🚀 go-pugleaf

On 1 Nov 90 22:33:35 GMT, jlg@lanl.gov (Jim Giles) said:

jlg> From article <PCG.90Nov1171210@odin.cs.aber.ac.uk>, by
jlg> pcg@cs.aber.ac.uk (Piercarlo Grandi):

pcg> In a way or another this implies that each language implies a virtual
pcg> machine. The task of the code generator is to map the virtual machine
pcg> onto the real machine as well as it can, which means faithfully
pcg> (semantics) and efficiently (pragmatics).

jlg> Unfortunately for your argument, there are few languages which have
jlg> definitions of this kind.  The definition of a language usually
jlg> implies a _class_ of virtual machines.

Yes, of course a virtual machine may be left unspecified here and there.
You are indeed allowed to view such a virtual machine as a possibly
infinite class of more precisely defined virtual machines. Nominalism.

jlg> The code generator is allowed to map any of these onto the real
jlg> hardware.  Any optimizer which generates code which faithfully
jlg> implements anything within the class of acceptable behaviours is
jlg> correct.

Again, the problem is not to show that aggressive optimization is
desirable if it assumed bug free. Indeed this idea is a red herring:

jlg> If an optimizer leaves the class of acceptable behaviours, it is
jlg> _broken_.

Write or find a compiler, optimizing or not, for a popular language that
is not _broken_ according to your definition.

Of course you cannot. Now, the interesting question is how much of a
liability in terms of reduced confidence is aggressive optimization.
Reasoning shows that in way of principle it lowers the reliability of a
compiler. Show that this is not true, or that if true there are
compensating advantages, and quantify yoru argument.

The burden of proof is on proponents of aggressive optimization, becasue
they have to overcome the obvious observation that anything that
enlarges or complicates a program will tend to decrease its reliability.

jlg> You are trying to limit the compiler to code transformations that
jlg> _you_ regard as straightforward.  In truth, the compiler is only
jlg> limited by the language definition.

Only in a theoretical world. It is also limited by what is cost
effective, in the sense above. The argument here is not that aggressive
optimizations are illegal or impossible; it is whether they are the best
alternative, when compared to straightforward code generation plus
source level rewriting assistants, from an engineering point of view.
--
Piercarlo "Peter" Grandi           | ARPA: pcg%uk.ac.aber.cs@nsfnet-relay.ac.uk
Dept of CS, UCW Aberystwyth        | UUCP: ...!mcsun!ukc!aber-cs!pcg
Penglais, Aberystwyth SY23 3BZ, UK | INET: pcg@cs.aber.ac.uk