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Optimizer removes ssub16 used to set GE flags

Mike Handler
Note: This was originally posted on 27th April 2009 at http://forums.arm.com

I'm using RVCT 3.0 compiler with the optimizer (e.g., -O3). My code has inline assembly.

Here's my problem: If the inline assembly uses a parallel subtract instruction (e.g., SSUB16) to set the GE flags for use by the SEL instruction, the optimizer removes the SSUB16.

It seems that the optimizer removes the SSUB16 because it doesn't see the register result being used, even though the GE results are indeed used.

Here's an example:
__inline int MAX16(int a1_a0, int b1_b0)
{
   register int maxVal16;
   __asm
   {
      ssub16  maxVal16, a1_a0, b1_b0
      sel     maxVal16, a1_a0, b1_b0
   };
   return(maxVal16);
}
int findMax(int a, int b )
{
   return (MAX16(a, b ));
}


The dump file has
    findMax
    $a
    .text
        0x00000000:    e6800fb1    ....    SEL      r0,r0,r1
        0x00000004:    e12fff1e    ../.    BX       lr



Is there a way to prevent this problem without adding cycles?   I can fix the problem by using a volatile ptr for the ssub16 result, but that adds extra cycles.

Thanks in advance for any help.
Parents
  • Note: This was originally posted on 28th April 2009 at http://forums.arm.com

    Sim,

    Thanks for the idea of using pragma push, pragma O0, pragma pop.

    Unfortunately, although the pragma's were a great idea for the simplified example I gave, it doesn't address my real situation.

    In reality, I have a routine with many inline asm calls and mixed with C code.

    If I put the pragma's surrounding the main routine per your example, I lose C-level optimization across the routine.

    If I put the pragma's within the inline asm routines themselves, or surrounding the inline asm calls in the C routine, those inline asm routines mysteriously disappear from the dump file output, as if they were optimized out.  That is, it doesn't seem like you can mix optimization levels within a routine--which seems understandable.

    So I'm still faced with the original problem.

    Thanks again.
Reply
  • Note: This was originally posted on 28th April 2009 at http://forums.arm.com

    Sim,

    Thanks for the idea of using pragma push, pragma O0, pragma pop.

    Unfortunately, although the pragma's were a great idea for the simplified example I gave, it doesn't address my real situation.

    In reality, I have a routine with many inline asm calls and mixed with C code.

    If I put the pragma's surrounding the main routine per your example, I lose C-level optimization across the routine.

    If I put the pragma's within the inline asm routines themselves, or surrounding the inline asm calls in the C routine, those inline asm routines mysteriously disappear from the dump file output, as if they were optimized out.  That is, it doesn't seem like you can mix optimization levels within a routine--which seems understandable.

    So I'm still faced with the original problem.

    Thanks again.
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