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Cortex M3 - Right Shift Bug

Pete Baston

Hi,

I'm using uVision 3.33, supplied as part of the Luminary Micro design contest. I've noticed the following problem when trying to right-shift a 16-bit unsigned value.

The code produced is:-

   176: unsigned short int16;
   177:
   178: void testfunc(void)
   179: {
0x0000072E BD10      POP      {r4,pc}
   180:         int16 >>= 1;
0x00000730 480A      LDR      r0,[pc,#40]  ; @0x0000075C
0x00000732 8800      LDRH     r0,[r0,#0x00]
0x00000734 4909      LDR      r1,[pc,#36]  ; @0x0000075C
0x00000736 8008      STRH     r0,[r1,#0x00]
   181: }

.. which does not do a right shift !

If I do a left shift, however, it works:-

   176: unsigned short int16;
   177:
   178: void testfunc(void)
   179: {
0x0000072E BD10      POP      {r4,pc}
   180:         int16 <<= 1;
0x00000730 480C      LDR      r0,[pc,#48]  ; @0x00000764
0x00000732 8800      LDRH     r0,[r0,#0x00]
0x00000734 F64F71FF  MOVW     r1,#0xFFFF
0x00000738 EA010040  AND      r0,r1,r0,LSL #1
0x0000073C 4909      LDR      r1,[pc,#36]  ; @0x00000764
0x0000073E 8008      STRH     r0,[r1,#0x00]
   181: }

.. with the LSL instruction clearly visible.

Is this a known bug in the Keil compiler ?

Thanks,

Pete Baston.

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