[llvm] [Loads] Respect UseDerefAtPointSemantics in isDerefAndAlignedPointer. (PR #123196)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Florian Hahn (fhahn)

<details>
<summary>Changes</summary>

If a pointer gets freed, it may not be dereferenceable any longer, even though there is a dominating dereferenceable assumption. As first step, only consider assumptions if the function doesn't free memory if UseDerefAtPointSemantics is used.

---
Full diff: https://github.com/llvm/llvm-project/pull/123196.diff


3 Files Affected:

- (modified) llvm/lib/Analysis/Loads.cpp (+4-1) 
- (modified) llvm/lib/IR/Value.cpp (+1-1) 
- (modified) llvm/test/Transforms/LoopVectorize/dereferenceable-info-from-assumption-constant-size.ll (+18-5) 


``````````diff
diff --git a/llvm/lib/Analysis/Loads.cpp b/llvm/lib/Analysis/Loads.cpp
index 7bbd469bd035d3..4cc93a8daeb0f1 100644
--- a/llvm/lib/Analysis/Loads.cpp
+++ b/llvm/lib/Analysis/Loads.cpp
@@ -25,6 +25,8 @@
 
 using namespace llvm;
 
+extern cl::opt<unsigned> UseDerefAtPointSemantics;
+
 static bool isAligned(const Value *Base, Align Alignment,
                       const DataLayout &DL) {
   return Base->getPointerAlignment(DL) >= Alignment;
@@ -168,7 +170,8 @@ static bool isDereferenceableAndAlignedPointer(
                                               Size, DL, CtxI, AC, DT, TLI,
                                               Visited, MaxDepth);
 
-  if (CtxI) {
+  if (CtxI &&
+      (!UseDerefAtPointSemantics || CtxI->getFunction()->doesNotFreeMemory())) {
     /// Look through assumes to see if both dereferencability and alignment can
     /// be proven by an assume if needed.
     RetainedKnowledge AlignRK;
diff --git a/llvm/lib/IR/Value.cpp b/llvm/lib/IR/Value.cpp
index b2ee75811fbb7d..03961f34a18b8f 100644
--- a/llvm/lib/IR/Value.cpp
+++ b/llvm/lib/IR/Value.cpp
@@ -36,7 +36,7 @@
 
 using namespace llvm;
 
-static cl::opt<unsigned> UseDerefAtPointSemantics(
+cl::opt<unsigned> UseDerefAtPointSemantics(
     "use-dereferenceable-at-point-semantics", cl::Hidden, cl::init(false),
     cl::desc("Deref attributes and metadata infer facts at definition only"));
 
diff --git a/llvm/test/Transforms/LoopVectorize/dereferenceable-info-from-assumption-constant-size.ll b/llvm/test/Transforms/LoopVectorize/dereferenceable-info-from-assumption-constant-size.ll
index 572511a5ffb926..7b41c9b3197aa6 100644
--- a/llvm/test/Transforms/LoopVectorize/dereferenceable-info-from-assumption-constant-size.ll
+++ b/llvm/test/Transforms/LoopVectorize/dereferenceable-info-from-assumption-constant-size.ll
@@ -1411,7 +1411,7 @@ exit:
 }
 
 ; %a may be freeed between the dereferenceable assumption and accesses.
-; FIXME: It is not safe to use with -use-dereferenceable-at-point-semantics.
+; It is not safe to use with -use-dereferenceable-at-point-semantics.
 define void @may_free_align_deref_assumption_in_header_constant_trip_count_loop_invariant_ptr(ptr noalias %a, ptr noalias %b, ptr noalias %c) {
 ; CHECK-LABEL: define void @may_free_align_deref_assumption_in_header_constant_trip_count_loop_invariant_ptr(
 ; CHECK-SAME: ptr noalias [[A:%.*]], ptr noalias [[B:%.*]], ptr noalias [[C:%.*]]) {
@@ -1422,16 +1422,29 @@ define void @may_free_align_deref_assumption_in_header_constant_trip_count_loop_
 ; CHECK:       [[VECTOR_PH]]:
 ; CHECK-NEXT:    br label %[[VECTOR_BODY:.*]]
 ; CHECK:       [[VECTOR_BODY]]:
-; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[PRED_LOAD_CONTINUE2:.*]] ]
 ; CHECK-NEXT:    [[TMP0:%.*]] = add i64 [[INDEX]], 0
 ; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i32, ptr [[B]], i64 [[TMP0]]
 ; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr inbounds i32, ptr [[TMP1]], i32 0
 ; CHECK-NEXT:    [[WIDE_LOAD:%.*]] = load <2 x i32>, ptr [[TMP2]], align 4
 ; CHECK-NEXT:    [[TMP3:%.*]] = icmp sge <2 x i32> [[WIDE_LOAD]], zeroinitializer
-; CHECK-NEXT:    [[TMP5:%.*]] = load i32, ptr [[A]], align 4
+; CHECK-NEXT:    [[TMP4:%.*]] = xor <2 x i1> [[TMP3]], splat (i1 true)
+; CHECK-NEXT:    [[TMP5:%.*]] = extractelement <2 x i1> [[TMP4]], i32 0
+; CHECK-NEXT:    br i1 [[TMP5]], label %[[PRED_LOAD_IF:.*]], label %[[PRED_LOAD_CONTINUE:.*]]
+; CHECK:       [[PRED_LOAD_IF]]:
 ; CHECK-NEXT:    [[TMP15:%.*]] = load i32, ptr [[A]], align 4
-; CHECK-NEXT:    [[TMP13:%.*]] = insertelement <2 x i32> poison, i32 [[TMP5]], i32 0
-; CHECK-NEXT:    [[TMP11:%.*]] = insertelement <2 x i32> [[TMP13]], i32 [[TMP15]], i32 1
+; CHECK-NEXT:    [[TMP7:%.*]] = insertelement <2 x i32> poison, i32 [[TMP15]], i32 0
+; CHECK-NEXT:    br label %[[PRED_LOAD_CONTINUE]]
+; CHECK:       [[PRED_LOAD_CONTINUE]]:
+; CHECK-NEXT:    [[TMP12:%.*]] = phi <2 x i32> [ poison, %[[VECTOR_BODY]] ], [ [[TMP7]], %[[PRED_LOAD_IF]] ]
+; CHECK-NEXT:    [[TMP13:%.*]] = extractelement <2 x i1> [[TMP4]], i32 1
+; CHECK-NEXT:    br i1 [[TMP13]], label %[[PRED_LOAD_IF1:.*]], label %[[PRED_LOAD_CONTINUE2]]
+; CHECK:       [[PRED_LOAD_IF1]]:
+; CHECK-NEXT:    [[TMP14:%.*]] = load i32, ptr [[A]], align 4
+; CHECK-NEXT:    [[TMP16:%.*]] = insertelement <2 x i32> [[TMP12]], i32 [[TMP14]], i32 1
+; CHECK-NEXT:    br label %[[PRED_LOAD_CONTINUE2]]
+; CHECK:       [[PRED_LOAD_CONTINUE2]]:
+; CHECK-NEXT:    [[TMP11:%.*]] = phi <2 x i32> [ [[TMP12]], %[[PRED_LOAD_CONTINUE]] ], [ [[TMP16]], %[[PRED_LOAD_IF1]] ]
 ; CHECK-NEXT:    [[PREDPHI:%.*]] = select <2 x i1> [[TMP3]], <2 x i32> [[WIDE_LOAD]], <2 x i32> [[TMP11]]
 ; CHECK-NEXT:    [[TMP8:%.*]] = getelementptr inbounds i32, ptr [[C]], i64 [[TMP0]]
 ; CHECK-NEXT:    [[TMP9:%.*]] = getelementptr inbounds i32, ptr [[TMP8]], i32 0

``````````

</details>


https://github.com/llvm/llvm-project/pull/123196