[CodeGen] Prevent nullptr deref in genAlternativeCodeSequence

A pointer dereference was added (D141302) above an assert that checks
whether the pointer is null. This commit moves the assert above the
dereference and transforms it into an llvm_unreachable to better express
the intent that certain switch cases should never be reached.

Differential Revision: https://reviews.llvm.org/D145599

1 files changed, 1 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/TargetInstrInfo.cpp b/llvm/lib/CodeGen/TargetInstrInfo.cpp
index e86724a..1ea5c07 100644
--- a/llvm/lib/CodeGen/TargetInstrInfo.cpp
+++ b/llvm/lib/CodeGen/TargetInstrInfo.cpp
@@ -1038,15 +1038,13 @@ void TargetInstrInfo::genAlternativeCodeSequence(
     Prev = MRI.getUniqueVRegDef(Root.getOperand(2).getReg());
     break;
   default:
-    break;
+    llvm_unreachable("Unknown pattern for machine combiner");
   }
 
   // Don't reassociate if Prev and Root are in different blocks.
   if (Prev->getParent() != Root.getParent())
     return;
 
-  assert(Prev && "Unknown pattern for machine combiner");
-
   reassociateOps(Root, *Prev, Pattern, InsInstrs, DelInstrs, InstIdxForVirtReg);
 }