Re: "Subtract C, add Ada"

[dobrien@seas.gwu.edu (David O'Brien)]

In article <3fnf28$s3f@Starbase.NeoSoft.COM> you wrote:
: It's a question of who optimizes your code.  In the 60s,
: when C was developed, compilers *couldn't* optimize code,
: so the programmer had to.  Many apparently-bizarre C
: capabilities are there to support optimization level -1
: (programmer does the optimizations).

C was designed in the 70's.  The earliest reference I could find to it
is the cc(1) UNIX 3rd Edition man page dated 3/15/72 from "A Quarter
Century of Unix".  The language from which C developed, B, was created
in 1970.  And the only published document on the B language, "CSTR #8 -
The Programming Language B", is dated Jan 1973.  BCPL, which B came
from, is from 1969.

So, I think that "compilers *couldn't* optimize code" may be an over
generalization.  Is this _really_ the case for 1970's compiler
technology?

: Consider the common C idiom to copy a null-terminated
: array, usually a string [syntax may not be quite right]:

:      while (*a++=*b++);

I am curious about programming culture.  Notice that when someone writes
something in C they feel they have to remove all white space.  When they
write in Ada they put in plenty and use [very] long names for every
thing.  Why is that???  No wonder people say C is hard to read.


: This is more efficient, with a non-optimizing compiler, than

:   while (*a)
:     { *a = *b; a=a+1; b=b+1}; /*or whatever is right syntax*/

: because
: * The operator ++ increments the value of while it's still
:   in a register.  The second code reads a from memory to
:   do the character assign, reads it again to increment it.

You are making a *major* assumption about the CPU this is running on.
On many smaller uP's there isn't that many registers.

: * ++ can use the assembler's increment instruction.  a=a+1
:   could not.

Since adding 1 to variable is so common place, it is trivial to add this
type of optimization (even in your parser).

: * Using the assignment as the loop expression indirectly
:   directs the compiler to keep the result of the assignment
:   in a register for reuse in the expression, instead
:   of writing it to memory, then reading it.

Again, you really are thinking with a preconceived notion of your
hardware.

-- David O'Brien	(dobrien@sea.gwu.edu)