crosscompiling and gdb

As an example I m trying to see how division is implemented on arm and arm64.

created a test.c

int main(void)
{
        int a = 10;
        int b = 20;
        return (a/b);
}

Make sure to comple with -o option always because a.out doesn't has symbols

To compile for arm64
/usr2/arunks/workspace/LA.BR.1.3.1/prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9/bin/aarch64-linux-android-gccc -o test64 test.c --sysroot=/usr2/arunks/workspace/LA.BR.1.3.1/prebuilts/ndk/9/platforms/android-17/arch-arm64/usr/

(gdb) disassemble main
Dump of assembler code for function main:
   0x00000000004005a8 <+0>:     sub     sp, sp, #0x10
   0x00000000004005ac <+4>:     mov     w0, #0xa                        // #10
   0x00000000004005b0 <+8>:     str     w0, [sp,#12]
   0x00000000004005b4 <+12>:    mov     w0, #0x14                       // #20
   0x00000000004005b8 <+16>:    str     w0, [sp,#8]
   0x00000000004005bc <+20>:    ldr     w1, [sp,#12]
   0x00000000004005c0 <+24>:    ldr     w0, [sp,#8]
   0x00000000004005c4 <+28>:    sdiv    w0, w1, w0
   0x00000000004005c8 <+32>:    add     sp, sp, #0x10
   0x00000000004005cc <+36>:    ret
End of assembler dump.

start gdb
/usr2/arunks/workspace/LA.BR.1.3.1/prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.8/bin/arm-linux-androideabi-gcc -o test test.c --sysroot=/usr2/arunks/workspace/LA.BR.1.3.1/prebuilts/ndk/9/platforms/android-17/arch-arm/usr/

To compile for arm
/usr2/arunks/workspace/LA.BR.1.3.1/prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.8/bin/arm-linux-androideabi-gdb test

start gdb
/usr2/arunks/workspace/LA.BR.1.3.1/prebuilts/gcc/linux-x86/arm/arm-linux-androideabi-4.8/bin/arm-linux-androideabi-gdb test

Dump of assembler code for function main:
   0x00008460 <+0>:     push    {r11, lr}
   0x00008464 <+4>:     add     r11, sp, #4
   0x00008468 <+8>:     sub     sp, sp, #8
   0x0000846c <+12>:    mov     r3, #10
   0x00008470 <+16>:    str     r3, [r11, #-8]
   0x00008474 <+20>:    mov     r3, #20
   0x00008478 <+24>:    str     r3, [r11, #-12]
   0x0000847c <+28>:    ldr     r0, [r11, #-8]
   0x00008480 <+32>:    ldr     r1, [r11, #-12]
   0x00008484 <+36>:    bl      0x8498 <__divsi3>
   0x00008488 <+40>:    mov     r3, r0
   0x0000848c <+44>:    mov     r0, r3
   0x00008490 <+48>:    sub     sp, r11, #4
   0x00008494 <+52>:    pop     {r11, pc}
End of assembler dump.

So arm32 implements division in software whereas arm64 has a dedicated instruction to do this.