PowerPC : GCC2 optimises better than GCC4???

[Mark Colby]

This sounds like a dumb question I know. However the following code
snippet results in many more machine instructions under 4.4.2 than under
2.9.5 (I am running a cygwin->PowerPC cross):

  typedef unsigned int U32;
  typedef union
  {
    U32 R;
    struct
    {
      U32 BF1:2;
      U32 :8;
      U32 BF2:2;
      U32 BF3:2;
      U32 :18;
    } B;
  } TEST_t;
  U32 testFunc(void)
  {
    TEST_t t;
    t.R=0;
    t.B.BF1=2;
    t.B.BF2=3;
    t.B.BF3=1;
    return t.R;
  }

Output under 4.4.2 (powerpc-eabi-gcc-4-4-2 -O3 -S gcc-test.cpp -o
gcc-test-442.s):

  li 0,2
  li 3,0
  rlwimi 3,0,30,0,1
  li 0,3
  rlwimi 3,0,20,10,11
  li 0,1
  rlwimi 3,0,18,12,13
  blr

Output under 2.9.5 (powerpc-eabi-gcc-2-9-5 -O3 -S gcc-test.cpp -o
gcc-test-295.s):

  lis 3,0x8034
  blr

Is there any way to improve this behaviour? I have been using 2.9.5 very
successfully for years and am now looking at 4.4.2, but have many such
examples in my code (for clarity of commenting and maintainability).

I have also noticed that 4.4.2 seems to use significantly larger stack
frames, and consequently more register-stacking instructions than 2.9.5
for the same functions. Am I missing something? Many thanks if you can
shed any light on this.

Mark


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