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Re: small integers
- To: "David A. Moon" <moon@CAMBRIDGE.APPLE.COM>
- Subject: Re: small integers
- From: Rob_MacLachlan@LISP-PMAX1.SLISP.CS.CMU.EDU
- Date: Sat, 10 Oct 92 00:09:22 -0400
- Cc: info-dylan@CAMBRIDGE.APPLE.COM
- In-reply-to: Your message of Fri, 09 Oct 92 16:38:00 -0500. <9210092040.AA20496@cambridge.apple.com>
Date: Fri, 09 Oct 92 16:38:00 EDT
From: "David A. Moon" <firstname.lastname@example.org>
Subject: Re: small integers
> Date: Fri, 09 Oct 92 03:22:39 -0400
> From: Scott_Fahlman@SEF-PMAX.SLISP.CS.CMU.EDU
To be successful, many implementations of Dylan are going to have to
integrate well with the system environment in which they find
themselves. It is impossible to play in the Macintosh world without
32-bit integers; [...] I imagine most other environments, such as most
dialects of Unix, also depend on 32-bit integers.
This is exactly why CMU CL has special code generators for signed and
unsigned 32bit arithmetic. As long as you are doing word-integer
arithmetic, there is no reason to invoke the overhead of arbitrary
I have given some though to how word-integers might be supported in Dylan.
This is really a corner of the foreign interface problem. Probably there
should be a fairly clear correspondence between C/C++ types and Dylan
classes; many new classes would need to be introduced as part of the
foreign interface. Some of those might be <int>, <unsigned-int>, etc.
And of course, those classes should work with C semantics, which I believe
ignores overflow totally.
The combination of the <small-integer> class, the preferred Dylan
programming style, and type inference in the compiler should eliminate
the need to allow for bignum operands in most places where that is
possible and the programmer cares about efficiency.
Well, that hasn't been our experience in Common Lisp. Our programming
style types all function arguments and instance slots, and our type
inference is good, yet we still find the need to insert many FIXNUM output
type assertions in complex expressions.
It may still be necessary to check for overflow, but that can certainly
be open-coded with small cost, especially when the overflow causes a
type error rather than continuing the computation with bignums. This
is what Pascal (at least on the Macintosh) does, so it can't be too
Yes, I agree that the cost of detecting overflow and signalling an error is
small, which is why I propose that <small-integer> overflow should always
signal an error. However, in an overflow-to-bignum scheme, the only way
that you could signal an error instead of proceeding is if there were an
output type assertion, which won't be true of intermediate results in
There are published trace-scheduling-like techniques that [...]
doesn't CMU's Common Lisp compiler do this?
Nope. Self-style loop splitting would also work, but we'd really rather
not waste compile time and code size dealing with overflows that never
happen. We just put in declarations until the compiler tells is it's
happy. This technology does work, I just think we can do better.
My question of whether it's feasible to eliminate the overflow checking
through compiler optimization most of the time hasn't been answered
yet. I already know it's feasible to eliminate the type checking, so
only the overflow checking remains.
Yes, the handing of overflow is all we are disagreeing about. My answer is
-- yes it is usually possible, but
-- type inference fails often enough to be annoying, and
-- declaring appropriate subranges imposes a burden not present in other