ARM constant folding bug?

["Jonathan S. Shapiro" <shap at eros-os dot com>]

This is observed on gcc-3.4.6. It may be a known issue, and/or it may be
fixed in later compilers.

For bringup purposes, I wrote an inline assembly hack to get the cross
compiler to tell me whether the target is little/big endian. The code
fragment is:

  const unsigned long ul = 0x04030201llu;

  if ( *((char *) &ul) == 0x1 )
    ASMDEF(LITTLE_ENDIAN);
  else
    ASMDEF(BIG_ENDIAN);

The expectation is that when run with -O2, the compiler should
constant-fold all of this stuff away, resulting in exactly one ASMDEF.
On i386 and m68k, the constant folding occurs as expected. For ARM it
does not(!).

I am not surprised that there is different behavior for different
targets, but I am very surprised that *this* behavior is different. This
optimization ought to be happening in the mid-end, and it ought to be
entirely machine independent.

Can somebody explain what is going on here?


shap