ARM's Changing Call Used Registers Causes Weird Bugs

[lin zuojian <manjian2006 at gmail dot com>]

Hi,
    in arm.c 
    static void
    arm_conditional_register_usage (void)
    ...
  if (TARGET_32BIT && TARGET_HARD_FLOAT && TARGET_VFP)
    {
      /* VFPv3 registers are disabled when earlier VFP
	 versions are selected due to the definition of
	 LAST_VFP_REGNUM.  */
      for (regno = FIRST_VFP_REGNUM;
	   regno <= LAST_VFP_REGNUM; ++ regno)
	{
	  fixed_regs[regno] = 0;
	  call_used_regs[regno] = regno < FIRST_VFP_REGNUM + 16
	    || regno >= FIRST_VFP_REGNUM + 32;
	}
    }
    
    these lines will change the called used registers, when using
    compiler flags like: -mfpu=neon.
    That causes weird bugs. Consider the situation in Android ARM
    architecture: I have a shared object supposed to run in a neon cpu,
    and -mfpu=neon added. But the system is not compiled using this
    flag. So when calling the system's library,  my code will risk using
    the clobbered d8-d16.
    The example will be:
    while (true) {
        struct my_struct s = {0}; // my_struct is 8 bytes long.
        call_system_library...
    }
    in this example. d8 is used to initialize s to zero. The assembly
    code like:
    push {d8} // because d8 is not call used.
    // the loop header
    vmov.i32 d8, #0
    // the loop body
    vstr d8, &s
    bl system_library
    b loop_body

    d8 is clobbered after branch link to system library, so the second
    loop will initialize s to random value, which causes crash.

    So I am forced to remove the -mfpu=neon for compatibility. My
    question is whether the gcc code show above confront to ARM
    standard. If so, why ARM make such a weird standard.
--
Lin Zuojian