[LLVMdev] ConstantFold 'undef xor undef'

[Jianzhou Zhao]

Hi,

At line 2292, lib/VMCore/ConstantFold.cpp (llvm2.7 release)

Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode,
                                              Constant *C1, Constant *C2) {
  ...
  // Handle UndefValue up front.
  if (isa<UndefValue>(C1) || isa<UndefValue>(C2)) {
    switch (Opcode) {
    case Instruction::Xor:
      if (isa<UndefValue>(C1) && isa<UndefValue>(C2))
        // Handle undef ^ undef -> 0 special case. This is a common
        // idiom (misuse).
        return Constant::getNullValue(C1->getType());
      // Fallthrough
    case Instruction::Add:

This function folds ‘undef xor undef’ into 0(getNullValue) at this
case. At http://llvm.org/docs/LangRef.html#undefvalues, undef xor
undef can also be evaluating to undef.

/////////////////////////////
  %A = xor undef, undef
  [...]
Safe:
  %A = undef
  [...]

This example points out that two undef operands are not necessarily
the same. [...], but the short answer is that an undef "variable" can
arbitrarily change its value over its "live range". This is true
because the "variable" doesn't actually have a live range. Instead,
the value is logically read from arbitrary registers that happen to be
around when needed, so the value is not necessarily consistent over
time.
/////////////////////////////

Which semantics is better? I guess both are fine because if we assume
these two def's are same, then it is 0 as
'ConstantFoldBinaryInstruction', while if we assume they are different
then it is equal to undef. But the second case seems to include the
first one. If we let undef xor undef to be undef, later we can use
this undef as 0, but also other values w.r.t contexts. Is there any
reason that ConstantFoldBinaryInstruction uses the first assumption?

Thanks.
-- 
Jianzhou