NAME

       optimization - Compiler optimization

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 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
       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:

       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.

       See also
           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: memory
       barriers. This is accomplished through adding a special 'memory' clobber to the inline 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:

       #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();
       }

       compiles with optimisations switched on (-Os) to

       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

       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.

       See also
           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:

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