Two days old build breakage on i386.

Karel Gardas karel.gardas at
Thu Jun 26 11:42:47 UTC 2014

On 06/26/14 12:37 PM, Johan Tibell wrote:
> I think it's reasonable to say my change triggers the issue, but I don't
> know why and I need someone who understand the register allocator better
> (e.g. Simon M) to at least give me some pointers on how to debug this.
> If this is causing more trouble than build bot breakages (which is bad
> enough), let me know and I'll either revert temporarily and or try to
> work around the issue.

it does not hurt me as long as it does not go to 7.10 release so it's 
enough time to fix that. So feel free not to revert your patch 
especially if you do have some time to look into real culprit in regs alloc.

> Also, how do I repro this if I don't have a x86 machine lying around?
> Have people been successful working on these kind of issues in a VM?

My simplest way is to install VirtualBox and inside it i386-ubuntu, give 
it 2 GB RAM, 15GB drive and this should get you going really quickly. 
Then you will need to:

sudo apt-get install gcc ghc make git (in ubuntu)

which should be enough and then just follow ghc's wiki to obtain source 
tree and bootstrap on linux...

Roughly it should be that IMHO. If not, I may do that myself and send 
you exact stepts to follow...


> On Thu, Jun 26, 2014 at 10:55 AM, Karel Gardas <karel.gardas at
> <mailto:karel.gardas at>> wrote:
>     Hello,
>     builders running on i386 building for this platform caught issue
>     which shows as a build breakage:
>     ghc-stage1: panic! (the 'impossible' happened)
>     (GHC version 7.9.20140624 for i386-unknown-linux):
>     RegAllocLinear.__allocRegsAndSpill: no spill candidates
>     allocating vreg: VirtualRegI n1Q6
>     assignment: [(c1PV,InMem 2),(n1Q5,InBoth (RealRegSingle 3)
>     0),(n1Q6,InMem 1),(n1Q7,InMem 3),(n1Q9,InReg (RealRegSingle 2))]
>     freeRegs: FreeRegs 4282318848
>     initFreeRegs: FreeRegs 4282318861
>     Please report this as a GHC bug:
>     <>
>     make[1]: ***
>     [libraries/ghc-prim/dist-__install/build/GHC/__PrimopWrappers.o] Error 1
>     libraries/ghc-prim/ <>: recipe for target
>     'libraries/ghc-prim/dist-__install/build/GHC/__PrimopWrappers.o' failed
>     Have a look for example on linux-i386 buildot log here:
>     <>
>     Anyway, this happens on Linux, FreeBSD and Solaris buildbots on i386
>     so it's OS independent and probably 32bit/i386 platform specific and
>     it's two days old breakage now. The last two night builds fail on
>     all mentioned buildbots. I'm not sure but perhaps:
>     commit d8abf85f8ca176854e9d5d0b12371c__4bc402aac3
>     Author: Johan Tibell <johan.tibell at
>     <mailto:johan.tibell at>>
>     Date:   Mon Jun 9 11:43:21 2014 +0200
>     triggers that issue? I'm not claiming that the commit is actual
>     culprit, this may be just recently un-hidden issue in linear regs
>     allocator on i386!
>     Thanks!
>     Karel

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