Sequence of 1-to-4 values with prefix sum to support Punycode decoding.

PiperOrigin-RevId: 642696557
Change-Id: Ia6b8e174ddb55e44bd082bf0d81d2f9c53c94016

5 files changed, 283 insertions, 0 deletions

diff --git a/CMake/AbseilDll.cmake b/CMake/AbseilDll.cmake
index 3c161ec0..87904542 100644
--- a/CMake/AbseilDll.cmake
+++ b/CMake/AbseilDll.cmake
@@ -122,6 +122,7 @@ set(ABSL_INTERNAL_DLL_FILES
   "debugging/symbolize.h"
   "debugging/internal/address_is_readable.cc"
   "debugging/internal/address_is_readable.h"
+  "debugging/internal/bounded_utf8_length_sequence.h"
   "debugging/internal/demangle.cc"
   "debugging/internal/demangle.h"
   "debugging/internal/demangle_rust.cc"
diff --git a/absl/debugging/BUILD.bazel b/absl/debugging/BUILD.bazel
index 670f7764..0cd8015c 100644
--- a/absl/debugging/BUILD.bazel
+++ b/absl/debugging/BUILD.bazel
@@ -225,6 +225,7 @@ cc_library(
         "internal/utf8_for_code_point.cc",
     ],
     hdrs = [
+        "internal/bounded_utf8_length_sequence.h",
         "internal/demangle.h",
         "internal/demangle_rust.h",
         "internal/utf8_for_code_point.h",
@@ -239,6 +240,20 @@ cc_library(
         "//absl/base",
         "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/numeric:bits",
+    ],
+)
+
+cc_test(
+    name = "bounded_utf8_length_sequence_test",
+    srcs = ["internal/bounded_utf8_length_sequence_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":demangle_internal",
+        "//absl/base:config",
+        "@com_google_googletest//:gtest",
+        "@com_google_googletest//:gtest_main",
     ],
 )
 
diff --git a/absl/debugging/CMakeLists.txt b/absl/debugging/CMakeLists.txt
index 8379b5b9..738ae4e3 100644
--- a/absl/debugging/CMakeLists.txt
+++ b/absl/debugging/CMakeLists.txt
@@ -195,6 +195,7 @@ absl_cc_library(
   NAME
     demangle_internal
   HDRS
+    "internal/bounded_utf8_length_sequence.h"
     "internal/demangle.h"
     "internal/demangle_rust.h"
     "internal/utf8_for_code_point.h"
@@ -206,12 +207,26 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::base
+    absl::bits
     absl::core_headers
   PUBLIC
 )
 
 absl_cc_test(
   NAME
+    bounded_utf8_length_sequence_test
+  SRCS
+    "internal/bounded_utf8_length_sequence_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::demangle_internal
+    absl::config
+    GTest::gmock_main
+)
+
+absl_cc_test(
+  NAME
     demangle_rust_test
   SRCS
     "internal/demangle_rust_test.cc"
diff --git a/absl/debugging/internal/bounded_utf8_length_sequence.h b/absl/debugging/internal/bounded_utf8_length_sequence.h
new file mode 100644
index 00000000..188e06c4
--- /dev/null
+++ b/absl/debugging/internal/bounded_utf8_length_sequence.h
@@ -0,0 +1,126 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_DEBUGGING_INTERNAL_BOUNDED_UTF8_LENGTH_SEQUENCE_H_
+#define ABSL_DEBUGGING_INTERNAL_BOUNDED_UTF8_LENGTH_SEQUENCE_H_
+
+#include <cstdint>
+
+#include "absl/base/config.h"
+#include "absl/numeric/bits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+
+// A sequence of up to max_elements integers between 1 and 4 inclusive, whose
+// insertion operation computes the sum of all the elements before the insertion
+// point.  This is useful in decoding Punycode, where one needs to know where in
+// a UTF-8 byte stream the n-th code point begins.
+//
+// BoundedUtf8LengthSequence is async-signal-safe and suitable for use in
+// symbolizing stack traces in a signal handler, provided max_elements is not
+// improvidently large.  For inputs of lengths accepted by the Rust demangler,
+// up to a couple hundred code points, InsertAndReturnSumOfPredecessors should
+// run in a few dozen clock cycles, on par with the other arithmetic required
+// for Punycode decoding.
+template <uint32_t max_elements>
+class BoundedUtf8LengthSequence {
+ public:
+  // Constructs an empty sequence.
+  BoundedUtf8LengthSequence() = default;
+
+  // Inserts `utf_length` at position `index`, shifting any existing elements at
+  // or beyond `index` one position to the right.  If the sequence is already
+  // full, the rightmost element is discarded.
+  //
+  // Returns the sum of the elements at positions 0 to `index - 1` inclusive.
+  // If `index` is greater than the number of elements already inserted, the
+  // excess positions in the range count 1 apiece.
+  //
+  // REQUIRES: index < max_elements and 1 <= utf8_length <= 4.
+  uint32_t InsertAndReturnSumOfPredecessors(
+      uint32_t index, uint32_t utf8_length) {
+    // The caller shouldn't pass out-of-bounds inputs, but if it does happen,
+    // clamp the values and try to continue.  If we're being called from a
+    // signal handler, the last thing we want to do is crash.  Emitting
+    // malformed UTF-8 is a lesser evil.
+    if (index >= max_elements) index = max_elements - 1;
+    if (utf8_length == 0 || utf8_length > 4) utf8_length = 1;
+
+    const uint32_t word_index = index/32;
+    const uint32_t bit_index = 2 * (index % 32);
+    const uint64_t ones_bit = uint64_t{1} << bit_index;
+
+    // Compute the sum of predecessors.
+    //   - Each value from 1 to 4 is represented by a bit field with value from
+    //     0 to 3, so the desired sum is index plus the sum of the
+    //     representations actually stored.
+    //   - For each bit field, a set low bit should contribute 1 to the sum, and
+    //     a set high bit should contribute 2.
+    //   - Another way to say the same thing is that each set bit contributes 1,
+    //     and each set high bit contributes an additional 1.
+    //   - So the sum we want is index + popcount(everything) + popcount(bits in
+    //     odd positions).
+    const uint64_t odd_bits_mask = 0xaaaaaaaaaaaaaaaa;
+    const uint64_t lower_seminibbles_mask = ones_bit - 1;
+    const uint64_t higher_seminibbles_mask = ~lower_seminibbles_mask;
+    const uint64_t same_word_bits_below_insertion =
+        rep_[word_index] & lower_seminibbles_mask;
+    int full_popcount = absl::popcount(same_word_bits_below_insertion);
+    int odd_popcount =
+        absl::popcount(same_word_bits_below_insertion & odd_bits_mask);
+    for (uint32_t j = word_index; j > 0; --j) {
+      const uint64_t word_below_insertion = rep_[j - 1];
+      full_popcount += absl::popcount(word_below_insertion);
+      odd_popcount += absl::popcount(word_below_insertion & odd_bits_mask);
+    }
+    const uint32_t sum_of_predecessors =
+        index + static_cast<uint32_t>(full_popcount + odd_popcount);
+
+    // Now insert utf8_length's representation, shifting successors up one
+    // place.
+    for (uint32_t j = max_elements/32 - 1; j > word_index; --j) {
+      rep_[j] = (rep_[j] << 2) | (rep_[j - 1] >> 62);
+    }
+    rep_[word_index] =
+        (rep_[word_index] & lower_seminibbles_mask) |
+        (uint64_t{utf8_length - 1} << bit_index) |
+        ((rep_[word_index] & higher_seminibbles_mask) << 2);
+
+    return sum_of_predecessors;
+  }
+
+ private:
+  // If the (32 * i + j)-th element of the represented sequence has the value k
+  // (0 <= j < 32, 1 <= k <= 4), then bits 2 * j and 2 * j + 1 of rep_[i]
+  // contain the seminibble (k - 1).
+  //
+  // In particular, the zero-initialization of rep_ makes positions not holding
+  // any inserted element count as 1 in InsertAndReturnSumOfPredecessors.
+  //
+  // Example: rep_ = {0xb1, ... the rest zeroes ...} represents the sequence
+  // (2, 1, 4, 3, ... the rest 1's ...).  Constructing the sequence of Unicode
+  // code points "Àa🂻中" = {U+00C0, U+0061, U+1F0BB, U+4E2D} (among many
+  // other examples) would yield this value of rep_.
+  static_assert(max_elements > 0 && max_elements % 32 == 0,
+                "max_elements must be a positive multiple of 32");
+  uint64_t rep_[max_elements/32] = {};
+};
+
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_DEBUGGING_INTERNAL_BOUNDED_UTF8_LENGTH_SEQUENCE_H_
diff --git a/absl/debugging/internal/bounded_utf8_length_sequence_test.cc b/absl/debugging/internal/bounded_utf8_length_sequence_test.cc
new file mode 100644
index 00000000..17a24fd1
--- /dev/null
+++ b/absl/debugging/internal/bounded_utf8_length_sequence_test.cc
@@ -0,0 +1,126 @@
+// Copyright 2024 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/debugging/internal/bounded_utf8_length_sequence.h"
+
+#include <cstdint>
+
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace debugging_internal {
+namespace {
+
+TEST(BoundedUtf8LengthSequenceTest, RemembersAValueOfOneCorrectly) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0);
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(1, 1), 1);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, RemembersAValueOfTwoCorrectly) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 2), 0);
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(1, 1), 2);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, RemembersAValueOfThreeCorrectly) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 3), 0);
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(1, 1), 3);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, RemembersAValueOfFourCorrectly) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 4), 0);
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(1, 1), 4);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, RemembersSeveralAppendedValues) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(1, 4), 1);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(2, 2), 5);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(3, 3), 7);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(4, 1), 10);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, RemembersSeveralPrependedValues) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 4), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 3), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 2), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(4, 1), 10);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(3, 1), 6);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(2, 1), 3);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(1, 1), 1);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, RepeatedInsertsShiftValuesOutTheRightEnd) {
+  BoundedUtf8LengthSequence<32> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 2), 0);
+  for (uint32_t i = 1; i < 31; ++i) {
+    ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0)
+        << "while moving the 2 into position " << i;
+    ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(31, 1), 32)
+        << "after moving the 2 into position " << i;
+  }
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0)
+      << "while moving the 2 into position 31";
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(31, 1), 31)
+      << "after moving the 2 into position 31";
+}
+
+TEST(BoundedUtf8LengthSequenceTest, InsertsIntoWord1LeaveWord0Untouched) {
+  BoundedUtf8LengthSequence<64> seq;
+  for (uint32_t i = 0; i < 32; ++i) {
+    ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(i, 2), 2 * i)
+        << "at index " << i;
+  }
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(32, 1), 64);
+  EXPECT_EQ(seq.InsertAndReturnSumOfPredecessors(32, 1), 64);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, InsertsIntoWord0ShiftValuesIntoWord1) {
+  BoundedUtf8LengthSequence<64> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(29, 2), 29);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(30, 3), 31);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(31, 4), 34);
+
+  // Pushing two 1's on the front moves the 3 and 4 into the high word.
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(34, 1), 31 + 2 + 3 + 4);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(32, 1), 31 + 2);
+}
+
+TEST(BoundedUtf8LengthSequenceTest, ValuesAreShiftedCorrectlyAmongThreeWords) {
+  BoundedUtf8LengthSequence<96> seq;
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(31, 3), 31);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(63, 4), 62 + 3);
+
+  // This insertion moves both the 3 and the 4 up a word.
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(0, 1), 0);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(65, 1), 63 + 3 + 4);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(64, 1), 63 + 3);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(33, 1), 32 + 3);
+  ASSERT_EQ(seq.InsertAndReturnSumOfPredecessors(32, 1), 32);
+}
+
+}  // namespace
+}  // namespace debugging_internal
+ABSL_NAMESPACE_END
+}  // namespace absl