Merge pull request #10900 from AnthonyLatsis/stable/20250601

[stable/20250601] Bring back deterministic hashing

Author: AnthonyLatsis

llvm/include/llvm/ADT/Hashing.h

@@ -45,7 +45,6 @@
 #define LLVM_ADT_HASHING_H
 
 #include "llvm/ADT/ADL.h"
-#include "llvm/Config/abi-breaking.h"
 #include "llvm/Support/DataTypes.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/SwapByteOrder.h"
@@ -128,6 +127,23 @@ hash_code hash_value(const std::basic_string<T> &arg);
 /// Compute a hash_code for a standard string.
 template <typename T> hash_code hash_value(const std::optional<T> &arg);
 
+/// Override the execution seed with a fixed value.
+///
+/// This hashing library uses a per-execution seed designed to change on each
+/// run with high probability in order to ensure that the hash codes are not
+/// attackable and to ensure that output which is intended to be stable does
+/// not rely on the particulars of the hash codes produced.
+///
+/// That said, there are use cases where it is important to be able to
+/// reproduce *exactly* a specific behavior. To that end, we provide a function
+/// which will forcibly set the seed to a fixed value. This must be done at the
+/// start of the program, before any hashes are computed. Also, it cannot be
+/// undone. This makes it thread-hostile and very hard to use outside of
+/// immediately on start of a simple program designed for reproducible
+/// behavior.
+void set_fixed_execution_hash_seed(uint64_t fixed_value);
+
+
 // All of the implementation details of actually computing the various hash
 // code values are held within this namespace. These routines are included in
 // the header file mainly to allow inlining and constant propagation.
@@ -307,17 +323,24 @@ struct hash_state {
   }
 };
 
-/// In LLVM_ENABLE_ABI_BREAKING_CHECKS builds, the seed is non-deterministic
-/// per process (address of a function in LLVMSupport) to prevent having users
-/// depend on the particular hash values. On platforms without ASLR, this is
-/// still likely non-deterministic per build.
+
+/// A global, fixed seed-override variable.
+///
+/// This variable can be set using the \see llvm::set_fixed_execution_seed
+/// function. See that function for details. Do not, under any circumstances,
+/// set or read this variable.
+extern uint64_t fixed_seed_override;
+
 inline uint64_t get_execution_seed() {
-#if LLVM_ENABLE_ABI_BREAKING_CHECKS
-  return static_cast<uint64_t>(
-      reinterpret_cast<uintptr_t>(&install_fatal_error_handler));
-#else
-  return 0xff51afd7ed558ccdULL;
-#endif
+  // FIXME: This needs to be a per-execution seed. This is just a placeholder
+  // implementation. Switching to a per-execution seed is likely to flush out
+  // instability bugs and so will happen as its own commit.
+  //
+  // However, if there is a fixed seed override set the first time this is
+  // called, return that instead of the per-execution seed.
+  const uint64_t seed_prime = 0xff51afd7ed558ccdULL;
+  static uint64_t seed = fixed_seed_override ? fixed_seed_override : seed_prime;
+  return seed;
 }
 
 

llvm/lib/Support/CMakeLists.txt

@@ -199,6 +199,7 @@ add_llvm_component_library(LLVMSupport
   FormatVariadic.cpp
   GlobPattern.cpp
   GraphWriter.cpp
+  Hashing.cpp
   HexagonAttributeParser.cpp
   HexagonAttributes.cpp
   InitLLVM.cpp

llvm/lib/Support/Hashing.cpp

@@ -0,0 +1,28 @@
+//===-------------- lib/Support/Hashing.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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file provides implementation bits for the LLVM common hashing
+// infrastructure. Documentation and most of the other information is in the
+// header file.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/Hashing.h"
+
+using namespace llvm;
+
+// Provide a definition and static initializer for the fixed seed. This
+// initializer should always be zero to ensure its value can never appear to be
+// non-zero, even during dynamic initialization.
+uint64_t llvm::hashing::detail::fixed_seed_override = 0;
+
+// Implement the function for forced setting of the fixed seed.
+// FIXME: Use atomic operations here so that there is no data race.
+void llvm::set_fixed_execution_hash_seed(uint64_t fixed_value) {
+  hashing::detail::fixed_seed_override = fixed_value;
+}

llvm/unittests/ADT/HashingTest.cpp

@@ -239,6 +239,78 @@ TEST(HashingTest, HashCombineRangeLengthDiff) {
   }
 }
 
+TEST(HashingTest, HashCombineRangeGoldenTest) {
+  struct { const char *s; uint64_t hash; } golden_data[] = {
+#if SIZE_MAX == UINT64_MAX || SIZE_MAX == UINT32_MAX
+    { "a",                                0xaeb6f9d5517c61f8ULL },
+    { "ab",                               0x7ab1edb96be496b4ULL },
+    { "abc",                              0xe38e60bf19c71a3fULL },
+    { "abcde",                            0xd24461a66de97f6eULL },
+    { "abcdefgh",                         0x4ef872ec411dec9dULL },
+    { "abcdefghijklm",                    0xe8a865539f4eadfeULL },
+    { "abcdefghijklmnopqrstu",            0x261cdf85faaf4e79ULL },
+    { "abcdefghijklmnopqrstuvwxyzabcdef", 0x43ba70e4198e3b2aULL },
+    { "abcdefghijklmnopqrstuvwxyzabcdef"
+      "abcdefghijklmnopqrstuvwxyzghijkl"
+      "abcdefghijklmnopqrstuvwxyzmnopqr"
+      "abcdefghijklmnopqrstuvwxyzstuvwx"
+      "abcdefghijklmnopqrstuvwxyzyzabcd", 0xdcd57fb2afdf72beULL },
+    { "a",                                0xaeb6f9d5517c61f8ULL },
+    { "aa",                               0xf2b3b69a9736a1ebULL },
+    { "aaa",                              0xf752eb6f07b1cafeULL },
+    { "aaaaa",                            0x812bd21e1236954cULL },
+    { "aaaaaaaa",                         0xff07a2cff08ac587ULL },
+    { "aaaaaaaaaaaaa",                    0x84ac949d54d704ecULL },
+    { "aaaaaaaaaaaaaaaaaaaaa",            0xcb2c8fb6be8f5648ULL },
+    { "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", 0xcc40ab7f164091b6ULL },
+    { "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+      "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+      "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+      "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
+      "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", 0xc58e174c1e78ffe9ULL },
+    { "z",                                0x1ba160d7e8f8785cULL },
+    { "zz",                               0x2c5c03172f1285d7ULL },
+    { "zzz",                              0x9d2c4f4b507a2ac3ULL },
+    { "zzzzz",                            0x0f03b9031735693aULL },
+    { "zzzzzzzz",                         0xe674147c8582c08eULL },
+    { "zzzzzzzzzzzzz",                    0x3162d9fa6938db83ULL },
+    { "zzzzzzzzzzzzzzzzzzzzz",            0x37b9a549e013620cULL },
+    { "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz", 0x8921470aff885016ULL },
+    { "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"
+      "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"
+      "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"
+      "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"
+      "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz", 0xf60fdcd9beb08441ULL },
+    { "a",                                0xaeb6f9d5517c61f8ULL },
+    { "ab",                               0x7ab1edb96be496b4ULL },
+    { "aba",                              0x3edb049950884d0aULL },
+    { "ababa",                            0x8f2de9e73a97714bULL },
+    { "abababab",                         0xee14a29ddf0ce54cULL },
+    { "ababababababa",                    0x38b3ddaada2d52b4ULL },
+    { "ababababababababababa",            0xd3665364219f2b85ULL },
+    { "abababababababababababababababab", 0xa75cd6afbf1bc972ULL },
+    { "abababababababababababababababab"
+      "abababababababababababababababab"
+      "abababababababababababababababab"
+      "abababababababababababababababab"
+      "abababababababababababababababab", 0x840192d129f7a22bULL }
+#else
+#error This test only supports 64-bit and 32-bit systems.
+#endif
+  };
+  for (unsigned i = 0; i < sizeof(golden_data)/sizeof(*golden_data); ++i) {
+    StringRef str = golden_data[i].s;
+    hash_code hash = hash_combine_range(str.begin(), str.end());
+#if 0 // Enable this to generate paste-able text for the above structure.
+    std::string member_str = "\"" + str.str() + "\",";
+    fprintf(stderr, " { %-35s 0x%016llxULL },\n",
+            member_str.c_str(), static_cast<uint64_t>(hash));
+#endif
+    EXPECT_EQ(static_cast<size_t>(golden_data[i].hash),
+              static_cast<size_t>(hash));
+  }
+}
+
 TEST(HashingTest, HashCombineBasicTest) {
   // Hashing a sequence of homogenous types matches range hashing.
   const int i1 = 42, i2 = 43, i3 = 123, i4 = 999, i5 = 0, i6 = 79;

llvm/utils/gn/secondary/llvm/lib/Support/BUILD.gn

@@ -94,6 +94,7 @@ static_library("Support") {
     "FormattedStream.cpp",
     "GlobPattern.cpp",
     "GraphWriter.cpp",
+    "Hashing.cpp",
     "HexagonAttributeParser.cpp",
     "HexagonAttributes.cpp",
     "InitLLVM.cpp",