This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Seeking suggestion

Jamie Prescott wrote: 
Is there a reason why something like this would not work?
    if (!TARGET_XXX2)
      emit_clobber(gen_rtx_REG(CCmode, CC_REGNUM));
    emit_insn(gen_addsi3_nc(operands[0], operands[1], operands[2]));

Yes. The optimizer will not know that addsi3_nc uses CC_REGNUM, as it is not mentioned, so the optimizer will not know that these two RTL instructions always need to remain next to each other. Any optimization pass that moves insns around may separate the add from the clobber resulting in broken code. This is what parallels are for, to make sure that the clobber and add stay together.

Jim

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]