const User *U = NULL;
unsigned Opcode = Instruction::UserOp1;
const Instruction *I = dyn_cast<Instruction>(V);
+ // Record if the value is defined in the same basic block.
+ //
+ // This information is crucial to know whether or not folding an
+ // operand is valid.
+ // Indeed, FastISel generates or reuses a virtual register for all
+ // operands of all instructions it selects. Obviously, the definition and
+ // its uses must use the same virtual register otherwise the produced
+ // code is incorrect.
+ // Before instruction selection, FunctionLoweringInfo::set sets the virtual
+ // registers for values that are alive across basic blocks. This ensures
+ // that the values are consistently set between across basic block, even
+ // if different instruction selection mechanisms are used (e.g., a mix of
+ // SDISel and FastISel).
+ // For values local to a basic block, the instruction selection process
+ // generates these virtual registers with whatever method is appropriate
+ // for its needs. In particular, FastISel and SDISel do not share the way
+ // local virtual registers are set.
+ // Therefore, this is impossible (or at least unsafe) to share values
+ // between basic blocks unless they use the same instruction selection
+ // method, which is not guarantee for X86.
+ // Moreover, things like hasOneUse could not be used accurately, if we
+ // allow to reference values across basic blocks whereas they are not
+ // alive across basic blocks initially.
bool InMBB = true;
if (I) {
Opcode = I->getOpcode();