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Not understanding the optimisation of the compiler

Steve Wool

We are evaluation Keil uVision V4.10 and ARMCC V4.1.0.481.

Our first test run didn't go so well.

With no optimisation, everything worked OK. Using optimisation 1 or above, the test failed.

For various reasons, I can not disclose the test source code. However, I will attempt to create a small self-contained example later on. The part of the test code that caused the problem is setting up an multi dimensional array, setting each element to '1' (ie. Array[x][y] = 1 with a for loop for both x and y).

In the meantime, I wonder if anyone has had a similar problem?

Using optimisation -O3, the assembler code in the debugger looks like this:

48: for ( y= 0 ; y < 8 ; y++ )
0x80000AAC E5C02000 STRB R2,[R0]
0x80000AB0 E2822001 ADD R2,R2,#0x00000001
0x80000AB4 E3A0C001 MOV R12,#0x00000001
0x80000AB8 E20220FF AND R2,R2,#0x000000FF
0x80000ABC E7C3C001 STRB R12,[R3,R1]
0x80000AC0 E5C02001 STRB R2,[R0,#0x0001]
0x80000AC4 EAFFFFFC B 0x80000ABC -- Isn't this an infinite loop???

Parents
  • 0 in reply to Steve Wool

    "Regardless of whether or not the variables used are volatile, and whether or not the compiler chooses to complete ignore the filling of the array, the compiler should [not] change a known loop with exit condition to a loop without an exit condition."

    That depends whether that loop in the particular context was equivalent to a loop without an exit condition.

    eg,

    void main( void )
{
   // Test filling an array
   for (x = 0; x < 100; x ++)
      for (y = 0; y < 8; y ++)
         array[x][y] = 1;

   // "Test finished - we're done"

   for( ;; ); // Embedded code - must not exit main!
}

    Again, without seeing the context, further discussion is impossible.

Reply
  • 0 in reply to Steve Wool

    "Regardless of whether or not the variables used are volatile, and whether or not the compiler chooses to complete ignore the filling of the array, the compiler should [not] change a known loop with exit condition to a loop without an exit condition."

    That depends whether that loop in the particular context was equivalent to a loop without an exit condition.

    eg,

    void main( void )
{
   // Test filling an array
   for (x = 0; x < 100; x ++)
      for (y = 0; y < 8; y ++)
         array[x][y] = 1;

   // "Test finished - we're done"

   for( ;; ); // Embedded code - must not exit main!
}

    Again, without seeing the context, further discussion is impossible.

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