Re: optimizing compiler against iaverage assembly programmer.

WStreett@shell.monmouth.com (Wilbur Streett) writes:
..................
>Good programmers in C understand what the CPU is up to underneath the
>covers, and probably can write Assembler if the need arises. So as
>has often been said, C is a portable assembler, and the real issue is
>if the programmer understands the base architecture of the system that
>the code is running on and the software architecture of the software
>created for the task at hand.


I think this is rapidly acquiring myth status. The C model of the
hardware is vintage 70s. For example the C idiom


int out;
err = f(in, &out);


presupposes a memory-to-memory architecture since pointers only point
to memory locations. There is also in C a presumption of simple
sequential execution. Modern architectures are register oriented and
have multiple units executing in parallel e.g. the Pentium Pro.


C compilers have to go to heroic lengths to match C code to what the
modern machine prefers e.g. in the above idiom a C compiler has to
decide whether it is safe to keep 'out' in a register and at what
points it must be flushed to memory in order to preserve the C model.


>BUT
>I doubt that you can optimize LISP as much as you can C or Assembler..


Functional languages make fewer assumptions about the hardware and (if
pure) are more easily parallelised so they can actually be optimised
better on modern architectures than C.


--
Anthony Shipman
TUSC Computer Systems Pty Ltd
E-mail: als@tusc.com.au
--

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