panic when compiling SHA

[Simon Marlow]

On 28/12/13 03:58, Ben Lippmeier wrote:
>
> On 27/12/2013, at 12:07 PM, Kazu Yamamoto (山本和彦) wrote:
>
>> Hi,
>>
>> When I tried to build the SHA library with GHC head on on 32bit Linux,
>> GHC head got panic. GHC 7.4.2 can build SHA on the same machine.
>>
>> Configuring SHA-1.6.1...
>> Building SHA-1.6.1...
>> Failed to install SHA-1.6.1
>> Last 10 lines of the build log ( /home/kazu/work/rpf/.cabal-sandbox/logs/SHA-1.6.1.log ):
>> Preprocessing library SHA-1.6.1...
>> [1 of 1] Compiling Data.Digest.Pure.SHA ( Data/Digest/Pure/SHA.hs, dist/dist-sandbox-ef3aaa11/build/Data/Digest/Pure/SHA.o )
>> ghc: panic! (the 'impossible' happened)
>>   (GHC version 7.7.20131202 for i386-unknown-linux):
>> 	regSpill: out of spill slots!
>>        regs to spill = 1129
>>        slots left    = 677
>
> There are only a fixed number of register spill slots, and when
> they're all used the compiler can't dynamically allocate more of
> them.

Not true any more in 7.8+ with the linear allocator.  I think it might 
still be true for the graph allocator, which is sadly suffering from a 
little bitrot and probably doesn't generate very good code with the new 
code generator.

So, avoiding -fregs-graph should work around this with 7.8.

Cheers,
	Simon


> This SHA benchmark is pathological in that the intermediate code expands to have many variables with long, overlapping live ranges. The underlying problem is really that the inliner and/or other optimisations have gone crazy and made a huge intermediate program. We *could* give it more spill slots, to make it compile, but the generated code would be horrible.
>
> Try turning down the optimisation level, reduce inliner keenness, or reduce SpecConstr flags.
>
> Ben.
>
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