Re: Patches to fix optimizer bug & C++ exceptions for GCC VAX backend

["Maciej W. Rozycki" <macro%linux-mips.org@localhost>]

On Fri, 8 Apr 2016, Jake Hamby wrote:

> Thanks for the info! I've discovered a few additional clues which should 
> help, namely the optimizer pass that's introducing the problem. Through 
> process of elimination, I discovered that adding "-fno-tree-ter" will 
> prevent the unrecognizable insn error. Strangely, I can't cause the 
> problem by using "-ftree-ter" and -O0, which seems odd, unless the code 
> is checking directly for a -O flag.

 You can't turn most optimisations on at -O0, you need to globally enable 
optimisation in the first place -- any -O setting will do, e.g. -O, -Os, 
etc., as per the GCC manual:

"Most optimizations are only enabled if an `-O' level is set on the
command line.  Otherwise they are disabled, even if individual
optimization flags are specified."

So to enable a single optimisation only you'll have to use e.g. -O 
combined with a list of negated -f options to disable this level's 
optimisation defaults.  Yes, I agree this sounds like an awkward UI; I 
guess it dates back to GCC's early days and nobody bothered to fix it.  
Maybe we need -Onone or suchlike.

> I'll continue to clean up the diffs that I've been working on, and send 
> out something when I've made more progress. I like the "cc" attr code 
> that I've added to fix the overaggressive elimination of CC0 tests, but 
> the problem is that now it's too conservative, because many insns are 
> defined as calls into C code which may or may not generate insns that 
> set the condition codes. I have a few ideas on how to clean up and 
> refactor that code, but first I had to convince myself that most of 
> what's in there now are actually useful optimizations, and they seem to 
> be.

 Good luck!

> Thanks for the help!

 You are welcome!

  Maciej