panic when compiling SHA

[Ben Lippmeier]

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.

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.