 PM, Brandon Simmons <
> brandon.m.simmons at gmail.com> wrote:
>>
>> The `primitive` package exports a lifted version of the undocumented
>> `touch#`
>>
>>
>> http://hackage.haskell.org/package/ghc-prim-0.3.1.0/docs/GHC-Prim.html
>>
>> which has type:
>>
>>     touch :: PrimMonad m => a -> m ()
>>
>> I'd like to know if this works correctly in general, or will it suffer
>> from the same gotches w/r/t unboxing as with addFinalizer and Weak
>> references? i.e. must it only be passed an unboxed type?
>>
>> Brandon
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>
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