[DebugInfo][InstrRef] Handle llvm.frameaddress intrinsics gracefully

When working out which instruction defines a value, the
instruction-referencing variable location code has a few special cases for
physical registers:
 * Arguments are never defined by instructions,
 * Constant physical registers always read the same value, are never def'd

This patch adds a third case for the llvm.frameaddress intrinsics: you can
read the framepointer in any block if you so choose, and use it as a
variable location, as shown in the added test.

This rather violates one of the assumptions behind instruction referencing,
that LLVM-ir shouldn't be able to read from an arbitrary register at some
arbitrary point in the program. The solution for now is to just emit a
DBG_PHI that reads the register value: this works, but if we wanted to do
something clever with DBG_PHIs in the future then this would probably get
in the way. As it stands, this patch avoids a crash.

Differential Revision: https://reviews.llvm.org/D106659

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/MachineFunction.cpp b/llvm/lib/CodeGen/MachineFunction.cpp
index dc4b1a6..0a454b6 100644
--- a/llvm/lib/CodeGen/MachineFunction.cpp
+++ b/llvm/lib/CodeGen/MachineFunction.cpp
@@ -1139,6 +1139,14 @@ auto MachineFunction::salvageCopySSA(MachineInstr &MI)
     // We can produce a DBG_PHI that identifies the constant physreg. Doesn't
     // matter where we put it, as it's constant valued.
     assert(CurInst->isCopy());
+  } else if (State.first == TRI.getFrameRegister(*this)) {
+    // LLVM IR is allowed to read the framepointer by calling a
+    // llvm.frameaddress.* intrinsic. We can support this by emitting a
+    // DBG_PHI $fp. This isn't ideal, because it extends the behaviours /
+    // position that DBG_PHIs appear at, limiting what can be done later.
+    // TODO: see if there's a better way of expressing these variable
+    // locations.
+    ;
   } else {
     // Assert that this is the entry block. If it isn't, then there is some
     // code construct we don't recognise that deals with physregs across