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\section{The Alpha Back End}

\subsection{Trap Shadows, Floating Exceptions, and Denormalized Numbers on the DEC Alpha}

\emph{By Andrew W. Appel and Lal George, Nov 28, 1995}

See section 4.7.5.1 of the \emph{Alpha Architecture Reference Manual}.

The Alpha has imprecise exceptions, meaning that if a floating
point instruction raises an IEEE exception, the exception may
not interrupt the processor until several successive instructions have
completed.  ML, on the other hand, may want a "precise" model
of floating point exceptions.

Furthermore, the Alpha hardware does not support denormalized numbers
(for gradual underflow'').  Instead, underflow always rounds to zero.
However, each floating operation (add, mult, etc.) has a trapping
variant that will raise an exception (imprecisely, of course) on
underflow; in that case, the instruction will produce a zero result
AND an exception will occur.  In fact, there are several variants
of each instruction; three variants of MULT are:
\begin{description}
\item[MULT  s1,s2,d]  truncate denormalized result to zero; no exception
\item[MULT/U  s1,s2,d] truncate denormalized result to zero; raise UNDERFLOW
\item[MULT/SU  s1,s2,d]  software completion, producing denormalized result
\end{description}

The hardware treats the \verb|MULT/U| and \verb|MULT/SU|
instructions identically,
truncating a denormalized result to zero and raising the UNDERFLOW
exception.  But the operating system, on an UNDERFLOW exception,
examines the faulting instruction to see if it's an \verb|/SU|
form, and if so,
recalculates \verb|s1*s2|, puts the right answer in \verb|d|, and continues,
all without invoking the user's signal handler.

Because most machines compute with denormalized numbers in hardware,
to maximize portability of SML programs, we use the \verb|MULT/SU| form.
(and \verb|ADD/SU|, \verb|SUB/SU|, etc.)  But to use this form successfully,
certain rules have to be followed.  Basically, d cannot be the same
register as s1 or s2, because the opsys needs to be able to
recalculate the operation using the original contents of s1 and s2,
and the MULT/SU instruction will overwrite d even if it traps.

More generally, we may want to have a sequence of floating-point
instructions.  The rules for such a sequence are:

1. The sequence should end with a \verb|TRAPB| (trap barrier) instruction.
(This could be relaxed somewhat, but certainly a \verb|TRAPB| would
be a good idea sometime before the next branch instruction or
update of an ML reference variable, or any other ML side effect.)
2. No instruction in the sequence should destroy any operand of itself
or of any previous instruction in the sequence.
3. No two instructions in the sequence should write the same destination
register.

We can achieve these conditions by the following trick in the
Alpha code generator.  Each instruction in the sequence will write
to a different temporary; this is guaranteed by the translation from
ML-RISC.  At the beginning of the sequence, we will put a special
pseudo-instruction (we call it \verb|DEFFREG|) that defines''
the destination
register of the arithmetic instruction.  If there are $K$ arithmetic
instructions in the sequence, then we'll insert $K$
\verb|DEFFREG| instructions
all at the beginning of the sequence.
Then, each arithop will not only define'' its destination temporary
but will use'' it as well.  When all these instructions are fed to
the liveness analyzer, the resulting interference graph will then
have inteference edges satisfying conditions 2 and 3 above.

Of course, \verb|DEFFREG| doesn't actually generate any code.  In our model
of the Alpha, every instruction generates exactly 4 bytes of code
except the span-dependent'' ones.  Therefore, we'll specify \verb|DEFFREG|
as a span-dependent instruction whose minimum and maximum sizes are zero.

At the moment, we do not group arithmetic operations into sequences;
that is, each arithop will be preceded by a single \verb|DEFFREG| and
followed by a \verb|TRAPB|.  To avoid the cost of all those \verb|TRAPB|'s,
we should improve this when we have time.  Warning:  Don't put more
than 31 instructions in the sequence, because they're all required
to write to different destination registers!

What about multiple traps?  For example, suppose a sequence of
instructions produces an Overflow and  a Divide-by-Zero exception?
ML would like to know only about the earliest trap, but the hardware
will report \emph{BOTH} traps to the operating system.  However, as long
as the rules above are followed (and the software-completion versions
of the arithmetic instructions are used), the operating system will
have enough information to know which instruction produced the
trap.  It is very probable that the operating system will report \emph{ONLY}
the earlier trap to the user process, but I'm not sure.

For a hint about what the operating system is doing in its own
trap-handler (with software completion), see section 6.3.2 of
\emph{OpenVMS Alpha Software}'' (Part II of the Alpha Architecture
Manual).  This stuff should apply to Unix (OSF1) as well as VMS.