Re: i370 port

["Ulrich Weigand" <uweigand at de dot ibm dot com>]

Paul Edwards wrote:

> One of the things that I experienced when porting 3.2.3 to MVS
> was that GCC was sensitive to the exact floating point representation.
> 
> Register selection was different depending on those slight differences.
> 
> Below is what documentation I have for it.  Dave Edwards, who
> wrote another S/370 emulator, was the one who discovered that.
> 
> Does anyone know if that was changed somewhere along the line?
> 
> BFN.  Paul.
> 
> 
> 
> 17. The assembler code generated by gccmvs when run on the
> PC is slightly different (even when the same parameters
> are used for code generation) from that when run on the
> mainframe, if -O2 is used instead of -Os. But functionally
> equivalent. This non-deterministic nature of the compiler
> is disconcerting. It seems to not always allocate registers
> consistently. This has been traced to floating point code
> in predict.c and local-alloc.c which is sensitive to the
> very small changes in floating point representation. This
> should be changed to include deltas when comparing floating
> point values. Here's an example of what's happening:

I agree that GCC output should not depend on details of the host
floating point representation.  (Ideally, the output of GCC built
as a cross-compiler should not depend on the host architecture
at all.)

However, it is hard to say whether such observations made on a
GCC 3.2 code base have any relevance to the current code -- for
example, local-alloc.c does not even exist any more, we now have
a completely new register allocator.

I'd recommend you go ahead with a port to current mainline and
verify whether you still see problems along those lines; if so,
it would be appropriate to open a bug report against GCC.

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU Toolchain for Linux on System z and Cell BE
  Ulrich.Weigand@de.ibm.com