[parisc-linux] Re: floating point exception error

[Jim Hull]

Randolph wrote:

> i looked at this some more.. it's not that the fcnv instruction is not
> implemented by the processor, but we seem to be falling into 
> one of the overflow/underflow cases... if you adjust the value being 
> converted (say remove one of the zeros), the program works without
trapping.
> 
> page 10-9 of the pa20 arch manual gives the conditions under which a
> floating point conversion op will cause an unimplemented exception.
> however my reading of the text is that an exception is only generated
if
> the overflow/underflow exceptions are enabled. i've tried explicitly 
> calling feclearexcept(FE_ALL_EXCEPT) before doing the fp op but it
still 
> causes the unimplemented exception trap.
> The kernel debugs seem to indicate the O/U exceptions are not set as 
> well....

Actually, the important architectural statement is this one, on p. 10-8:

  If an implementation chooses not to execute an instruction, the
  instruction signals an unimplemented exception.  An unimplemented
  exception always causes a delayed trap on a later floating-point
  instruction.  It does not change the Status Register Flag bits and
  cannot be disabled.

What p. 10-9 is describing are the conditions under which the processor
is *required* to "go unimplemented" (i.e., to take an unimplemented
exception).  But what p. 10-8 says is that the processor is *allowed* to
"go unimplemented" on any FP instruction at any time for any reason.

A totally unrealistic, but still allowed, implementation would be for
all multiply-add instructions to "go unimplemented" on Tuesdays, and all
multiply-subtract instructions to do so on Thursdays.  A more realistic
example would be for a processor to "go unimplemented" for certain hard
corner-case combinations of operands and/or rounding modes.

In the particular case of the fcnv-unsigned-to-float instruction being
discussed in this thread, all PA-8xxx processors "go unimplemented" if
the MSB is 1 in the source operand.  This explains why the trap
disappears when you "remove one of the zeros" from the operand.

What it does not explain is why the original message reported a
difference between a PA-8600 and a PA-8700.  According to every internal
HP processor document and PA-RISC FP designer I've been able to track
down, this area of the design hasn't been changed since the original
PA-8000, so there shouldn't be any differences in behavior.

Can someone repeat the experiment on PA-8600 and a PA-8700 machine that
are configured identically (kernel, glibc, test program, etc.)?

 -- Jim
    HP PA-RISC/IPF Processor Architect