[Bug target/70676] suboptimal code generation on AVR

[night_ghost at ykoctpa dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70676

--- Comment #2 from night_ghost at ykoctpa dot ru ---
*** testcase for call+ret: (all code from
https://github.com/night-ghost/minimosd-extra/tree/master/MinimOsd_Extra),
compile with  -fno-optimize-sibling-calls

#include <Arduino.h>

class SPI
{
  public:
    SPI(void);
    static byte transfer(byte);
};

SPI::SPI()
{
#define SCK_PIN   13
#define MISO_PIN  12
#define MOSI_PIN  11

  // initialize the SPI pins
  pinMode(SCK_PIN, OUTPUT);
  pinMode(MOSI_PIN, OUTPUT);
  pinMode(MISO_PIN, INPUT);
}

byte SPI::transfer(byte value){
  SPDR = value;
  while (!(SPSR & (1<<SPIF))) ;
  return SPDR;
}


byte MAX_read(byte addr){
  SPI::transfer(addr);
  return SPI::transfer(0xff);
}



*** testcase of splitting tail, compile without  -fno-optimize-sibling-calls

#include <Arduino.h>
#define NOINLINE __attribute__ ((noinline))

struct Point {
    byte x;
    byte y;
};

typedef struct Point point;

static boolean inline is_alt(point p){
    return (p.y & 0x40);
}

static void NOINLINE osd_printf_1(PGM_P fmt, float f){
    Serial.printf_P(fmt, f);
}

static void NOINLINE osd_printf_2(PGM_P fmt, float f, byte c){
    Serial.printf_P(fmt, f);
    Serial.write(c);
}

static void panBatteryPercent(point p){

    float val=2.0;


   if (is_alt(p))
       osd_printf_1(PSTR("%2.0f%%"),val);
    else
       osd_printf_2(PSTR("%4.0f\x01"),val,0x15);
    }

}

>Provided you use -mrelax (or link with -Wl,--relax), the linker tries to 
>replace [R]CALL+RET by [R]JMP instruction

Thanks for good key, it saves  700bytes of code (from 30642 to 29942)