arm-none-eabi-gcc C Pointers

Question

So working with C in the arm-none-eabi-gcc. I have been having an issue with pointers, they don't seem to exists. Perhaps I'm passing the wrong cmds to the compiler.

Here is an example.

    unsigned int * gpuPointer = GetGPU_Pointer(framebufferAddress);
    unsigned int color = 16;
    int y = 768;
    int x = 1024;

    while(y >= 0)
    {
        while(x >= 0)
        {
            *gpuPointer = color;
            color = color + 2;
            x--;
        }

        color++;
        y--;
        x = 1024;
    }

and the output from the disassembler.

81c8:   ebffffc3    bl  80dc <GetGPU_Pointer>
81cc:   e3a0c010    mov ip, #16 ; 0x10
81d0:   e28c3b02    add r3, ip, #2048   ; 0x800
81d4:   e2833002    add r3, r3, #2  ; 0x2
81d8:   e1a03803    lsl r3, r3, #16
81dc:   e1a01823    lsr r1, r3, #16
81e0:   e1a0300c    mov r3, ip
81e4:   e1a02003    mov r2, r3
81e8:   e2833002    add r3, r3, #2  ; 0x2
81ec:   e1a03803    lsl r3, r3, #16
81f0:   e1a03823    lsr r3, r3, #16
81f4:   e1530001    cmp r3, r1
81f8:   1afffff9    bne 81e4 <setup_framebuffer+0x5c>

Shouldn't there be a str cmd around 81e4? To add further the GetGPU_Pointer is coming from an assembler file but there is a declaration as so.

extern unsigned int * GetGPU_Pointer(unsigned int framebufferAddress);

My gut feeling is its something absurdly simple but I'm missing it.

Answer 1

You never change the value of gpuPointer and you haven't declared it to point to a volatile. So from the compiler's perspective you are overwriting a single memory location (*gpuPointer) 768*1024 times, but since you never use the value you are writing into it, the compiler is entitled to optimize by doing a single write at the end of the loop.