[flang-commits] [flang] c019372 - [flang][runtime] Add dynamically allocated temporary storage

Tom Eccles via flang-commits flang-commits at lists.llvm.org
Thu May 18 04:04:37 PDT 2023


Author: Tom Eccles
Date: 2023-05-18T10:48:45Z
New Revision: c01937296b01f512e9074730e04ad0bedb40d089

URL: https://github.com/llvm/llvm-project/commit/c01937296b01f512e9074730e04ad0bedb40d089
DIFF: https://github.com/llvm/llvm-project/commit/c01937296b01f512e9074730e04ad0bedb40d089.diff

LOG: [flang][runtime] Add dynamically allocated temporary storage

These functions will be used as part of the HLFIR lowering for
forall/where. The contents of the API were requested by @jeanPerier.

The API is designed around that use case, assuming that the caller knows
through some side channel what size to allocate for boxes returned from
the pop() function.

Differential Revision: https://reviews.llvm.org/D150050

Added: 
    flang/include/flang/Runtime/temporary-stack.h
    flang/runtime/temporary-stack.cpp
    flang/unittests/Runtime/TemporaryStack.cpp

Modified: 
    flang/runtime/CMakeLists.txt
    flang/unittests/Runtime/CMakeLists.txt

Removed: 
    


################################################################################
diff  --git a/flang/include/flang/Runtime/temporary-stack.h b/flang/include/flang/Runtime/temporary-stack.h
new file mode 100644
index 0000000000000..3c1a4ea11a435
--- /dev/null
+++ b/flang/include/flang/Runtime/temporary-stack.h
@@ -0,0 +1,67 @@
+//===-- include/flang/Runtime/temporary-stack.h -----------------*- 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
+//
+//===----------------------------------------------------------------------===//
+// Runtime functions for storing a dynamically resizable number of temporaries.
+// For use in HLFIR lowering.
+//===----------------------------------------------------------------------===//
+
+#ifndef FORTRAN_RUNTIME_TEMPORARY_STACK_H_
+#define FORTRAN_RUNTIME_TEMPORARY_STACK_H_
+
+#include "flang/Runtime/entry-names.h"
+#include <stdint.h>
+
+namespace Fortran::runtime {
+class Descriptor;
+extern "C" {
+
+// Stores both the descriptor and a copy of the value in a dynamically resizable
+// data structure identified by opaquePtr. All value stacks must be destroyed
+// at the end of their lifetime and not used afterwards.
+// Popped descriptors point to the copy of the value, not the original address
+// of the value. This copy is dynamically allocated, it is up to the caller to
+// free the value pointed to by the box. The copy operation is a simple memcpy.
+// The sourceFile and line number used when creating the stack are shared for
+// all operations.
+// Opaque pointers returned from these are incompatible with those returned by
+// the flavours for storing descriptors.
+[[nodiscard]] void *RTNAME(CreateValueStack)(
+    const char *sourceFile = nullptr, int line = 0);
+void RTNAME(PushValue)(void *opaquePtr, const Descriptor &value);
+// Note: retValue should be large enough to hold the right number of dimensions,
+// and the optional descriptor addendum
+void RTNAME(PopValue)(void *opaquePtr, Descriptor &retValue);
+// Return the i'th element into retValue (which must be the right size). An
+// exact copy of this descriptor remains in this storage so this one should not
+// be deallocated
+void RTNAME(ValueAt)(void *opaquePtr, uint64_t i, Descriptor &retValue);
+void RTNAME(DestroyValueStack)(void *opaquePtr);
+
+// Stores descriptors value in a dynamically resizable data structure identified
+// by opaquePtr. All descriptor stacks must be destroyed at the end of their
+// lifetime and not used afterwards.
+// Popped descriptors are identical to those which were pushed.
+// The sourceFile and line number used when creating the stack are shared for
+// all operations.
+// Opaque pointers returned from these are incompatible with those returned by
+// the flavours for storing both descriptors and values.
+[[nodiscard]] void *RTNAME(CreateDescriptorStack)(
+    const char *sourceFile = nullptr, int line = 0);
+void RTNAME(PushDescriptor)(void *opaquePtr, const Descriptor &value);
+// Note: retValue should be large enough to hold the right number of dimensions,
+// and the optional descriptor addendum
+void RTNAME(PopDescriptor)(void *opaquePtr, Descriptor &retValue);
+// Return the i'th element into retValue (which must be the right size). An
+// exact copy of this descriptor remains in this storage so this one should not
+// be deallocated
+void RTNAME(DescriptorAt)(void *opaquePtr, uint64_t i, Descriptor &retValue);
+void RTNAME(DestroyDescriptorStack)(void *opaquePtr);
+
+} // extern "C"
+} // namespace Fortran::runtime
+
+#endif // FORTRAN_RUNTIME_TEMPORARY_STACK_H_

diff  --git a/flang/runtime/CMakeLists.txt b/flang/runtime/CMakeLists.txt
index 2a7e74c175533..47166692e2795 100644
--- a/flang/runtime/CMakeLists.txt
+++ b/flang/runtime/CMakeLists.txt
@@ -144,6 +144,7 @@ add_flang_library(FortranRuntime
   stop.cpp
   sum.cpp
   support.cpp
+  temporary-stack.cpp
   terminator.cpp
   time-intrinsic.cpp
   tools.cpp

diff  --git a/flang/runtime/temporary-stack.cpp b/flang/runtime/temporary-stack.cpp
new file mode 100644
index 0000000000000..9107eb7654db4
--- /dev/null
+++ b/flang/runtime/temporary-stack.cpp
@@ -0,0 +1,222 @@
+//===-- runtime/temporary-stack.cpp ---------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// Implements std::vector like storage for a dynamically resizable number of
+// temporaries. For use in HLFIR lowering.
+
+#include "flang/Runtime/temporary-stack.h"
+#include "terminator.h"
+#include "flang/ISO_Fortran_binding.h"
+#include "flang/Runtime/assign.h"
+#include "flang/Runtime/descriptor.h"
+#include "flang/Runtime/memory.h"
+
+namespace {
+
+using namespace Fortran::runtime;
+
+// the number of elements to allocate when first creating the vector
+constexpr size_t INITIAL_ALLOC = 8;
+
+/// To store C style data. Does not run constructors/destructors.
+/// Not using std::vector to avoid linking the runtime library to stdc++
+template <bool COPY_VALUES> class DescriptorStorage final {
+  using size_type = uint64_t; // see checkedMultiply()
+
+  size_type capacity_{0};
+  size_type size_{0};
+  Descriptor **data_{nullptr};
+  Terminator terminator_;
+
+  // return true on overflow
+  static bool checkedMultiply(size_type x, size_type y, size_type &res);
+
+  void resize(size_type newCapacity);
+
+  Descriptor *cloneDescriptor(const Descriptor &source);
+
+public:
+  DescriptorStorage(const char *sourceFile, int line);
+  ~DescriptorStorage();
+
+  // `new` but using the runtime allocation API
+  static inline DescriptorStorage *allocate(const char *sourceFile, int line) {
+    Terminator term{sourceFile, line};
+    void *ptr = AllocateMemoryOrCrash(term, sizeof(DescriptorStorage));
+    return new (ptr) DescriptorStorage{sourceFile, line};
+  }
+
+  // `delete` but using the runtime allocation API
+  static inline void destroy(DescriptorStorage *instance) {
+    instance->~DescriptorStorage();
+    FreeMemory(instance);
+  }
+
+  // clones a descriptor into this storage
+  void push(const Descriptor &source);
+
+  // out must be big enough to hold a descriptor of the right rank and addendum
+  void pop(Descriptor &out);
+
+  // out must be big enough to hold a descriptor of the right rank and addendum
+  void at(size_type i, Descriptor &out);
+};
+
+using ValueStack = DescriptorStorage</*COPY_VALUES=*/true>;
+using DescriptorStack = DescriptorStorage</*COPY_VALUES=*/false>;
+} // namespace
+
+template <bool COPY_VALUES>
+bool DescriptorStorage<COPY_VALUES>::checkedMultiply(
+    size_type x, size_type y, size_type &res) {
+  // TODO: c++20 [[unlikely]]
+  if (x > UINT64_MAX / y) {
+    return true;
+  }
+  res = x * y;
+  return false;
+}
+
+template <bool COPY_VALUES>
+void DescriptorStorage<COPY_VALUES>::resize(size_type newCapacity) {
+  if (newCapacity <= capacity_) {
+    return;
+  }
+  size_type bytes;
+  if (checkedMultiply(newCapacity, sizeof(Descriptor *), bytes)) {
+    terminator_.Crash("temporary-stack: out of memory");
+  }
+  Descriptor **newData =
+      static_cast<Descriptor **>(AllocateMemoryOrCrash(terminator_, bytes));
+  memcpy(newData, data_, capacity_ * sizeof(Descriptor *));
+  FreeMemory(data_);
+  data_ = newData;
+  capacity_ = newCapacity;
+}
+
+template <bool COPY_VALUES>
+Descriptor *DescriptorStorage<COPY_VALUES>::cloneDescriptor(
+    const Descriptor &source) {
+  const std::size_t bytes = source.SizeInBytes();
+  void *memory = AllocateMemoryOrCrash(terminator_, bytes);
+  Descriptor *desc = new (memory) Descriptor{source};
+  return desc;
+}
+
+template <bool COPY_VALUES>
+DescriptorStorage<COPY_VALUES>::DescriptorStorage(
+    const char *sourceFile, int line)
+    : terminator_{sourceFile, line} {
+  resize(INITIAL_ALLOC);
+}
+
+template <bool COPY_VALUES>
+DescriptorStorage<COPY_VALUES>::~DescriptorStorage() {
+  for (size_type i = 0; i < size_; ++i) {
+    Descriptor *element = data_[i];
+    if constexpr (COPY_VALUES) {
+      element->Destroy(false, true);
+    }
+    FreeMemory(element);
+  }
+  FreeMemory(data_);
+}
+
+template <bool COPY_VALUES>
+void DescriptorStorage<COPY_VALUES>::push(const Descriptor &source) {
+  if (size_ == capacity_) {
+    size_type newSize;
+    if (checkedMultiply(capacity_, 2, newSize)) {
+      terminator_.Crash("temporary-stack: out of address space");
+    }
+    resize(newSize);
+  }
+  data_[size_] = cloneDescriptor(source);
+  Descriptor &box = *data_[size_];
+  size_ += 1;
+
+  if constexpr (COPY_VALUES) {
+    // copy the data pointed to by the box
+    box.set_base_addr(nullptr);
+    box.Allocate();
+    RTNAME(AssignTemporary)
+    (box, source, terminator_.sourceFileName(), terminator_.sourceLine());
+  }
+}
+
+template <bool COPY_VALUES>
+void DescriptorStorage<COPY_VALUES>::pop(Descriptor &out) {
+  if (size_ == 0) {
+    terminator_.Crash("temporary-stack: pop empty storage");
+  }
+  size_ -= 1;
+  Descriptor *ptr = data_[size_];
+  out = *ptr; // Descriptor::operator= handles the 
diff erent sizes
+  FreeMemory(ptr);
+}
+
+template <bool COPY_VALUES>
+void DescriptorStorage<COPY_VALUES>::at(size_type i, Descriptor &out) {
+  if (i >= size_) {
+    terminator_.Crash("temporary-stack: out of bounds access");
+  }
+  Descriptor *ptr = data_[i];
+  out = *ptr; // Descriptor::operator= handles the 
diff erent sizes
+}
+
+inline static ValueStack *getValueStorage(void *opaquePtr) {
+  return static_cast<ValueStack *>(opaquePtr);
+}
+inline static DescriptorStack *getDescriptorStorage(void *opaquePtr) {
+  return static_cast<DescriptorStack *>(opaquePtr);
+}
+
+namespace Fortran::runtime {
+extern "C" {
+void *RTNAME(CreateValueStack)(const char *sourceFile, int line) {
+  return ValueStack::allocate(sourceFile, line);
+}
+
+void RTNAME(PushValue)(void *opaquePtr, const Descriptor &value) {
+  getValueStorage(opaquePtr)->push(value);
+}
+
+void RTNAME(PopValue)(void *opaquePtr, Descriptor &value) {
+  getValueStorage(opaquePtr)->pop(value);
+}
+
+void RTNAME(ValueAt)(void *opaquePtr, uint64_t i, Descriptor &value) {
+  getValueStorage(opaquePtr)->at(i, value);
+}
+
+void RTNAME(DestroyValueStack)(void *opaquePtr) {
+  ValueStack::destroy(getValueStorage(opaquePtr));
+}
+
+void *RTNAME(CreateDescriptorStack)(const char *sourceFile, int line) {
+  return DescriptorStack::allocate(sourceFile, line);
+}
+
+void RTNAME(PushDescriptor)(void *opaquePtr, const Descriptor &value) {
+  getDescriptorStorage(opaquePtr)->push(value);
+}
+
+void RTNAME(PopDescriptor)(void *opaquePtr, Descriptor &value) {
+  getDescriptorStorage(opaquePtr)->pop(value);
+}
+
+void RTNAME(DescriptorAt)(void *opaquePtr, uint64_t i, Descriptor &value) {
+  getValueStorage(opaquePtr)->at(i, value);
+}
+
+void RTNAME(DestroyDescriptorStack)(void *opaquePtr) {
+  DescriptorStack::destroy(getDescriptorStorage(opaquePtr));
+}
+
+} // extern "C"
+} // namespace Fortran::runtime

diff  --git a/flang/unittests/Runtime/CMakeLists.txt b/flang/unittests/Runtime/CMakeLists.txt
index bba52706ea2d4..23f02aa751246 100644
--- a/flang/unittests/Runtime/CMakeLists.txt
+++ b/flang/unittests/Runtime/CMakeLists.txt
@@ -25,6 +25,7 @@ add_flang_unittest(FlangRuntimeTests
   RuntimeCrashTest.cpp
   Stop.cpp
   Time.cpp
+  TemporaryStack.cpp
   Transformational.cpp
 )
 

diff  --git a/flang/unittests/Runtime/TemporaryStack.cpp b/flang/unittests/Runtime/TemporaryStack.cpp
new file mode 100644
index 0000000000000..8ca5a27a4c15c
--- /dev/null
+++ b/flang/unittests/Runtime/TemporaryStack.cpp
@@ -0,0 +1,265 @@
+//===--- flang/unittests/Runtime/TemporaryStack.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 "gtest/gtest.h"
+#include "tools.h"
+#include "flang/ISO_Fortran_binding.h"
+#include "flang/Runtime/allocatable.h"
+#include "flang/Runtime/cpp-type.h"
+#include "flang/Runtime/descriptor.h"
+#include "flang/Runtime/temporary-stack.h"
+#include "flang/Runtime/type-code.h"
+#include <vector>
+
+using namespace Fortran::runtime;
+
+// true if two descriptors are otherwise identical, except for 
diff erent data
+// pointers. The pointed-to elements are bit for bit identical.
+static void descriptorAlmostEqual(
+    const Descriptor &lhs, const Descriptor &rhs) {
+  const Fortran::ISO::CFI_cdesc_t &lhsRaw = lhs.raw();
+  const Fortran::ISO::CFI_cdesc_t &rhsRaw = rhs.raw();
+
+  ASSERT_EQ(lhs.ElementBytes() == rhs.ElementBytes(), true);
+  ASSERT_EQ(lhsRaw.version == rhsRaw.version, true);
+  ASSERT_EQ(lhs.rank() == rhs.rank(), true);
+  ASSERT_EQ(lhs.type() == rhs.type(), true);
+  ASSERT_EQ(lhsRaw.attribute == rhsRaw.attribute, true);
+
+  ASSERT_EQ(memcmp(lhsRaw.dim, rhsRaw.dim, lhs.rank()) == 0, true);
+  const std::size_t bytes = lhs.Elements() * lhs.ElementBytes();
+  ASSERT_EQ(memcmp(lhsRaw.base_addr, rhsRaw.base_addr, bytes) == 0, true);
+
+  const DescriptorAddendum *lhsAdd = lhs.Addendum();
+  const DescriptorAddendum *rhsAdd = rhs.Addendum();
+  if (lhsAdd) {
+    ASSERT_NE(rhsAdd, nullptr);
+    ASSERT_EQ(lhsAdd->SizeInBytes() == rhsAdd->SizeInBytes(), true);
+    ASSERT_EQ(memcmp(lhsAdd, rhsAdd, lhsAdd->SizeInBytes()) == 0, true);
+  } else {
+    ASSERT_EQ(rhsAdd, nullptr);
+  }
+}
+
+TEST(TemporaryStack, ValueStackBasic) {
+  const TypeCode code{CFI_type_int32_t};
+  constexpr size_t elementBytes = 4;
+  constexpr size_t rank = 2;
+  void *const descriptorPtr = reinterpret_cast<void *>(0xdeadbeef);
+  const SubscriptValue extent[rank]{42, 24};
+
+  StaticDescriptor<rank> testDescriptorStorage[3];
+  Descriptor &inputDesc{testDescriptorStorage[0].descriptor()};
+  Descriptor &outputDesc{testDescriptorStorage[1].descriptor()};
+  Descriptor &outputDesc2{testDescriptorStorage[2].descriptor()};
+  inputDesc.Establish(code, elementBytes, descriptorPtr, rank, extent);
+
+  inputDesc.Allocate();
+  ASSERT_EQ(inputDesc.IsAllocated(), true);
+  uint32_t *inputData = static_cast<uint32_t *>(inputDesc.raw().base_addr);
+  for (std::size_t i = 0; i < inputDesc.Elements(); ++i) {
+    inputData[i] = i;
+  }
+
+  void *storage = RTNAME(CreateValueStack)(__FILE__, __LINE__);
+  ASSERT_NE(storage, nullptr);
+
+  RTNAME(PushValue)(storage, inputDesc);
+
+  RTNAME(ValueAt)(storage, 0, outputDesc);
+  descriptorAlmostEqual(inputDesc, outputDesc);
+
+  RTNAME(PopValue)(storage, outputDesc2);
+  descriptorAlmostEqual(inputDesc, outputDesc2);
+
+  RTNAME(DestroyValueStack)(storage);
+}
+
+static unsigned max(unsigned x, unsigned y) {
+  if (x > y) {
+    return x;
+  }
+  return y;
+}
+
+TEST(TemporaryStack, ValueStackMultiSize) {
+  constexpr unsigned numToTest = 42;
+  const TypeCode code{CFI_type_int32_t};
+  constexpr size_t elementBytes = 4;
+  SubscriptValue extent[CFI_MAX_RANK];
+
+  std::vector<OwningPtr<Descriptor>> inputDescriptors;
+  inputDescriptors.reserve(numToTest);
+
+  void *storage = RTNAME(CreateValueStack)(__FILE__, __LINE__);
+  ASSERT_NE(storage, nullptr);
+
+  // create descriptors with and without adendums
+  auto getAdendum = [](unsigned i) { return i % 2; };
+  // create descriptors with varying ranks
+  auto getRank = [](unsigned i) { return max(i % 8, 1); };
+
+  // push descriptors of varying sizes and contents
+  for (unsigned i = 0; i < numToTest; ++i) {
+    const bool adendum = getAdendum(i);
+    const size_t rank = getRank(i);
+    for (unsigned dim = 0; dim < rank; ++dim) {
+      extent[dim] = ((i + dim) % 8) + 1;
+    }
+
+    const OwningPtr<Descriptor> &desc =
+        inputDescriptors.emplace_back(Descriptor::Create(code, elementBytes,
+            nullptr, rank, extent, CFI_attribute_allocatable, adendum));
+
+    // Descriptor::Establish doesn't initialise the extents if baseaddr is null
+    for (unsigned dim = 0; dim < rank; ++dim) {
+      Fortran::ISO::CFI_dim_t &boxDims = desc->raw().dim[dim];
+      boxDims.lower_bound = 1;
+      boxDims.extent = extent[dim];
+      boxDims.sm = elementBytes;
+    }
+    desc->Allocate();
+
+    // fill the array with some data to test
+    for (uint32_t i = 0; i < desc->Elements(); ++i) {
+      uint32_t *data = static_cast<uint32_t *>(desc->raw().base_addr);
+      ASSERT_NE(data, nullptr);
+      data[i] = i;
+    }
+
+    RTNAME(PushValue)(storage, *desc.get());
+  }
+
+  const TypeCode boolCode{CFI_type_Bool};
+  // peek and test each descriptor
+  for (unsigned i = 0; i < numToTest; ++i) {
+    const OwningPtr<Descriptor> &input = inputDescriptors[i];
+    const bool adendum = getAdendum(i);
+    const size_t rank = getRank(i);
+
+    // buffer to return the descriptor into
+    OwningPtr<Descriptor> out = Descriptor::Create(
+        boolCode, 1, nullptr, rank, extent, CFI_attribute_other, adendum);
+
+    (void)input;
+    RTNAME(ValueAt)(storage, i, *out.get());
+    descriptorAlmostEqual(*input, *out);
+  }
+
+  // pop and test each descriptor
+  for (unsigned i = numToTest; i > 0; --i) {
+    const OwningPtr<Descriptor> &input = inputDescriptors[i - 1];
+    const bool adendum = getAdendum(i - 1);
+    const size_t rank = getRank(i - 1);
+
+    // buffer to return the descriptor into
+    OwningPtr<Descriptor> out = Descriptor::Create(
+        boolCode, 1, nullptr, rank, extent, CFI_attribute_other, adendum);
+
+    RTNAME(PopValue)(storage, *out.get());
+    descriptorAlmostEqual(*input, *out);
+  }
+
+  RTNAME(DestroyValueStack)(storage);
+}
+
+TEST(TemporaryStack, DescriptorStackBasic) {
+  const TypeCode code{CFI_type_Bool};
+  constexpr size_t elementBytes = 4;
+  constexpr size_t rank = 2;
+  void *const descriptorPtr = reinterpret_cast<void *>(0xdeadbeef);
+  const SubscriptValue extent[rank]{42, 24};
+
+  StaticDescriptor<rank> testDescriptorStorage[3];
+  Descriptor &inputDesc{testDescriptorStorage[0].descriptor()};
+  Descriptor &outputDesc{testDescriptorStorage[1].descriptor()};
+  Descriptor &outputDesc2{testDescriptorStorage[2].descriptor()};
+  inputDesc.Establish(code, elementBytes, descriptorPtr, rank, extent);
+
+  void *storage = RTNAME(CreateDescriptorStack)(__FILE__, __LINE__);
+  ASSERT_NE(storage, nullptr);
+
+  RTNAME(PushDescriptor)(storage, inputDesc);
+
+  RTNAME(DescriptorAt)(storage, 0, outputDesc);
+  ASSERT_EQ(
+      memcmp(&inputDesc, &outputDesc, testDescriptorStorage[0].byteSize), 0);
+
+  RTNAME(PopDescriptor)(storage, outputDesc2);
+  ASSERT_EQ(
+      memcmp(&inputDesc, &outputDesc2, testDescriptorStorage[0].byteSize), 0);
+
+  RTNAME(DestroyDescriptorStack)(storage);
+}
+
+TEST(TemporaryStack, DescriptorStackMultiSize) {
+  constexpr unsigned numToTest = 42;
+  const TypeCode code{CFI_type_Bool};
+  constexpr size_t elementBytes = 4;
+  const uintptr_t ptrBase = 0xdeadbeef;
+  SubscriptValue extent[CFI_MAX_RANK];
+
+  std::vector<OwningPtr<Descriptor>> inputDescriptors;
+  inputDescriptors.reserve(numToTest);
+
+  void *storage = RTNAME(CreateDescriptorStack)(__FILE__, __LINE__);
+  ASSERT_NE(storage, nullptr);
+
+  // create descriptors with and without adendums
+  auto getAdendum = [](unsigned i) { return i % 2; };
+  // create descriptors with varying ranks
+  auto getRank = [](unsigned i) { return max(i % CFI_MAX_RANK, 1); };
+
+  // push descriptors of varying sizes and contents
+  for (unsigned i = 0; i < numToTest; ++i) {
+    const bool adendum = getAdendum(i);
+    const size_t rank = getRank(i);
+    for (unsigned dim = 0; dim < rank; ++dim) {
+      extent[dim] = max(i - dim, 1);
+    }
+
+    // varying pointers
+    void *const ptr = reinterpret_cast<void *>(ptrBase + i * elementBytes);
+
+    const OwningPtr<Descriptor> &desc =
+        inputDescriptors.emplace_back(Descriptor::Create(code, elementBytes,
+            ptr, rank, extent, CFI_attribute_other, adendum));
+    RTNAME(PushDescriptor)(storage, *desc.get());
+  }
+
+  const TypeCode intCode{CFI_type_int8_t};
+  // peek and test each descriptor
+  for (unsigned i = 0; i < numToTest; ++i) {
+    const OwningPtr<Descriptor> &input = inputDescriptors[i];
+    const bool adendum = getAdendum(i);
+    const size_t rank = getRank(i);
+
+    // buffer to return the descriptor into
+    OwningPtr<Descriptor> out = Descriptor::Create(
+        intCode, 1, nullptr, rank, extent, CFI_attribute_other, adendum);
+
+    RTNAME(DescriptorAt)(storage, i, *out.get());
+    ASSERT_EQ(memcmp(input.get(), out.get(), input->SizeInBytes()), 0);
+  }
+
+  // pop and test each descriptor
+  for (unsigned i = numToTest; i > 0; --i) {
+    const OwningPtr<Descriptor> &input = inputDescriptors[i - 1];
+    const bool adendum = getAdendum(i - 1);
+    const size_t rank = getRank(i - 1);
+
+    // buffer to return the descriptor into
+    OwningPtr<Descriptor> out = Descriptor::Create(
+        intCode, 1, nullptr, rank, extent, CFI_attribute_other, adendum);
+
+    RTNAME(PopDescriptor)(storage, *out.get());
+    ASSERT_EQ(memcmp(input.get(), out.get(), input->SizeInBytes()), 0);
+  }
+
+  RTNAME(DestroyDescriptorStack)(storage);
+}


        


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