Re: [patch/RFA] multiarch INSTRUCTION_NULLIFIED

[Andrew Cagney <cagney@gnu.org>]

Randolph Chung wrote:
>>   i was trying to figure out that piece of commented out code too and why
>>   it was changed. looked through cvs history but didn't find it. I'm not
>>   particularly fond of introducing new almost-arch-specific gdbarch
>>   methods either, but this does seem to be doing slightly different things
>>   than the existing ones. OTOH this is almost a "cosmetic" feature, so one
>>   alternative is to drop the INSTRUCTION_NULLIFIED logic completely....
>>
>>Perhaps that isn't such a bad idea if it doesn't confuse GDB too much.
> 
> 
> i did some more investigations.... turns out this is not a cosmetic
> piece of code at all :)
> 
> suppose we have a function that ended with a branch-with-nullify-next
> instruction back to the caller. if you did a "step" on the branch, and
> we don't skip the nullified instruction, we would end up on the
> nullified instruction which actually belongs to the next function.
> 
> for example:
> 
> Dump of assembler code for function call_with_trampolines:
> 0x0001217c <call_with_trampolines+0>:   copy r3,r1
> 0x00012180 <call_with_trampolines+4>:   copy sp,r3
> 0x00012184 <call_with_trampolines+8>:   stw,ma r1,40(,sp)
> 0x00012188 <call_with_trampolines+12>:  ldi -28,r19
> 0x0001218c <call_with_trampolines+16>:  fstd fr5,r19(,r3)
> 0x00012190 <call_with_trampolines+20>:  ldi -28,r19
> 0x00012194 <call_with_trampolines+24>:  fldd r19(,r3),fr22
> 0x00012198 <call_with_trampolines+28>:  fcpy,dbl fr22,fr4
> 0x0001219c <call_with_trampolines+32>:  ldo 40(r3),sp
> 0x000121a0 <call_with_trampolines+36>:  ldw,mb -40(,sp),r3
> 0x000121a4 <call_with_trampolines+40>:  bv,n r0(rp)
> End of assembler dump.
> (gdb) disassemble 0x121a8
> Dump of assembler code for function marker_indirect_call:
> 0x000121a8 <marker_indirect_call+0>:    copy r3,r1
> 0x000121ac <marker_indirect_call+4>:    copy sp,r3
> 0x000121b0 <marker_indirect_call+8>:    stw,ma r1,40(,sp)
> 0x000121b4 <marker_indirect_call+12>:   ldo 40(r3),sp
> 0x000121b8 <marker_indirect_call+16>:   ldw,mb -40(,sp),r3
> 0x000121bc <marker_indirect_call+20>:   bv,n r0(rp)
> 
> if we are at 0x121a4, and we do a step, it will stop at
> "marker_indirect_call" instead of back at the caller of
> "call_with_trampolines". since the insn at 0x121a8 is not actually
> executed in this call sequence, the correct thing to do is to blindly
> step past the nullified instruction before we make any decision on what
> to do.
> 
> here's an updated patch to multiarch INSTRUCTION_NULLIFIED, with updated
> comments and a somewhat cleaner interface.
> 
> comments? ok to check in?

I'm still not convinced (it's me and not mark you need to convince here ;-).

Is it possible to create a core file with the PC sitting on the 
nullified instruction, and if such a beast is created, how can GDB 
correctly handle it - GDB clearly can't single step the core file.  If 
that problem is solved, this method becomes redundant.

Andrew