On 2021-11-14 03:10, Phil Smith III wrote:
Unoptimised code typically has redundant instructions.
For example, a store instruction to finish an assignment
operation might be followed by a load from the same place
in a following statement.
That I could believe--the unoptimized code is dealing with the
statements as
discrete code snippets. But this was code generated for a single,
simple
statement.
No it isn't.
It contains a declaration and an initialization.
Is this bad? You would not notice the time to execute
the redundant LR instruction.
If that statement were true, there would be no need for optimization.
Rubbish.
An LR instruction is one of the smallest and fastest.
This is code that will run billions of times,
The body of the procedure will take far longer than the "LR".
Compared to that code, the time taken by the LR will be negligible.
And if it were to be run "billions" of times,
you wouldn't be running it unoptimised.
often millions in a single run;
yes, we will feel it.
No you won't.
Again, my point (well, one of them) was that this is
really pretty grim code: I'd downmark a student who produced it.
If you want better code, specify optimisation.
Corollary: the compiler generates...deliberately poor code without
optimization? Again, why?
Because you asked it to.
You specified no optimisation.
Note that I had the same ARCH() value on both versions.
That's irrelevant.
So why wouldn't the
original code use the same fancy new instruction?
Because you didn't ask for optimisation.
I would have assumed that optimization meant considering how statements
interact, not generating lousy code for each on purpose.
Specifying optimisation does not generate lousy code "on purpose".
Have you got your lines crossed?
In any case, optimisation can involve optimisation within a statement;
moving code from inside a loop to outside a loop;
and improving the transition between statements.
But I am not a compiler creator by any stretch of the imagination.
Maybe I'm
missing some design principle here, which is why I asked in the first
place...
