[Local][InstCombine][GVN] Handle !noundef metadata in combineMetadata

Handle !noundef metadata in comhineMetadata. The behavior of violating
!noundef metadata is undefined behavior. So if K dominates J, we can
preserve it uncontionally, otherwise, we preserve it if both K and J
have !noundef metadata been set.

This patch also makes !noundef metadata added in KnownIDs when doing
instruction combine for phi node or global value numbering for loads.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D142801

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index b6c6c5a..624907a 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -2713,6 +2713,11 @@ void llvm::combineMetadata(Instruction *K, const Instruction *J,
       case LLVMContext::MD_preserve_access_index:
         // Preserve !preserve.access.index in K.
         break;
+      case LLVMContext::MD_noundef:
+        // If K does move, keep noundef if it is present in both instructions.
+        if (DoesKMove)
+          K->setMetadata(Kind, JMD);
+        break;
     }
   }
   // Set !invariant.group from J if J has it. If both instructions have it
@@ -2819,7 +2824,8 @@ void llvm::patchReplacementInstruction(Instruction *I, Value *Repl) {
       LLVMContext::MD_noalias,         LLVMContext::MD_range,
       LLVMContext::MD_fpmath,          LLVMContext::MD_invariant_load,
       LLVMContext::MD_invariant_group, LLVMContext::MD_nonnull,
-      LLVMContext::MD_access_group,    LLVMContext::MD_preserve_access_index};
+      LLVMContext::MD_access_group,    LLVMContext::MD_preserve_access_index,
+      LLVMContext::MD_noundef};
   combineMetadata(ReplInst, I, KnownIDs, false);
 }