/* Copyright (c) 2010 Jan Waclawek
   Copyright (c) 2010 Joerg Wunsch
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/* $Id: optimize.dox 2160 2010-06-10 20:45:42Z joerg_wunsch $ */

/** \page optimization Compiler optimization

\section optim_code_reorder Problems with reordering code
\author Jan Waclawek

Programs contain sequences of statements, and a naive compiler would
execute them exactly in the order as they are written. But an
optimizing compiler is free to \e reorder the statements - or even
parts of them - if the resulting "net effect" is the same. The
"measure" of the "net effect" is what the standard calls "side
effects", and is accomplished exclusively through accesses (reads and
writes) to variables qualified as \c volatile. So, as long as all
volatile reads and writes are to the same addresses and in the same
order (and writes write the same values), the program is correct,
regardless of other operations in it. (One important point to note
here is, that time duration between consecutive volatile accesses is
not considered at all.)

Unfortunately, there are also operations which are not covered by
volatile accesses. An example of this in avr-gcc/avr-libc are the
cli() and sei() macros defined in <avr/interrupt.h>, which convert
directly to the respective assembler mnemonics through the __asm__()
statement. These don't constitute a variable access at all, not even
volatile, so the compiler is free to move them around. Although there
is a "volatile" qualifier which can be attached to the __asm__()
statement, its effect on (re)ordering is not clear from the
documentation (and is more likely only to prevent complete removal by
the optimiser), as it (among other) states:

<em>Note that even a volatile asm instruction can be moved
relative to other code, including across jump instructions. [...]
Similarly, you can't expect a sequence of volatile asm instructions to
remain perfectly consecutive.</em>

\sa http://gcc.gnu.org/onlinedocs/gcc-4.3.4/gcc/Extended-Asm.html

There is another mechanism which can be used to achieve something
similar: <em>memory barriers</em>. This is accomplished through adding a
special "memory" clobber to the inline \c asm statement, and ensures that
all variables are flushed from registers to memory before the
statement, and then re-read after the statement. The purpose of memory
barriers is slightly different than to enforce code ordering: it is
supposed to ensure that there are no variables "cached" in registers,
so that it is safe to change the content of registers e.g. when
switching context in a multitasking OS (on "big" processors with
out-of-order execution they also imply usage of special instructions
which force the processor into "in-order" state (this is not the case
of AVRs)).

However, memory barrier works well in ensuring that all volatile
accesses before and after the barrier occur in the given order with
respect to the barrier. However, it does not ensure the compiler
moving non-volatile-related statements across the barrier. Peter
Dannegger provided a nice example of this effect:

\code
#define cli() __asm volatile( "cli" ::: "memory" )
#define sei() __asm volatile( "sei" ::: "memory" )

unsigned int ivar;

void test2( unsigned int val )
{
  val = 65535U / val;

  cli();

  ivar = val;

  sei();
}
\endcode

compiles with optimisations switched on (-Os) to

\verbatim
00000112 <test2>:
 112:	bc 01       	movw	r22, r24
 114:	f8 94       	cli
 116:	8f ef       	ldi	r24, 0xFF	; 255
 118:	9f ef       	ldi	r25, 0xFF	; 255
 11a:	0e 94 96 00 	call	0x12c	; 0x12c <__udivmodhi4>
 11e:	70 93 01 02 	sts	0x0201, r23
 122:	60 93 00 02 	sts	0x0200, r22
 126:	78 94       	sei
 128:	08 95       	ret
\endverbatim

where the potentially slow division is moved across cli(),
resulting in interrupts to be disabled longer than intended. Note,
that the volatile access occurs in order with respect to cli() or
sei(); so the "net effect" required by the standard is achieved as
intended, it is "only" the timing which is off. However, for most of
embedded applications, timing is an important, sometimes critical
factor.

\sa https://www.mikrocontroller.net/topic/65923

Unfortunately, at the moment, in avr-gcc (nor in the C standard),
there is no mechanism to enforce complete match of written and
executed code ordering - except maybe of switching the optimization
completely off (-O0), or writing all the critical code in assembly.

To sum it up:

\li memory barriers ensure proper ordering of volatile accesses
\li memory barriers don't ensure statements with no volatile accesses to be reordered across the barrier

*/
