[avr-libc-dev] [bug #14616] definition of sei() in interrupts.h is faulty

[anonymous]

Follow-up Comment #4, bug #14616 (project avr-libc):

Qualificator 'volatile' works fine in the above example.
Look, please, an expanded variant:

  int acc;
  int val;
  int volatile acc_v;
  int volatile val_v;

foo1(): no protection, loop is fully optimized:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  void foo1 (void)
  {
      unsigned char i;
      for (i = 10; i; i--) {
          asm volatile ("cli");
          acc += val;                   /* nonvolatile both     */
          asm volatile ("sei");
      }
  }

  .L6:
  /* #APP */
        cli
  /* #NOAPP */
        add r24,r18
        adc r25,r19
  /* #APP */
        sei
  /* #NOAPP */
        subi r20,lo8(-(-1))
        brne .L6

foo2(): acc_v is protected, only val fetch is optimized:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  void foo2 (void)
  {
      unsigned char i;
      for (i = 10; i; i--) {
          asm volatile ("cli");
          acc_v += val;                 /* acc_v is volatile    */
          asm volatile ("sei");
      }
  }

  .L15:
  /* #APP */
        cli
  /* #NOAPP */
        lds r24,acc_v
        lds r25,(acc_v)+1
        add r24,r18
        adc r25,r19
        sts (acc_v)+1,r25
        sts acc_v,r24
  /* #APP */
        sei
  /* #NOAPP */
        subi r20,lo8(-(-1))
        brne .L15

foo3(): val_v is protected, acc i/o is optimized:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  void foo3 (void)
  {
      unsigned char i;
      for (i = 10; i; i--) {
          asm volatile ("cli");
          acc += val_v;                 /* val_v is volatile */
          asm volatile ("sei");
      }
  }

  .L23:
  /* #APP */
        cli
  /* #NOAPP */
        lds r24,val_v
        lds r25,(val_v)+1
        add r18,r24
        adc r19,r25
  /* #APP */
        sei
  /* #NOAPP */
        subi r20,lo8(-(-1))
        brne .L23

foo4(): both are protected, all i/o is not optimized:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  void foo4 (void)
  {
      unsigned char i;
      for (i = 10; i; i--) {
          asm volatile ("cli");
          acc_v += val_v;               /* both are volatile    */
          asm volatile ("sei");
      }
  }

  .L32:
  /* #APP */
        cli
  /* #NOAPP */
        lds r24,acc_v
        lds r25,(acc_v)+1
        lds r18,val_v
        lds r19,(val_v)+1
        add r24,r18
        adc r25,r19
        sts (acc_v)+1,r25
        sts acc_v,r24
  /* #APP */
        sei
  /* #NOAPP */
        subi r20,lo8(-(-1))
        brne .L32

Is it possible to give an example, where 'volatile' variable is
not work (with simple sei()) ?

2.
Yes, sei() looks as function call. But there are many other
"almost functions", that ignore possible side effects.
For example:

  #include <stdlib.h>
  int acc, val;

  void foo (void)
  {
      unsigned char i;
      for (i = 10; i; i--) {
          val = abs(val);
          acc += val;
      }
  }

Without 'volatile': no protection, full optimization:
  .L7:
        mov r20,r24
        mov r21,r25
        sbrs r25,7
        rjmp .L6
        clr r20
        clr r21
        sub r20,r24
        sbc r21,r25
  .L6:
        mov r25,r21
        mov r24,r20
        add r18,r20
        adc r19,r21
        subi r22,lo8(-(-1))
        brne .L7

Dmitry.


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