linux-powerpc : panic in stage1 compiler

[erikd]

I was chasing a segfault that was occuring in the stage2 compiler (bug #5111) and Simon Marlowe suggested I try and find something that fails in the stage1 compiler instead as that would be easier to debug.

I ran the test suite with the stage1 compiler on both x86_64 and powerpc and found that the test testsuite/tests/ghc-regress/rts/2783.hs failed for powerpc but not on x86_64.

The testsuite/tests/ghc-regress/rts/2783.run.stderr file contains the following:

[1 of 1] Compiling Main             ( 2783.hs, 2783.o )
ghc-stage1: panic! (the 'impossible' happened)
  (GHC version 7.1.20110412 for powerpc-unknown-linux):
	LlvmCodeGen.Reg: GlobalReg (VanillaReg 7 VGcPtr) not supported!

I'm willing to invest some time on this, but will probably need some guidance.

[erikd]

Currently trying the following patch.

--- a/compiler/llvmGen/LlvmCodeGen/Regs.hs
+++ b/compiler/llvmGen/LlvmCodeGen/Regs.hs
@@ -38,6 +38,7 @@ lmGlobalReg suf reg
         VanillaReg 4 _ -> wordGlobal $ "R4" ++ suf
         VanillaReg 5 _ -> wordGlobal $ "R5" ++ suf
         VanillaReg 6 _ -> wordGlobal $ "R6" ++ suf
+        VanillaReg 7 _ -> wordGlobal $ "R7" ++ suf
         SpLim          -> wordGlobal $ "SpLim" ++ suf
         FloatReg 1     -> floatGlobal $"F1" ++ suf
         FloatReg 2     -> floatGlobal $"F2" ++ suf

[erikd]

And with that patch I get:

[1 of 1] Compiling Main             ( 2783.hs, 2783.o )
ghc-stage1: panic! (the 'impossible' happened)
  (GHC version 7.1.20110412 for powerpc-unknown-linux):
	LlvmCodeGen.Reg: GlobalReg ( -> VanillaReg 8 VGcPtr) not supported!

Obviously no simple solution there.

[simonpj]

David, might you be able look at this? Thanks! Simon

[dterei]

Sure. What is the background? I read the Erik is working on restoring PPC support but I assume that is being done using the NCG? This is a bug in the LLVM code generator. Erik are you working on the NCG for PPC or trying to get LLVM to support PPC? If the former this bug probably isn't relevant to you.

If working on LLVM I would get an unregisterised build working first which shouldn't trigger this bug since no registers will be in use. A registered build will require modification to LLVM itself.

[erikd]

Yes, I'd like to get LLVM working for powerpc. Unfortunately un-registered builds seem broken at the moment (see bug #5134), but I do have a working 7.0.3 ghc.

I'm willing to do the required modification to LLVM and push it upstream.

[dterei]

Hmm OK. I'm heading to bed now but will look into this bug and 5134 tomorrow. Getting unregistered working is defiantly the first step so I wouldn't worry about patching LLVM for now.

[simonmar]

While this is something worth fixing, it is almost certainly *not* related to #5111, which was your original problem, because we don't build the stage 2 compiler with LLVM.

[dterei]

OK I've 'fixed' this bug. However don't expect LLVM to work with a registered build of PowerPC. If you want, create a new top level trac bug to track LLVM for PowerPC. Once Ian has 5134 fixed though start working on unregistered. Once that is done start working on registered without TABLES_NEXT_TO_CODE enabled. Once that is working, get TNTC working.