[BranchFolding] Document why replacing HashMachineInstr with hash_code doesn't work

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240415 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/BranchFolding.cpp b/lib/CodeGen/BranchFolding.cpp
index d3d276ff0df7e4c21cde47e3fd445a5c865eccf2..618266731c067ad6219af8b25e96b82923a04b51 100644 (file)
--- a/lib/CodeGen/BranchFolding.cpp
+++ b/lib/CodeGen/BranchFolding.cpp
@@ -270,7 +270,9 @@ static unsigned HashMachineInstr(const MachineInstr *MI) {
   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: