On Wed, Mar 14, 2018 at 2:23 PM, Lawrence Crowl <Lawrence@crowl.org> wrote:
On 3/14/18, Hyman Rosen <hyman.rosen@gmail.com> wrote:
> On Mar 14, 2018, Lawrence Crowl <Lawrence@crowl.org> wrote:
Second, and most imporant, C's pointer and array model enabled
programmers to write in source code the kinds of optimizations that
compilers of the day were not doing.  In particular, compilers were
not doing strength reduction in loops.  In practice, a C program
could run in half the time of a Pascal program.  Consequently, the
need for using assembler was lower, which avoided the significant
cost of writing in assembler.  That optimization ability is exactly
why C became popular.

Notice that this is an optimization written by the programmer.

The most popular quote about C back in the day was "trust the programmer."
The notion that parts of a program should be ignored because the compiler
decides their behavior is undefined and therefore don't run is antithetical to that.

The language has undefined behavior because it was targeted to systems
in which instruction sets did radically different things.

But each of those systems did something.  The compiler world generate the
obvious (remember, transparency) instructions corresponding to the code,
and the program would get whatever the result was.  There was never a
notion that undefined behavior gave the compiler permission to act as if the
program never executed undefined behavior.

As usual, Ada was on top of this.  Ada95 defined the concept of bounded error:
The general idea is that the behavior is not fully determined but nevertheless
falls within well-defined bounds.


Compiler writers are often under intense pressure to squeeze more
performance out of programs.  They have made legal code faster at the
expense of illegal programs.  In doing so, they did not change the
language.  On the surface, that is reasonable.  The major problem was
that they started performing an entirely new class of optimizations
without warning anyone.

The fundamental error is claiming that undefined behavior constitutes an
"illegal program", or rather, that behavior which may not be definable in the
same way in every environment should constitute an illegal program.  It's
my contention that the optimizationist sect of compiler writers are the people
who pushed for making such unportable programs illegal to begin with.

I have no sympathy for people who press to get the wrong answer faster.
It's the patricide asking for mercy as an orphan here.