[ValueTracking] Treat phi as underlying obj when not decomposing further (#84339)

At the moment, getUnderlyingObjects simply continues for phis that do
not refer to the same underlying object in loops, without adding them to
the list of underlying objects, effectively ignoring those phis.

Instead of ignoring those phis, add them to the list of underlying
objects. This fixes a miscompile where LoopAccessAnalysis fails to
identify a memory dependence, because no underlying objects can be found
for a set of memory accesses.

Fixes https://github.com/llvm/llvm-project/issues/82665.

PR: https://github.com/llvm/llvm-project/pull/84339
(cherry picked from commit b274b23665dec30f3ae4fb83ccca8b77e6d3ada3)

2 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 412115e..9f9451e 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -5986,6 +5986,8 @@ void llvm::getUnderlyingObjects(const Value *V,
       if (!LI || !LI->isLoopHeader(PN->getParent()) ||
           isSameUnderlyingObjectInLoop(PN, LI))
         append_range(Worklist, PN->incoming_values());
+      else
+        Objects.push_back(P);
       continue;
     }
 
diff --git a/llvm/test/Analysis/LoopAccessAnalysis/underlying-object-loop-varying-phi.ll b/llvm/test/Analysis/LoopAccessAnalysis/underlying-object-loop-varying-phi.ll
index 1a5a6ac..106dc8c 100644
--- a/llvm/test/Analysis/LoopAccessAnalysis/underlying-object-loop-varying-phi.ll
+++ b/llvm/test/Analysis/LoopAccessAnalysis/underlying-object-loop-varying-phi.ll
@@ -7,8 +7,13 @@ target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
 define void @indirect_ptr_recurrences_read_write(ptr %A, ptr %B) {
 ; CHECK-LABEL: 'indirect_ptr_recurrences_read_write'
 ; CHECK-NEXT:    loop:
-; CHECK-NEXT:      Memory dependences are safe
+; CHECK-NEXT:      Report: unsafe dependent memory operations in loop. Use #pragma clang loop distribute(enable) to allow loop distribution to attempt to isolate the offending operations into a separate loop
+; CHECK-NEXT:  Unsafe indirect dependence.
 ; CHECK-NEXT:      Dependences:
+; CHECK-NEXT:        IndidrectUnsafe:
+; CHECK-NEXT:            %l = load i32, ptr %ptr.recur, align 4, !tbaa !4 ->
+; CHECK-NEXT:            store i32 %xor, ptr %ptr.recur, align 4, !tbaa !4
+; CHECK-EMPTY:
 ; CHECK-NEXT:      Run-time memory checks:
 ; CHECK-NEXT:      Grouped accesses:
 ; CHECK-EMPTY: