[Loads] Support dereference for non-constant offset

Improve isDereferenceableAndAlignedInLoop API to consider non-constant
offset feeding into the first access in the loop.

Author: annamthomas

llvm/lib/Analysis/Loads.cpp

@@ -361,29 +361,29 @@ bool llvm::isDereferenceableAndAlignedInLoop(
     AccessSize = MaxPtrDiff;
     AccessSizeSCEV = PtrDiff;
   } else if (auto *MinAdd = dyn_cast<SCEVAddExpr>(AccessStart)) {
-    if (MinAdd->getNumOperands() != 2)
-      return false;
+    const auto *NewBase = dyn_cast<SCEVUnknown>(SE.getPointerBase(MinAdd));
+    const auto *OffsetSCEV = SE.removePointerBase(MinAdd);
 
-    const auto *Offset = dyn_cast<SCEVConstant>(MinAdd->getOperand(0));
-    const auto *NewBase = dyn_cast<SCEVUnknown>(MinAdd->getOperand(1));
-    if (!Offset || !NewBase)
+    if (!OffsetSCEV || !NewBase)
       return false;
 
-    // The following code below assumes the offset is unsigned, but GEP
-    // offsets are treated as signed so we can end up with a signed value
-    // here too. For example, suppose the initial PHI value is (i8 255),
-    // the offset will be treated as (i8 -1) and sign-extended to (i64 -1).
-    if (Offset->getAPInt().isNegative())
+    if (!SE.isKnownNonNegative(OffsetSCEV))
       return false;
 
     // For the moment, restrict ourselves to the case where the offset is a
     // multiple of the requested alignment and the base is aligned.
     // TODO: generalize if a case found which warrants
-    if (Offset->getAPInt().urem(Alignment.value()) != 0)
+    auto *OffsetSCEVTy = OffsetSCEV->getType();
+    if (!SE.isKnownPredicate(
+            ICmpInst::ICMP_EQ,
+            SE.getURemExpr(OffsetSCEV,
+                           SE.getConstant(OffsetSCEVTy, Alignment.value())),
+            SE.getZero(OffsetSCEVTy)))
       return false;
-
-    AccessSize = MaxPtrDiff + Offset->getAPInt();
-    AccessSizeSCEV = SE.getAddExpr(PtrDiff, Offset);
+    AccessSizeSCEV = SE.getAddExpr(PtrDiff, OffsetSCEV);
+    const auto *Offset = dyn_cast<SCEVConstant>(OffsetSCEV);
+    AccessSize = MaxPtrDiff + (Offset ? Offset->getAPInt()
+                                      : SE.getUnsignedRangeMax(OffsetSCEV));
     Base = NewBase->getValue();
   } else
     return false;

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

@@ -562,38 +562,19 @@ define void @deref_assumption_loop_access_start_variable(i8 %v, ptr noundef %P,
 ; CHECK-NEXT:    [[TMP1:%.*]] = add i64 [[IV_START]], [[N_VEC]]
 ; 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:    [[OFFSET_IDX:%.*]] = add i64 [[IV_START]], [[INDEX]]
-; CHECK-NEXT:    [[TMP6:%.*]] = getelementptr inbounds i32, ptr [[B]], i64 [[OFFSET_IDX]]
-; CHECK-NEXT:    [[WIDE_LOAD:%.*]] = load <2 x i32>, ptr [[TMP6]], align 1
-; CHECK-NEXT:    [[TMP8:%.*]] = icmp sge <2 x i32> [[WIDE_LOAD]], zeroinitializer
-; CHECK-NEXT:    [[TMP4:%.*]] = xor <2 x i1> [[TMP8]], 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:    [[TMP16:%.*]] = add i64 [[OFFSET_IDX]], 0
-; CHECK-NEXT:    [[TMP7:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[TMP16]]
-; CHECK-NEXT:    [[TMP19:%.*]] = load i32, ptr [[TMP7]], align 1
-; CHECK-NEXT:    [[TMP9:%.*]] = insertelement <2 x i32> poison, i32 [[TMP19]], i32 0
-; CHECK-NEXT:    br label %[[PRED_LOAD_CONTINUE]]
-; CHECK:       [[PRED_LOAD_CONTINUE]]:
-; CHECK-NEXT:    [[TMP10:%.*]] = phi <2 x i32> [ poison, %[[VECTOR_BODY]] ], [ [[TMP9]], %[[PRED_LOAD_IF]] ]
-; CHECK-NEXT:    [[TMP11:%.*]] = extractelement <2 x i1> [[TMP4]], i32 1
-; CHECK-NEXT:    br i1 [[TMP11]], label %[[PRED_LOAD_IF1:.*]], label %[[PRED_LOAD_CONTINUE2]]
-; CHECK:       [[PRED_LOAD_IF1]]:
-; CHECK-NEXT:    [[TMP12:%.*]] = add i64 [[OFFSET_IDX]], 1
-; CHECK-NEXT:    [[TMP13:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[TMP12]]
-; CHECK-NEXT:    [[TMP14:%.*]] = load i32, ptr [[TMP13]], align 1
-; CHECK-NEXT:    [[TMP15:%.*]] = insertelement <2 x i32> [[TMP10]], i32 [[TMP14]], i32 1
-; CHECK-NEXT:    br label %[[PRED_LOAD_CONTINUE2]]
-; CHECK:       [[PRED_LOAD_CONTINUE2]]:
-; CHECK-NEXT:    [[WIDE_LOAD1:%.*]] = phi <2 x i32> [ [[TMP10]], %[[PRED_LOAD_CONTINUE]] ], [ [[TMP15]], %[[PRED_LOAD_IF1]] ]
-; CHECK-NEXT:    [[PREDPHI:%.*]] = select <2 x i1> [[TMP8]], <2 x i32> [[WIDE_LOAD]], <2 x i32> [[WIDE_LOAD1]]
-; CHECK-NEXT:    [[TMP17:%.*]] = getelementptr inbounds i32, ptr [[C]], i64 [[OFFSET_IDX]]
-; CHECK-NEXT:    store <2 x i32> [[PREDPHI]], ptr [[TMP17]], align 1
+; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr i32, ptr [[A]], i64 [[OFFSET_IDX]]
+; CHECK-NEXT:    [[TMP7:%.*]] = getelementptr inbounds i32, ptr [[B]], i64 [[OFFSET_IDX]]
+; CHECK-NEXT:    [[WIDE_LOAD:%.*]] = load <2 x i32>, ptr [[TMP7]], align 1
+; CHECK-NEXT:    [[TMP4:%.*]] = icmp sge <2 x i32> [[WIDE_LOAD]], zeroinitializer
+; CHECK-NEXT:    [[WIDE_LOAD1:%.*]] = load <2 x i32>, ptr [[TMP2]], align 1
+; CHECK-NEXT:    [[PREDPHI:%.*]] = select <2 x i1> [[TMP4]], <2 x i32> [[WIDE_LOAD]], <2 x i32> [[WIDE_LOAD1]]
+; CHECK-NEXT:    [[TMP5:%.*]] = getelementptr inbounds i32, ptr [[C]], i64 [[OFFSET_IDX]]
+; CHECK-NEXT:    store <2 x i32> [[PREDPHI]], ptr [[TMP5]], align 1
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 2
-; CHECK-NEXT:    [[TMP18:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
-; CHECK-NEXT:    br i1 [[TMP18]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP14:![0-9]+]]
+; CHECK-NEXT:    [[TMP6:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
+; CHECK-NEXT:    br i1 [[TMP6]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP14:![0-9]+]]
 ; CHECK:       [[MIDDLE_BLOCK]]:
 ; CHECK-NEXT:    [[CMP_N:%.*]] = icmp eq i64 [[TMP3]], [[N_VEC]]
 ; CHECK-NEXT:    br i1 [[CMP_N]], label %[[EXIT:.*]], label %[[SCALAR_PH]]
@@ -620,7 +601,6 @@ define void @deref_assumption_loop_access_start_variable(i8 %v, ptr noundef %P,
 ; CHECK:       [[EXIT]]:
 ; CHECK-NEXT:    ret void
 ;
-
 entry:
   %a = getelementptr i8, ptr %P, i64 16
   %cmp = icmp slt i64 %iv.start, %N