Initializer expression optimization

I am having a problem with the Keil compiler not performing an optimization that looks trivial. I am using Keil 4.50. The issue can be reproduced with the following simple code:

extern int g(int arg1);
extern int extint1;

int f( int arg1)
{
      int a;
      int b;

      a = g(arg1);
      b = (int) &extint1 + a - a + 10 -5;

      return b;
}

When compiled with maximum optimization (O2 or O3), I would expect the compiler to optimize away the ‘a-a’ in the computation of b, but it doesn’t. When compiled with –O3 the result is

; generated by ARM C/C++ Compiler, 4.1 [Build 791]
; commandline ArmCC [--list --debug -c --asm --interleave -ocompileroptimization.o --asm_dir=.\ --list_dir=.\ --depend=compileroptimization.d --cpu=Cortex-M3 --apcs=interwork -O3 -IC:\Keil\ARM\CMSIS\Include -IC:\Keil\ARM\INC\STMicroelectronics --omf_browse=compileroptimization.crf CompilerOptimization.c]
                          THUMB

                          AREA ||.text||, CODE, READONLY, ALIGN=2

                  f PROC
;;;3
;;;4      int f( int arg1)
000000  b510              PUSH     {r4,lr}
;;;5      {
;;;6        int a;
;;;7        int b;
;;;8
;;;9        a = g(arg1);
000002  f7fffffe          BL       g
;;;10       b = (int) &extint1 + a - a + 10 -5;
000006  4902              LDR      r1,|L1.16|
000008  4401              ADD      r1,r1,r0
00000a  1a08              SUBS     r0,r1,r0
00000c  1d40              ADDS     r0,r0,#5
;;;11
;;;12       return b;
;;;13     }
00000e  bd10              POP      {r4,pc}
                          ENDP

                  |L1.16|
                          DCD      extint1

Note the ADD r1,r1,r0 immediately followed by the SUBS r0,r1,r0; not what I expect from an optimizer. Without the presence of extint1, the compiler optimizes correctly.
The trivial answer is of course not to write code that does ‘a-a’, but that is not a realistic option in my project. I need to rely on the compiler to optimize this code.

Does anyone have an idea on why the compiler behaves like this and how I can let it optimize this correctly?