Re: [PATCH] Improve initial RTL generation for loops

[Roger Sayle <roger at eyesopen dot com>]

> > 2002-09-10  Roger Sayle  <roger@eyesopen.com>
> >
> >	* stmt.c (expand_end_loop):  Don't rotate the loop when there
> >	are no instructions in the test, i.e. the loop is unconditional.
> >	(expand_exit_loop_if_false):  Optimize RTL generation of loop
> >	tests when the condition is always true or always false.
> >	* c-semantics.c (genrtl_do_stmt):  Optimize RTL generation of
> >	do-loops when the condition is always true.
> >	(genrtl_for_stmt):  Optimize RTL gen.eration of for-loops when
> >	the for-expression is empty.
>
> My only concern is that we may not be handling fixups correctly; don't
> we still potentially need to expand fixups for a loop with a always-false
> condition when we leave the loop nesting?  If so, does that work properly?

Thanks for the warning.  I've just rechecked my patch with respect
to fix-up issues.  Fortunately, if the loop condition is true, we
don't output a jump so nothing needs to be fixed up.  If the condition
is always false, we call expand_exit_loop, which takes care of the
fixups for us.

The potential problems with fixups only occur with conditional branches
such that one edge exits the loop and the other doesn't.  In this case
any additional code required to correct the stack on exit can't be placed
immediately of the conditional jump (because it'll be executed for both
the taken and untaken paths).  Hence, the reason for the jump over jump!
Unconditional jumps out of loops don't have this problem.

I now see that you're right that I can't just replace the "jump over
jump" completely.  However, we could try and spot cases where the
exit label is already known, so we can check whether any fixup code
is required.


Roger
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