[LICM] Only create load in pre-header when promoting load.

When only a store is sunk, there is no need to create a load in the
pre-header, as the result of the load will never get used.

The dead load can can introduce UB, if the function is marked as
writeonly.

Fixes llvm#51248.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D123473

Author: fhahn

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -2156,21 +2156,24 @@ bool llvm::promoteLoopAccessesToScalars(
 
   // Set up the preheader to have a definition of the value.  It is the live-out
   // value from the preheader that uses in the loop will use.
-  LoadInst *PreheaderLoad = new LoadInst(
-      AccessTy, SomePtr, SomePtr->getName() + ".promoted",
-      Preheader->getTerminator());
-  if (SawUnorderedAtomic)
-    PreheaderLoad->setOrdering(AtomicOrdering::Unordered);
-  PreheaderLoad->setAlignment(Alignment);
-  PreheaderLoad->setDebugLoc(DebugLoc());
-  if (AATags)
-    PreheaderLoad->setAAMetadata(AATags);
-  SSA.AddAvailableValue(Preheader, PreheaderLoad);
-
-  MemoryAccess *PreheaderLoadMemoryAccess = MSSAU.createMemoryAccessInBB(
-      PreheaderLoad, nullptr, PreheaderLoad->getParent(), MemorySSA::End);
-  MemoryUse *NewMemUse = cast<MemoryUse>(PreheaderLoadMemoryAccess);
-  MSSAU.insertUse(NewMemUse, /*RenameUses=*/true);
+  LoadInst *PreheaderLoad = nullptr;
+  if (FoundLoadToPromote) {
+    PreheaderLoad =
+        new LoadInst(AccessTy, SomePtr, SomePtr->getName() + ".promoted",
+                     Preheader->getTerminator());
+    if (SawUnorderedAtomic)
+      PreheaderLoad->setOrdering(AtomicOrdering::Unordered);
+    PreheaderLoad->setAlignment(Alignment);
+    PreheaderLoad->setDebugLoc(DebugLoc());
+    if (AATags)
+      PreheaderLoad->setAAMetadata(AATags);
+
+    MemoryAccess *PreheaderLoadMemoryAccess = MSSAU.createMemoryAccessInBB(
+        PreheaderLoad, nullptr, PreheaderLoad->getParent(), MemorySSA::End);
+    MemoryUse *NewMemUse = cast<MemoryUse>(PreheaderLoadMemoryAccess);
+    MSSAU.insertUse(NewMemUse, /*RenameUses=*/true);
+    SSA.AddAvailableValue(Preheader, PreheaderLoad);
+  }
 
   if (VerifyMemorySSA)
     MSSAU.getMemorySSA()->verifyMemorySSA();
@@ -2181,7 +2184,7 @@ bool llvm::promoteLoopAccessesToScalars(
   if (VerifyMemorySSA)
     MSSAU.getMemorySSA()->verifyMemorySSA();
   // If the SSAUpdater didn't use the load in the preheader, just zap it now.
-  if (PreheaderLoad->use_empty())
+  if (PreheaderLoad && PreheaderLoad->use_empty())
     eraseInstruction(*PreheaderLoad, *SafetyInfo, MSSAU);
 
   return true;

llvm/test/Transforms/LICM/scalar-promote.ll

@@ -605,10 +605,9 @@ Out:
 define void @test_sink_store_only() writeonly {
 ; CHECK-LABEL: @test_sink_store_only(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[GLB_PROMOTED:%.*]] = load i8, i8* @glb, align 1
 ; CHECK-NEXT:    br label [[LOOP_HEADER:%.*]]
 ; CHECK:       loop.header:
-; CHECK-NEXT:    [[DIV1:%.*]] = phi i8 [ [[GLB_PROMOTED]], [[ENTRY:%.*]] ], [ [[DIV:%.*]], [[LOOP_LATCH:%.*]] ]
+; CHECK-NEXT:    [[DIV1:%.*]] = phi i8 [ undef, [[ENTRY:%.*]] ], [ [[DIV:%.*]], [[LOOP_LATCH:%.*]] ]
 ; CHECK-NEXT:    [[I:%.*]] = phi i8 [ 0, [[ENTRY]] ], [ [[ADD:%.*]], [[LOOP_LATCH]] ]
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i8 [[I]], 4
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP_LATCH]], label [[EXIT:%.*]]