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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: Florian Hahn (fhahn)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/84339.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/ValueTracking.cpp (+2) 
- (modified) llvm/test/Analysis/LoopAccessAnalysis/underlying-object-loop-varying-phi.ll (+6-1) 


``````````diff
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 52ae9f034e5d34..b9c046ea9fe832 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -6123,6 +6123,8 @@ 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 1a5a6ac08d4045..106dc8c13a49fa 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:

``````````

</details>


https://github.com/llvm/llvm-project/pull/84339