[FunctionAttrs] Handle ConstantRange overflow in memset initializes inference (#145739)

Avoid constructing invalid ConstantRange when Offset + Length in memset
overflows signed 64-bit integer space. This prevents assertion failures
when inferring the initializes attribute.

Fixes #140345

Author: xgupta

llvm/lib/Transforms/IPO/FunctionAttrs.cpp

@@ -675,14 +675,24 @@ ArgumentAccessInfo getArgumentAccessInfo(const Instruction *I,
       [](Value *Length,
          std::optional<int64_t> Offset) -> std::optional<ConstantRange> {
     auto *ConstantLength = dyn_cast<ConstantInt>(Length);
-    if (ConstantLength && Offset &&
-        ConstantLength->getValue().isStrictlyPositive()) {
-      return ConstantRange(
-          APInt(64, *Offset, true),
-          APInt(64, *Offset + ConstantLength->getSExtValue(), true));
+    if (ConstantLength && Offset) {
+      int64_t Len = ConstantLength->getSExtValue();
+
+      // Reject zero or negative lengths
+      if (Len <= 0)
+        return std::nullopt;
+
+      APInt Low(64, *Offset, true);
+      bool Overflow;
+      APInt High = Low.sadd_ov(APInt(64, Len, true), Overflow);
+      if (Overflow)
+        return std::nullopt;
+
+      return ConstantRange(Low, High);
     }
     return std::nullopt;
   };
+
   if (auto *SI = dyn_cast<StoreInst>(I)) {
     if (SI->isSimple() && &SI->getOperandUse(1) == ArgUse.U) {
       // Get the fixed type size of "SI". Since the access range of a write

llvm/test/Transforms/FunctionAttrs/initializes.ll

@@ -649,3 +649,17 @@ define void @range_overflows_signed_64_bit_int(ptr %arg) {
   store i32 0, ptr %getelementptr
   ret void
 }
+
+; We should bail if the memset range overflows a signed 64-bit int.
+define void @memset_large_offset_nonzero_size(ptr %dst) {
+; CHECK: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(argmem: write)
+; CHECK-LABEL: define void @memset_large_offset_nonzero_size(
+; CHECK-SAME: ptr writeonly captures(none) [[DST:%.*]]) #[[ATTR0]] {
+; CHECK-NEXT:    [[OFFSET:%.*]] = getelementptr inbounds i8, ptr [[DST]], i64 9223372036854775805
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr [[OFFSET]], i8 0, i64 3, i1 false)
+; CHECK-NEXT:    ret void
+;
+  %offset = getelementptr inbounds i8, ptr %dst, i64 9223372036854775805
+  call void @llvm.memset.p0.i64(ptr %offset, i8 0, i64 3, i1 false)
+  ret void
+}