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Re: [PATCH] MIPS: Fix unwinding through signal frames on -EB o32.

David Daney <> writes:
> Richard Sandiford wrote:
>> David Daney <> writes:
>>> When I previously changed the mips_fallback_frame_state to return an 
>>> adjusted $pc value, I overlooked the big-endian o32 case.  For this case 
>>> (and only this case), we were adding 4 to $pc and also adding 4 to the 
>>> cfa resulting in a total adjustment of 8.  This was causing the libjava 
>>> Array_3 test to fail as the reported $pc had been moved outside of the 
>>> catch block resulting in the exception being caught at the wrong place.
>> Ah, thanks, is that why that test was failing?  I hadn't got around
>> to it yet...
>>> The fix is to base the $pc calculation on the unadjusted cfa value and 
>>> use a new (possibly adjusted) variable 'reg_base' as the base for the 
>>> rest of the registers.
>> Well, going back to the review of the original endian-correction patch:
>> your experience seems to suggest even more strongly that it would be
>> best not to adjust "new_cfa" at all, and simply add a separate offset
>> to the register accesses.  I.e.: set reg_offset to 0 or 4, and add
>> "reg_offset" to:
>>    (_Unwind_Ptr)&(sc->sc_regs[i]) - new_cfa
>> I think that's equivalent to what you're doing, so the patch is
>> preapproved with that change, if it works.
> It is equivalent, and should work.  Unfortunately the O2 is quite slow, 
> so testing will take some time...
> Perhaps I will only rebuild libgcc, that will at least save the 3 days 
> needed to bootstrap.

Sorry ;/  FWIW, I tested the revised patch below with qemu on
mips-linux-gnu, and like you say, it fixed the 4 Array_3 failures
without introducing any others.  I went ahead and installed it;
hope that's OK.


2007-10-18  David Daney  <>

	* config/mips/linux-unwind.h (mips_fallback_frame_state): Use new
	reg_offset variable to calculate register locations.

Index: gcc/config/mips/linux-unwind.h
--- gcc/config/mips/linux-unwind.h	2007-10-18 17:36:47.000000000 +0100
+++ gcc/config/mips/linux-unwind.h	2007-10-18 17:36:54.000000000 +0100
@@ -52,7 +52,7 @@ mips_fallback_frame_state (struct _Unwin
   u_int32_t *pc = (u_int32_t *) context->ra;
   struct sigcontext *sc;
-  _Unwind_Ptr new_cfa;
+  _Unwind_Ptr new_cfa, reg_offset;
   int i;
   /* 24021061 li v0, 0x1061 (rt_sigreturn)*/
@@ -85,22 +85,24 @@ mips_fallback_frame_state (struct _Unwin
     return _URC_END_OF_STACK;
-  new_cfa = (_Unwind_Ptr)sc;
+  new_cfa = (_Unwind_Ptr) sc;
   fs->regs.cfa_how = CFA_REG_OFFSET;
   fs->regs.cfa_reg = STACK_POINTER_REGNUM;
   fs->regs.cfa_offset = new_cfa - (_Unwind_Ptr) context->cfa;
-#if _MIPS_SIM == _ABIO32 && defined __MIPSEB__
   /* On o32 Linux, the register save slots in the sigcontext are
      eight bytes.  We need the lower half of each register slot,
      so slide our view of the structure back four bytes.  */
-  new_cfa -= 4;
+#if _MIPS_SIM == _ABIO32 && defined __MIPSEB__
+  reg_offset = 4;
+  reg_offset = 0;
   for (i = 0; i < 32; i++) {
     fs->regs.reg[i].how = REG_SAVED_OFFSET;
-      = (_Unwind_Ptr)&(sc->sc_regs[i]) - new_cfa;
+      = (_Unwind_Ptr)&(sc->sc_regs[i]) + reg_offset - new_cfa;
   /* The PC points to the faulting instruction, but the unwind tables
      expect it point to the following instruction.  We compensate by

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