[SelectionDAGBuilder] Look for appropriate INLINEASM_BR instruction to verify (#152591)

Partially fix #149023.
The original code `MRI.def_begin(Reg)->getParent()` may return the
incorrect MI, as the physical register `Reg` may have multiple
definitions.
This patch selects the correct MI to verify by comparing the MBB of each
definition.

New testcase hangs with -O1/2/3 enabled. The BranchFolding may be to
blame.

1 files changed, 9 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 7aa1fad..6256ebc 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -12744,17 +12744,22 @@ static Register FollowCopyChain(MachineRegisterInfo &MRI, Register Reg) {
   assert(MI->getOpcode() == TargetOpcode::COPY &&
          "start of copy chain MUST be COPY");
   Reg = MI->getOperand(1).getReg();
+
+  // If the copied register in the first copy must be virtual.
+  assert(Reg.isVirtual() && "expected COPY of virtual register");
   MI = MRI.def_begin(Reg)->getParent();
+
   // There may be an optional second copy.
   if (MI->getOpcode() == TargetOpcode::COPY) {
     assert(Reg.isVirtual() && "expected COPY of virtual register");
     Reg = MI->getOperand(1).getReg();
     assert(Reg.isPhysical() && "expected COPY of physical register");
-    MI = MRI.def_begin(Reg)->getParent();
+  } else {
+    // The start of the chain must be an INLINEASM_BR.
+    assert(MI->getOpcode() == TargetOpcode::INLINEASM_BR &&
+           "end of copy chain MUST be INLINEASM_BR");
   }
-  // The start of the chain must be an INLINEASM_BR.
-  assert(MI->getOpcode() == TargetOpcode::INLINEASM_BR &&
-         "end of copy chain MUST be INLINEASM_BR");
+
   return Reg;
 }