Re: MIPS div_trap vs. general conditional traps

[Jeffrey A Law <law at cygnus dot com>]

  In message <200009062017.NAA06786@kayak.mcgary.org>you write:
  > I implemented conditional traps for MIPS using the MIPS
  > conditional-trap machine instructions for ISA>=2 and using the MIPS
  > `break' instruction for ISA==1.  I posted the patch last night.
Sounds like a good thing.


  > Since then, I have noticed that the MIPS div_trap also generates
  > TRAP_IFs, but always translates them to conditional branches to MIPS
  > BREAK instructions.  Is there any reason why we shouldn't use
  > conditional-trap machine instructions for div_trap as well, falling
  > back to conditional branches to BREAK only for ISA==1 ?
Because the expansion of division now done by the compiler needed to be
equivalent to what used to be done by the assembler in terms of 
break vs trap.

  > One reason not to do it is that operating systems might expect div
  > traps to come as BREAKs (EXC == 9), rather than as traps (EXC == 13).
Precisely.

  > However, since I doubt that OSes do anything other than terminate the
  > program that gets a div_trap, and BREAK and trap exceptions probably
  > both terminate just as effectively,
You lose the context as to why you had to stop the process.  ie, we use
different break #s for the different reasons why we might stop the
target.

  > For backward compatibility, we could always provide a `-m' option to
  > force branches to BREAKs.
We shouldn't break backwards compatibility to start with.

  > Another reason not to do it, and to favor branches to BREAK even for
  > general conditional traps is if the conditional-trap machine
  > instructions are serializing.  My MIPS manuals are silent on that
  > issue.
No clue on this one.
jeff