MachineSSAUpdater: insert IMPLICIT_DEF at top of basic block

Summary:
When getValueInMiddleOfBlock happens to be called for a basic block
that has no incoming value at all, an IMPLICIT_DEF is inserted in that
block via GetValueAtEndOfBlockInternal. This IMPLICIT_DEF must be at
the top of its basic block or it will likely not reach the use that
the caller intends to insert.

Issue: https://github.com/GPUOpen-Drivers/llpc/issues/204

Reviewers: arsenm, rampitec

Subscribers: jvesely, wdng, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68183

llvm-svn: 374040

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/MachineSSAUpdater.cpp b/llvm/lib/CodeGen/MachineSSAUpdater.cpp
index 3370b52ede4..258a5f9e048 100644
--- a/llvm/lib/CodeGen/MachineSSAUpdater.cpp
+++ b/llvm/lib/CodeGen/MachineSSAUpdater.cpp
@@ -292,7 +292,7 @@ public:
                               MachineSSAUpdater *Updater) {
     // Insert an implicit_def to represent an undef value.
     MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
-                                        BB, BB->getFirstTerminator(),
+                                        BB, BB->getFirstNonPHI(),
                                         Updater->VRC, Updater->MRI,
                                         Updater->TII);
     return NewDef->getOperand(0).getReg();