"RegAllocLinear.getStackSlotFor: out of stack slots" when compiling SHA1.hs in darcs

[apstrand]

I got the following error when compiling darcs under win32/cygwin with ghc 6.8.2 and profiling turned on.

$ make VERBOSE=1
ghc -DPACKAGE_NAME=\"darcs\" -DPACKAGE_TARNAME=\"darcs\" -DPACKAGE_VERSION=\"2.0.0pre3\" -DPACKAGE_STRING=\"darcs\ 2.0.0pre3\" -DPACKAGE_BUGREPORT=\"bugs@darcs.net\" -DSTDC_HEADERS=1 -DHAVE_SYS_TYPES_H=1 -DHAVE_SYS_STAT_H=1 -DHAVE_STDLIB_H=1 -DHAVE_STRING_H=1 -DHAVE_MEMORY_H=1 -DHAVE_STRINGS_H=1 -DHAVE_INTTYPES_H=1 -DHAVE_STDINT_H=1 -DHAVE_UNISTD_H=1 -Isrc/win32 -cpp  -package regex-compat -package QuickCheck -package mtl -package parsec -package html -package containers -package pretty -O -funbox-strict-fields  -prof -auto-all -Wall -Werror -I. -I./src -i./src -isrc/win32 -Isrc/win32 -DWIN32
-DHAVE_MAPI -no-auto-all -funfolding-use-threshold20 -c src/SHA1.lhs
ghc.exe: panic! (the 'impossible' happened)
  (GHC version 6.8.2 for i386-unknown-mingw32):
        RegAllocLinear.getStackSlotFor: out of stack slots

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

make: *** [src/SHA1.o] Error 1

Context for the darcs version I tried to compile is attached.

[apstrand]

Darcs version

[igloo]

Standalone example

[igloo]

I've added a standalone example:

$ ghc -O -funbox-strict-fields -prof -funfolding-use-threshold20 -c SHA1.lhs
ghc.exe: panic! (the 'impossible' happened)
  (GHC version 6.8.2 for i386-unknown-mingw32):
        RegAllocLinear.getStackSlotFor: out of stack slots

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

[simonmar]

I'm curious as to whether this is new or not (I suspect not). The SHA1 code is known to be a good stress test for the register allocator, in fact Ben Lippmeier used it extensively while developing the new register allocator.

You can work around the problem with -fregs-graph (to enable the new graph-colouring register allocator).

What's happening is the register allocator is exhausting the fixed number of spill slots we have; and there are currently 8k's worth. It doesn't re-use spill slots, which is one bug, but also there are no optimisations that attempt to reduce register pressue, which could be considered another bug.

Given that we want to concentrate efforts on the new allocator, and we have a workaround for this issue, I don't think it's worth investing significant time in fixing the old register allocator, so I'm moving the milestone to 6.10.

[alatter]

using the attached file:

{{{$ ghc -O -funbox-strict-fields -prof -funfolding-use-threshold20 -c SHA1.lhs -fregs-graph +RTS -K10M ghc-6.9.20080126: panic! (the 'impossible' happened)

(GHC version 6.9.20080126 for i386-unknown-linux):

regSpill: out of spill slots!

regs to spill = 814 slots left = 677

Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug}}}

The +RTS -K10M is needed to prevent stack overflow.

[guest]

Duplicated on Ubuntu 8.04 (32 bit intel), with ghc6.9.20080615. No -prof was used -- just bailed during compiling darcs-2.0.2.

-fregs-graph produced a stack overflow message +RTS -K10M produced the error noted by alatter.

[igloo]

Closing in favour of #2790.