compiler optimizes out pointer to function when address is assigned manually

Question

I need to call a function at address 0xDD2:

// foo.h
void foo(void) __at(0xDD2);

// foo.c
#include "foo.h"
void foo(void)
{
    // some code
}

This code works:

#include "foo.h"
void main(void)
{
    void (*a)(void) = &foo;
    a();
}

However, this one doesn't:

#include "foo.h"
void main(void)
{
    void (*a)(void) = (void (*)(void))(0x0DD2);
    a();
}

The compiler (XC8) says: main.c:5:: warning: (759) expression generates no code and debugger passes these lines while debugging.

I need second one (call function just by its address). Why compiler optimizes it out? Is there any mistake in pointer assignment? Changing optimization level of compiler didn't help.

Answer 1

This seems to do what you asked for:

/*
 * File:   main.c
 * Author: dan1138
 * Target: PIC18F47Q10
 * Compiler: XC8 v2.20
 * 
 * See: https://stackoverflow.com/questions/62282036/compiler-optimizes-out-pointer-to-function-when-address-is-assigned-manually/62297967#62297967
 *
 * Created on June 9, 2020, 11:42 PM
 */
#pragma config FEXTOSC = OFF, RSTOSC = HFINTOSC_64MHZ, CLKOUTEN = OFF
#pragma config CSWEN = ON, FCMEN = OFF, MCLRE = EXTMCLR, PWRTE = OFF
#pragma config LPBOREN = OFF, BOREN = SBORDIS, BORV = VBOR_190
#pragma config ZCD = OFF, PPS1WAY = OFF, STVREN = ON, XINST = OFF
#pragma config WDTCPS = WDTCPS_31, WDTE = OFF, WDTCWS = WDTCWS_7, WDTCCS = SC
#pragma config WRT0 = OFF, WRT1 = OFF, WRT2 = OFF, WRT3 = OFF
#pragma config WRT4 = OFF, WRT5 = OFF, WRT6 = OFF, WRT7 = OFF
#pragma config WRTC = OFF, WRTB = OFF, WRTD = OFF
#pragma config SCANE = ON, LVP = OFF, CP = OFF, CPD = OFF
#pragma config EBTR0 = OFF, EBTR1 = OFF, EBTR2 = OFF, EBTR3 = OFF
#pragma config EBTR4 = OFF, EBTR5 = OFF, EBTR6 = OFF, EBTR7 = OFF
#pragma config EBTRB = OFF

#include <xc.h>

void main(void)
{
    extern void foo(void) __at(0xDD2);
    void (* volatile a)(void) = foo;

    a();
}    

Disassembly:

!void main(void)
!{
!    extern void foo(void) __at(0xDD2);
!    void (* volatile a)(void) = foo;
0x1FFD4: MOVLW 0xD2
0x1FFD6: MOVWF __pcstackCOMRAM, ACCESS
0x1FFD8: MOVLW 0xD
0x1FFDA: MOVWF 0x2, ACCESS
0x1FFDC: MOVLW 0x0
0x1FFDE: MOVWF 0x3, ACCESS
!    a();
0x1FFE0: RCALL 0xFFE6
0x1FFE2: GOTO 0x1FFFA
0x1FFE4: NOP
0x1FFE6: PUSH
0x1FFE8: MOVWF PCLATH, ACCESS
0x1FFEA: MOVF __pcstackCOMRAM, W, ACCESS
0x1FFEC: MOVWF TOS, ACCESS
0x1FFEE: MOVF 0x2, W, ACCESS
0x1FFF0: MOVWF TOSH, ACCESS
0x1FFF2: MOVF 0x3, W, ACCESS
0x1FFF4: MOVWF TOSU, ACCESS
0x1FFF6: MOVF PCLATH, W, ACCESS
0x1FFF8: RETURN 0
!}
0x1FFFA: BSF _ccovbit2_1, 0, ACCESS
0x1FFFC: GOTO 0x0
0x1FFFE: NOP