[flang] add API to copy and update descriptors for assumed ranks (#93305)

When passing assumed-rank around, the lower bounds, dynamic type and
attribute must sometimes be updated to match the dummy attributes. See
https://github.com/llvm/llvm-project/blob/main/flang/docs/AssumedRank.md#annex-1---descriptor-temporary-for-the-dummy-arguments
for more details.

Doing it inline would require generating many instructions and block CFG
at the LLVM IR dialect level in codegen. Go for a simple runtime API
instead.
A matching fir.rebox_assumed_rank operation will be created and will
allow for easier future optimizations when inlining is done in FIR.

2 files changed, 59 insertions, 0 deletions

diff --git a/flang/unittests/Runtime/CMakeLists.txt b/flang/unittests/Runtime/CMakeLists.txt
index f7caaca..ed047b08 100644
--- a/flang/unittests/Runtime/CMakeLists.txt
+++ b/flang/unittests/Runtime/CMakeLists.txt
@@ -25,6 +25,7 @@ add_flang_unittest(FlangRuntimeTests
   Reduction.cpp
   RuntimeCrashTest.cpp
   Stop.cpp
+  Support.cpp
   Time.cpp
   TemporaryStack.cpp
   Transformational.cpp
diff --git a/flang/unittests/Runtime/Support.cpp b/flang/unittests/Runtime/Support.cpp
new file mode 100644
index 0000000..fa2a233
--- /dev/null
+++ b/flang/unittests/Runtime/Support.cpp
@@ -0,0 +1,58 @@
+//===-- flang/unittests/Runtime/Support.cpp ----------------------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "flang/Runtime/support.h"
+#include "gtest/gtest.h"
+#include "tools.h"
+#include "flang/Runtime/descriptor.h"
+
+using namespace Fortran::runtime;
+using Fortran::common::TypeCategory;
+TEST(CopyAndUpdateDescriptor, Basic) {
+  auto x{MakeArray<TypeCategory::Integer, 4>(
+      std::vector<int>{2, 3}, std::vector<std::int32_t>{0, 1, 2, 3, 4, 5})};
+  x->GetDimension(0).SetLowerBound(11);
+  x->GetDimension(1).SetLowerBound(12);
+
+  StaticDescriptor<2, false> statDesc;
+  Descriptor &result{statDesc.descriptor()};
+
+  RTNAME(CopyAndUpdateDescriptor)
+  (result, *x, nullptr, CFI_attribute_pointer, LowerBoundModifier::Preserve);
+  ASSERT_EQ(result.rank(), 2);
+  EXPECT_EQ(result.raw().base_addr, x->raw().base_addr);
+  EXPECT_TRUE(result.IsPointer());
+  EXPECT_EQ(result.GetDimension(0).Extent(), x->GetDimension(0).Extent());
+  EXPECT_EQ(
+      result.GetDimension(0).LowerBound(), x->GetDimension(0).LowerBound());
+  EXPECT_EQ(result.GetDimension(1).Extent(), x->GetDimension(1).Extent());
+  EXPECT_EQ(
+      result.GetDimension(1).LowerBound(), x->GetDimension(1).LowerBound());
+
+  RTNAME(CopyAndUpdateDescriptor)
+  (result, *x, nullptr, CFI_attribute_allocatable,
+      LowerBoundModifier::SetToZeroes);
+  ASSERT_EQ(result.rank(), 2);
+  EXPECT_EQ(result.raw().base_addr, x->raw().base_addr);
+  EXPECT_TRUE(result.IsAllocatable());
+  EXPECT_EQ(result.GetDimension(0).Extent(), x->GetDimension(0).Extent());
+  EXPECT_EQ(result.GetDimension(0).LowerBound(), 0);
+  EXPECT_EQ(result.GetDimension(1).Extent(), x->GetDimension(1).Extent());
+  EXPECT_EQ(result.GetDimension(1).LowerBound(), 0);
+
+  RTNAME(CopyAndUpdateDescriptor)
+  (result, *x, nullptr, CFI_attribute_other, LowerBoundModifier::SetToOnes);
+  ASSERT_EQ(result.rank(), 2);
+  EXPECT_EQ(result.raw().base_addr, x->raw().base_addr);
+  EXPECT_FALSE(result.IsAllocatable());
+  EXPECT_FALSE(result.IsPointer());
+  EXPECT_EQ(result.GetDimension(0).Extent(), x->GetDimension(0).Extent());
+  EXPECT_EQ(result.GetDimension(0).LowerBound(), 1);
+  EXPECT_EQ(result.GetDimension(1).Extent(), x->GetDimension(1).Extent());
+  EXPECT_EQ(result.GetDimension(1).LowerBound(), 1);
+}