🚀 go-pugleaf

In article <4ijp75$19v@solutions.solon.com>
Andrew Dalgleish <andrewd@axonet.com.au> writes:
<snip>
AD: Aren't these functionally identical? 

Yes, they are. Question is can you write a program that changes all
uses of goto's to the corresponding structured blocks?

AD: 
AD:   do
AD:     statement;  
AD:   while (exp);
AD: 
AD:   label:
AD:     statement;
AD:   if (exp)
AD:     goto label;
AD: 
AD: If a compiler can't optimize this, I reserve the right to refuse 
AD: to trust it's optimizations on more complex constructs.

Try the above problem, and you will see why the compiler may have
trouble dealing with unstructured code. 

Let us take your example. In the structured form of a loop, a compiler
can specifically check for expressions that are constants and pull
them out of the loop. In the second form, it has to first check that
no other statement branches to the label from anywhere (in which case
it may or may not be expressible as a simple while loop), and only
then make the determination of what are constants. In fact, loop
optimizations are usually easier than control flow analyses that
you need to optimize the latter.

If I were designing a compiler, why should I spend time and effort
writing complicated code to recognize a quintessentially bad
programming construct? In this case, it turns out that the loop is
easy to recognize, and many compilers will probably do so: but this is
a very specific case.

Why not say what you mean? That helps the person reading the code, and
often helps the compiler. And, before you give your opinion as to what
you trust and what you do not, please try to think of the poor
compiler writer as well. 

Cheers
Tanmoy
--
tanmoy@qcd.lanl.gov(128.165.23.46) DECNET: BETA::"tanmoy@lanl.gov"(1.218=1242)
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