[doc] empty loops

[Nathan Sidwell <nathan at codesourcery dot com>]

this patch clarifies why empty loops might be removed.  It comes
from answering http://gcc.gnu.org/ml/gcc-help/2004-11/msg00169.html

ok?

nathan
--
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2004-11-24  Nathan Sidwell  <nathan@codesourcery.com>

	* trouble.texi (Non-bugs): Clarify empty loop removal.

Index: doc/trouble.texi
===================================================================
RCS file: /cvs/gcc/gcc/gcc/doc/trouble.texi,v
retrieving revision 1.29
diff -c -3 -p -r1.29 trouble.texi
*** doc/trouble.texi	5 Nov 2004 01:36:57 -0000	1.29
--- doc/trouble.texi	24 Nov 2004 14:45:33 -0000
*************** to have a delay, so deleting them will n
*** 1217,1229 ****
  faster.
  
  However, the rationale here is that optimization of a nonempty loop
! cannot produce an empty one, which holds for C but is not always the
! case for C++.
  
! @opindex funroll-loops
! Moreover, with @option{-funroll-loops} small ``empty'' loops are already
! removed, so the current behavior is both sub-optimal and inconsistent
! and will change in the future.
  
  @item
  Making side effects happen in the same order as in some other compiler.
--- 1217,1244 ----
  faster.
  
  However, the rationale here is that optimization of a nonempty loop
! cannot produce an empty one. This held for carefully written C compiled
! with less powerful optimizers but is not always the case for carefully
! written C++ or with more powerful optimizers.
! 
! Thus GCC will remove loops whenever it can determine no operations
! occurring within the loop are externally visible (apart from the time
! taken to execute them, of course).  Be aware of this when performing
! timing tests, for instance the following loop can be completely removed,
! provided @code{some_expression} can provably not change any global state.
  
! @smallexample
! @{
!    int sum = 0;
!    int ix;
! 
!    for (ix = 0; ix != 10000; ix++)
!       sum += some_expression;
! @}
! @end smallexample
! 
! Even though @code{sum} is accumulated in the loop, no use is made of
! that summation, so the accumulation can be removed.
  
  @item
  Making side effects happen in the same order as in some other compiler.