ALL compilers?

[COTTRELL, JAMES]

> > ...even trivial compilers will generate identical code for
> >   if(p)			/* BAD */
> >   if(p != (anytype *)0)	/* GOOD */
> 
> ALL C COMPILERS GENERATE IDENTICAL CODE FOR THOSE TWO CASES.  Anything that
> does *not* generate identical code for them - even if that means comparing
> the bits stored in "p" with the bit pattern of 0x555555 - is not a C
> compiler.  It is a compiler for a language which resembles C in some ways
> but isn't C.
>	Guy Harris

This is probably quibbling. It all depends on the degree of optimization
in the compiler. Lets take the two statements (on a PDP-11):

	`if (p)'	vs	`if (p != 0)'
asm:	tst	p		cmp	p,#0
	beq	false		beq	false

Now a smart optimizer knows how to optimize the compare with zero into
a test, but a simple, literal-minded do-what-I-say not what-I-mean
one semester three credit course compiler might not optimize.
It get hairier with something like:

	`if (a = b)'	vs	`if ((a = b) != 0)'
asm:	mov	a,b		mov	a,b
				cmp	a,#0 (or tst a)
	beq	false		beq	false

Here we can optimize away the compare instruxion entirely.
The point is that both would be C compilers, one would just be better.

Guy is right, however, in that programmers should not get hooked
on details like this. Both are *conceptually* equivalent & I would
bet that all *real* C compilers do optimize them to *be* equivalent.
I used to worry about multiplying & dividing by powers of two &
wrote them as shifts until I saw that the optimizer did that for me.

	jim		cottrell at nbs
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