Optimising C code for small size - sharing static variables?

Question

I have two functions, both are similar to this:

void Bit_Delay()
{
    //this is a tuned tight loop for 8 MHz to generate timings for 9600 baud
    volatile char z = 12;
    
    while(z)
    {
        z++;
        z++;
        z++;
        z++;
        z -= 5;
    }
}

(The second function is analogous instead it uses 18 instead of 12 for the counter).

The code works flawlessly as it is (with z appearing locally to each function internally), but I'm trying to cram a little more functionality into my executable before I hit the (horribly) limited FLASH memory available.

My thought was to promote the z variable to be a global one (a volatile static). Because these two functions are effectively atomic operations (it's a single-threaded CPU and there are no interrupts at play to interfere), I figured that these two functions could share the single variable, thus saving a tiny bit of stack manipulation.

This didn't work. It is clear that the compiler is optimising-out much of the code related to z completely! The code then fails to function properly (running far too fast), and the size of the compiled binary drops to about 50% or so.

I realised that I needed the z variable to be marked volatile to prevent the compiler from removing code it knows is counting a fixed (and thus reducible to a constant) number each time.

Question:

Can I optimise this any further, and trick the compiler into keeping both functions intact? I'm compiling with "-Os" (optimise for small binary).

Here's the entire program verbatim for those playing along at home...

#include <avr/io.h>

#define RX_PIN (1 << PORTB0) //physical pin 3
#define TX_PIN (1 << PORTB1) //physical pin 1

void Bit_Delay()
{
    //this is a tuned tight loop for 8 MHz to generate timings for 9600 baud
    volatile char z = 12;
    
    while(z)
    {
        z++;
        z++;
        z++;
        z++;
        z -= 5;
    }
}

void Serial_TX_Char(char c)
{
    char i;
    
    //start bit
    PORTB &= ~TX_PIN;
    Bit_Delay();
    
    for(i = 0 ; i < 8 ; i++)
    {
        //output the data bits, LSB first
        if(c & 0x01)
            PORTB |= TX_PIN;
        else
            PORTB &= ~TX_PIN;
        
        c >>= 1;
        Bit_Delay();
    }

    //stop bit  
    PORTB |= TX_PIN;
    Bit_Delay();
}

char Serial_RX_Char()
{
    char retval = 0;
    volatile char z = 18; //1.5 bits delay

    //wait for idle high
    while((PINB & RX_PIN) == 0)
    {}
    
    //wait for start bit falling-edge
    while((PINB & RX_PIN) != 0)
    {}

    //1.5 bits delay
    while(z)
    {
        z++;
        z++;
        z++;
        z++;
        z -= 5;
    }

    for(z = 0 ; z < 8 ; z++)
    {
        retval >>= 1; //make space for the new bit
        retval |= (PINB & RX_PIN) << (8 - RX_PIN); //get the bit and store it
        Bit_Delay();
    }
    
    return retval;      
}

int main(void)
{
    CCP = 0xd8; //protection signature for clock registers (see datasheet)
    CLKPSR = 0x00; //set the clock prescaler to "div by 1"
    DDRB |= TX_PIN;
    PORTB |= TX_PIN; //idle high
        
    while (1) 
        Serial_TX_Char(Serial_RX_Char() ^ 0x20);
}

The target CPU is an Atmel ATTiny5 microcontroller, the code above uses up 94.1% of the FLASH memory! If you connect to the chip using a serial port at 9600 Baud, 8N1, you can type characters in and it returns them with bit 0x20 flipped (uppercase to lowercase and vice-versa).

This is not a serious project of course, I'm just experimenting to see how much functionality I could cram into this chip. I'm not going to bother with rewriting this in assembly, I seriously doubt I could do a better job than GCC's optimiser!

EDIT

@Frank asked about the IDE / compiler I'm using...

Microchip Studio (7.0.2542)

The "All Options" string that is passed to the compiler avr-gcc...

-x c -funsigned-char -funsigned-bitfields -DDEBUG  -I"C:\Program Files (x86)\Atmel\Studio\7.0\Packs\atmel\ATtiny_DFP\1.8.332\include"  -Os -ffunction-sections -fdata-sections -fpack-struct -fshort-enums -g2 -Wall -mmcu=attiny5 -B "C:\Program Files (x86)\Atmel\Studio\7.0\Packs\atmel\ATtiny_DFP\1.8.332\gcc\dev\attiny5" -c -std=gnu99 -MD -MP -MF "$(@:%.o=%.d)" -MT"$(@:%.o=%.d)" -MT"$(@:%.o=%.o)" 

Answer 1

I question the following assumption:

This didn't work. It is clear that the compiler is optimising-out much of the code related to z completely! The code then fails to function properly (running far too fast), and the size of the compiled binary drops to about 50% or so.

Looking at https://gcc.godbolt.org/z/sKdz3h8oP, it seems like the loops are actually being performed, however, for whatever reason each z++, when using a global volatile z goes from:

subi r28,lo8(-(1))
sbci r29,hi8(-(1))
ld r20,Y
subi r28,lo8((1))
sbci r29,hi8((1))
subi r20,lo8(-(1))
subi r28,lo8(-(1))
sbci r29,hi8(-(1))
st Y,r20
subi r28,lo8((1))
sbci r29,hi8((1))

to:

lds r20,z
subi r20,lo8(-(1))
sts z,r20

You will need to recalibrate your 12, 18, and 5 constants to get your baud rate correct (since fewer instructions are executed in each loop), but the logic is there in the compiled version.

To be clear: This looks really weird to me, the local volatile version is clearly not being compiled correctly. I did find an old gcc bug along these lines: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=33970, but it seems to not cover the local variable case.