[BranchFolding] Document why replacing HashMachineInstr with hash_code doesn't work
llvm-svn: 240415
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index d3d276f..6182667 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -270,7 +270,9 @@
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &Op = MI->getOperand(i);
- // Merge in bits from the operand if easy.
+ // Merge in bits from the operand if easy. We can't use MachineOperand's
+ // hash_code here because it's not deterministic and we sort by hash value
+ // later.
unsigned OperandHash = 0;
switch (Op.getType()) {
case MachineOperand::MO_Register: