[Bug middle-end/65082] Wasted cycles when using a register based varible

["NickParker at Eaton dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65082

--- Comment #3 from NickParker at Eaton dot com ---

register uint16_t r4 asm ("r4");
register uint16_t r6 asm ("r6");
volatile int8_t localOscCosine;
volatile int8_t acInput;

void pllExec(void)
{
  int16_t mix_output_s2=0;
  r4 += r6;
  localOscCosine = pgm_read_byte(&cosine7b[r4 >> 8]);
  mix_output_s2 = (localOscCosine * acInput); // GCC-AVR it give 16-bits....
};


---------------------------------------
results in.....


void pllExec(void)
  39:pll.c         **** {
  15                       .loc 1 39 0
  16                       .cfi_startproc
  17                   /* prologue: function */
  18                   /* frame size = 0 */
  19                   /* stack size = 0 */
  20                   .L__stack_usage = 0
  21                   .LVL0:
  40:pll.c         ****   //int16_t ss;
  41:pll.c         ****   int16_t mix_output_s2=0;
  42:pll.c         ****   r4 += r6;
  22                       .loc 1 42 0
  23 0000 F301              movw r30,r6
  24 0002 E40D              add r30,r4
  25 0004 F51D              adc r31,r5
  26 0006 2F01              movw r4,r30
  27                   .LVL1:
  28                   .LBB2: