[Loads] Check if Ptr can be freed between Assume and CtxI. (#161255)

When using information from dereferenceable assumptions, we need to make
sure that the memory is not freed between the assume and the specified
context instruction. Instead of just checking canBeFreed, check if there
any calls that may free between the assume and the context instruction.

Note that this also adjusts the context instruction to be the terminator
in the loop predecessor, if there is one and it is a branch (to avoid
things like invoke).

PR: llvm/llvm-project#161255

Author: fhahn

llvm/lib/Analysis/Loads.cpp

@@ -37,17 +37,13 @@ static bool isDereferenceableAndAlignedPointerViaAssumption(
     function_ref<bool(const RetainedKnowledge &RK)> CheckSize,
     const DataLayout &DL, const Instruction *CtxI, AssumptionCache *AC,
     const DominatorTree *DT) {
-  // Dereferenceable information from assumptions is only valid if the value
-  // cannot be freed between the assumption and use. For now just use the
-  // information for values that cannot be freed in the function.
-  // TODO: More precisely check if the pointer can be freed between assumption
-  // and use.
-  if (!CtxI || Ptr->canBeFreed())
+  if (!CtxI)
     return false;
   /// Look through assumes to see if both dereferencability and alignment can
   /// be proven by an assume if needed.
   RetainedKnowledge AlignRK;
   RetainedKnowledge DerefRK;
+  bool PtrCanBeFreed = Ptr->canBeFreed();
   bool IsAligned = Ptr->getPointerAlignment(DL) >= Alignment;
   return getKnowledgeForValue(
       Ptr, {Attribute::Dereferenceable, Attribute::Alignment}, *AC,
@@ -56,7 +52,11 @@ static bool isDereferenceableAndAlignedPointerViaAssumption(
           return false;
         if (RK.AttrKind == Attribute::Alignment)
           AlignRK = std::max(AlignRK, RK);
-        if (RK.AttrKind == Attribute::Dereferenceable)
+
+        // Dereferenceable information from assumptions is only valid if the
+        // value cannot be freed between the assumption and use.
+        if ((!PtrCanBeFreed || willNotFreeBetween(Assume, CtxI)) &&
+            RK.AttrKind == Attribute::Dereferenceable)
           DerefRK = std::max(DerefRK, RK);
         IsAligned |= AlignRK && AlignRK.ArgValue >= Alignment.value();
         if (IsAligned && DerefRK && CheckSize(DerefRK))
@@ -390,7 +390,11 @@ bool llvm::isDereferenceableAndAlignedInLoop(
   } else
     return false;
 
-  Instruction *HeaderFirstNonPHI = &*L->getHeader()->getFirstNonPHIIt();
+  Instruction *CtxI = &*L->getHeader()->getFirstNonPHIIt();
+  if (BasicBlock *LoopPred = L->getLoopPredecessor()) {
+    if (isa<BranchInst>(LoopPred->getTerminator()))
+      CtxI = LoopPred->getTerminator();
+  }
   return isDereferenceableAndAlignedPointerViaAssumption(
              Base, Alignment,
              [&SE, AccessSizeSCEV, &LoopGuards](const RetainedKnowledge &RK) {
@@ -399,9 +403,9 @@ bool llvm::isDereferenceableAndAlignedInLoop(
                    SE.applyLoopGuards(AccessSizeSCEV, *LoopGuards),
                    SE.applyLoopGuards(SE.getSCEV(RK.IRArgValue), *LoopGuards));
              },
-             DL, HeaderFirstNonPHI, AC, &DT) ||
+             DL, CtxI, AC, &DT) ||
          isDereferenceableAndAlignedPointer(Base, Alignment, AccessSize, DL,
-                                            HeaderFirstNonPHI, AC, &DT);
+                                            CtxI, AC, &DT);
 }
 
 static bool suppressSpeculativeLoadForSanitizers(const Instruction &CtxI) {

llvm/test/Transforms/LoopVectorize/dereferenceable-info-from-assumption-constant-size.ll

@@ -1182,31 +1182,13 @@ define void @deref_assumption_in_header_constant_trip_count_nofree_via_context(p
 ; CHECK:       [[VECTOR_PH]]:
 ; CHECK-NEXT:    br label %[[VECTOR_BODY:.*]]
 ; CHECK:       [[VECTOR_BODY]]:
-; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[PRED_LOAD_CONTINUE2:.*]] ]
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr i32, ptr [[A]], i64 [[INDEX]]
 ; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr inbounds i32, ptr [[B]], i64 [[INDEX]]
 ; CHECK-NEXT:    [[WIDE_LOAD1:%.*]] = load <2 x i32>, ptr [[TMP0]], align 4
-; CHECK-NEXT:    [[TMP2:%.*]] = icmp slt <2 x i32> [[WIDE_LOAD1]], zeroinitializer
-; CHECK-NEXT:    [[TMP13:%.*]] = extractelement <2 x i1> [[TMP2]], i32 0
-; CHECK-NEXT:    br i1 [[TMP13]], label %[[PRED_LOAD_IF:.*]], label %[[PRED_LOAD_CONTINUE:.*]]
-; CHECK:       [[PRED_LOAD_IF]]:
-; CHECK-NEXT:    [[TMP3:%.*]] = add i64 [[INDEX]], 0
-; CHECK-NEXT:    [[TMP4:%.*]] = getelementptr i32, ptr [[A]], i64 [[TMP3]]
-; CHECK-NEXT:    [[TMP5:%.*]] = load i32, ptr [[TMP4]], align 4
-; CHECK-NEXT:    [[TMP6:%.*]] = insertelement <2 x i32> poison, i32 [[TMP5]], i32 0
-; CHECK-NEXT:    br label %[[PRED_LOAD_CONTINUE]]
-; CHECK:       [[PRED_LOAD_CONTINUE]]:
-; CHECK-NEXT:    [[TMP7:%.*]] = phi <2 x i32> [ poison, %[[VECTOR_BODY]] ], [ [[TMP6]], %[[PRED_LOAD_IF]] ]
-; CHECK-NEXT:    [[TMP8:%.*]] = extractelement <2 x i1> [[TMP2]], i32 1
-; CHECK-NEXT:    br i1 [[TMP8]], label %[[PRED_LOAD_IF1:.*]], label %[[PRED_LOAD_CONTINUE2]]
-; CHECK:       [[PRED_LOAD_IF1]]:
-; CHECK-NEXT:    [[TMP9:%.*]] = add i64 [[INDEX]], 1
-; CHECK-NEXT:    [[TMP10:%.*]] = getelementptr i32, ptr [[A]], i64 [[TMP9]]
-; CHECK-NEXT:    [[TMP11:%.*]] = load i32, ptr [[TMP10]], align 4
-; CHECK-NEXT:    [[TMP12:%.*]] = insertelement <2 x i32> [[TMP7]], i32 [[TMP11]], i32 1
-; CHECK-NEXT:    br label %[[PRED_LOAD_CONTINUE2]]
-; CHECK:       [[PRED_LOAD_CONTINUE2]]:
-; CHECK-NEXT:    [[WIDE_LOAD:%.*]] = phi <2 x i32> [ [[TMP7]], %[[PRED_LOAD_CONTINUE]] ], [ [[TMP12]], %[[PRED_LOAD_IF1]] ]
-; CHECK-NEXT:    [[PREDPHI:%.*]] = select <2 x i1> [[TMP2]], <2 x i32> [[WIDE_LOAD]], <2 x i32> [[WIDE_LOAD1]]
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp sge <2 x i32> [[WIDE_LOAD1]], zeroinitializer
+; CHECK-NEXT:    [[WIDE_LOAD2:%.*]] = load <2 x i32>, ptr [[TMP1]], align 4
+; CHECK-NEXT:    [[PREDPHI:%.*]] = select <2 x i1> [[TMP2]], <2 x i32> [[WIDE_LOAD1]], <2 x i32> [[WIDE_LOAD2]]
 ; CHECK-NEXT:    [[TMP14:%.*]] = getelementptr inbounds i32, ptr [[C]], i64 [[INDEX]]
 ; CHECK-NEXT:    store <2 x i32> [[PREDPHI]], ptr [[TMP14]], align 4
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 2

llvm/test/Transforms/LoopVectorize/single-early-exit-deref-assumptions.ll

@@ -504,24 +504,35 @@ exit:
 define i64 @early_exit_alignment_and_deref_known_via_assumption_with_constant_size_nofree_via_context(ptr noalias %p1, ptr noalias %p2) nosync {
 ; CHECK-LABEL: define i64 @early_exit_alignment_and_deref_known_via_assumption_with_constant_size_nofree_via_context(
 ; CHECK-SAME: ptr noalias [[P1:%.*]], ptr noalias [[P2:%.*]]) #[[ATTR1:[0-9]+]] {
-; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:  [[ENTRY:.*:]]
 ; CHECK-NEXT:    call void @llvm.assume(i1 true) [ "align"(ptr [[P1]], i64 4), "dereferenceable"(ptr [[P1]], i64 1024) ]
 ; CHECK-NEXT:    call void @llvm.assume(i1 true) [ "align"(ptr [[P2]], i64 4), "dereferenceable"(ptr [[P2]], i64 1024) ]
-; CHECK-NEXT:    br label %[[LOOP:.*]]
-; CHECK:       [[LOOP]]:
-; CHECK-NEXT:    [[INDEX1:%.*]] = phi i64 [ [[INDEX_NEXT:%.*]], %[[LOOP_INC:.*]] ], [ 0, %[[ENTRY]] ]
+; CHECK-NEXT:    br label %[[VECTOR_PH:.*]]
+; CHECK:       [[VECTOR_PH]]:
+; CHECK-NEXT:    br label %[[VECTOR_BODY:.*]]
+; CHECK:       [[VECTOR_BODY]]:
+; CHECK-NEXT:    [[INDEX1:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT3:%.*]], %[[VECTOR_BODY]] ]
 ; CHECK-NEXT:    [[ARRAYIDX2:%.*]] = getelementptr inbounds i8, ptr [[P1]], i64 [[INDEX1]]
-; CHECK-NEXT:    [[LD1:%.*]] = load i8, ptr [[ARRAYIDX2]], align 1
+; CHECK-NEXT:    [[WIDE_LOAD:%.*]] = load <4 x i8>, ptr [[ARRAYIDX2]], align 1
 ; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i8, ptr [[P2]], i64 [[INDEX1]]
-; CHECK-NEXT:    [[LD2:%.*]] = load i8, ptr [[TMP1]], align 1
-; CHECK-NEXT:    [[CMP3:%.*]] = icmp eq i8 [[LD1]], [[LD2]]
-; CHECK-NEXT:    br i1 [[CMP3]], label %[[LOOP_INC]], label %[[LOOP_END:.*]]
-; CHECK:       [[LOOP_INC]]:
-; CHECK-NEXT:    [[INDEX_NEXT]] = add i64 [[INDEX1]], 1
-; CHECK-NEXT:    [[EXITCOND:%.*]] = icmp ne i64 [[INDEX_NEXT]], 1024
-; CHECK-NEXT:    br i1 [[EXITCOND]], label %[[LOOP]], label %[[LOOP_END]]
+; CHECK-NEXT:    [[WIDE_LOAD2:%.*]] = load <4 x i8>, ptr [[TMP1]], align 1
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp ne <4 x i8> [[WIDE_LOAD]], [[WIDE_LOAD2]]
+; CHECK-NEXT:    [[INDEX_NEXT3]] = add nuw i64 [[INDEX1]], 4
+; CHECK-NEXT:    [[TMP3:%.*]] = freeze <4 x i1> [[TMP2]]
+; CHECK-NEXT:    [[TMP4:%.*]] = call i1 @llvm.vector.reduce.or.v4i1(<4 x i1> [[TMP3]])
+; CHECK-NEXT:    [[TMP5:%.*]] = icmp eq i64 [[INDEX_NEXT3]], 1024
+; CHECK-NEXT:    [[TMP6:%.*]] = or i1 [[TMP4]], [[TMP5]]
+; CHECK-NEXT:    br i1 [[TMP6]], label %[[MIDDLE_SPLIT:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP7:![0-9]+]]
+; CHECK:       [[MIDDLE_SPLIT]]:
+; CHECK-NEXT:    br i1 [[TMP4]], label %[[VECTOR_EARLY_EXIT:.*]], label %[[MIDDLE_BLOCK:.*]]
+; CHECK:       [[MIDDLE_BLOCK]]:
+; CHECK-NEXT:    br label %[[LOOP_END:.*]]
+; CHECK:       [[VECTOR_EARLY_EXIT]]:
+; CHECK-NEXT:    [[TMP7:%.*]] = call i64 @llvm.experimental.cttz.elts.i64.v4i1(<4 x i1> [[TMP2]], i1 true)
+; CHECK-NEXT:    [[TMP8:%.*]] = add i64 [[INDEX1]], [[TMP7]]
+; CHECK-NEXT:    br label %[[LOOP_END]]
 ; CHECK:       [[LOOP_END]]:
-; CHECK-NEXT:    [[RETVAL:%.*]] = phi i64 [ [[INDEX1]], %[[LOOP]] ], [ -1, %[[LOOP_INC]] ]
+; CHECK-NEXT:    [[RETVAL:%.*]] = phi i64 [ -1, %[[MIDDLE_BLOCK]] ], [ [[TMP8]], %[[VECTOR_EARLY_EXIT]] ]
 ; CHECK-NEXT:    ret i64 [[RETVAL]]
 ;
 entry: