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 //
 // Copyright 2017 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.
 //
 // -----------------------------------------------------------------------------
 // File: optimization.h
 // -----------------------------------------------------------------------------
 //
 // This header file defines portable macros for performance optimization.
 
 #ifndef ABSL_BASE_OPTIMIZATION_H_
 #define ABSL_BASE_OPTIMIZATION_H_
 
 #include <assert.h>
 
 #include "absl/base/config.h"
 #include "absl/base/options.h"
 
 // ABSL_BLOCK_TAIL_CALL_OPTIMIZATION
 //
 // Instructs the compiler to avoid optimizing tail-call recursion. This macro is
 // useful when you wish to preserve the existing function order within a stack
 // trace for logging, debugging, or profiling purposes.
 //
 // Example:
 //
 //   int f() {
 //     int result = g();
 //     ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
 //     return result;
 //   }
 #if defined(__pnacl__)
 #define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() if (volatile int x = 0) { (void)x; }
 #elif defined(__clang__)
 // Clang will not tail call given inline volatile assembly.
 #define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() __asm__ __volatile__("")
 #elif defined(__GNUC__)
 // GCC will not tail call given inline volatile assembly.
 #define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() __asm__ __volatile__("")
 #elif defined(_MSC_VER)
 #include <intrin.h>
 // The __nop() intrinsic blocks the optimisation.
 #define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() __nop()
 #else
 #define ABSL_BLOCK_TAIL_CALL_OPTIMIZATION() if (volatile int x = 0) { (void)x; }
 #endif
 
 // ABSL_CACHELINE_SIZE
 //
 // Explicitly defines the size of the L1 cache for purposes of alignment.
 // Setting the cacheline size allows you to specify that certain objects be
 // aligned on a cacheline boundary with `ABSL_CACHELINE_ALIGNED` declarations.
 // (See below.)
 //
 // NOTE: this macro should be replaced with the following C++17 features, when
 // those are generally available:
 //
 //   * `std::hardware_constructive_interference_size`
 //   * `std::hardware_destructive_interference_size`
 //
 // See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0154r1.html
 // for more information.
 #if defined(__GNUC__)
 // Cache line alignment
 #if defined(__i386__) || defined(__x86_64__)
 #define ABSL_CACHELINE_SIZE 64
 #elif defined(__powerpc64__)
 #define ABSL_CACHELINE_SIZE 128
 #elif defined(__aarch64__)
 // We would need to read special register ctr_el0 to find out L1 dcache size.
 // This value is a good estimate based on a real aarch64 machine.
 #define ABSL_CACHELINE_SIZE 64
 #elif defined(__arm__)
 // Cache line sizes for ARM: These values are not strictly correct since
 // cache line sizes depend on implementations, not architectures.  There
 // are even implementations with cache line sizes configurable at boot
 // time.
 #if defined(__ARM_ARCH_5T__)
 #define ABSL_CACHELINE_SIZE 32
 #elif defined(__ARM_ARCH_7A__)
 #define ABSL_CACHELINE_SIZE 64
 #endif
 #endif
 #endif
 
 #ifndef ABSL_CACHELINE_SIZE
 // A reasonable default guess.  Note that overestimates tend to waste more
 // space, while underestimates tend to waste more time.
 #define ABSL_CACHELINE_SIZE 64
 #endif
 
 // ABSL_CACHELINE_ALIGNED
 //
 // Indicates that the declared object be cache aligned using
 // `ABSL_CACHELINE_SIZE` (see above). Cacheline aligning objects allows you to
 // load a set of related objects in the L1 cache for performance improvements.
 // Cacheline aligning objects properly allows constructive memory sharing and
 // prevents destructive (or "false") memory sharing.
 //
 // NOTE: callers should replace uses of this macro with `alignas()` using
 // `std::hardware_constructive_interference_size` and/or
 // `std::hardware_destructive_interference_size` when C++17 becomes available to
 // them.
 //
 // See http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0154r1.html
 // for more information.
 //
 // On some compilers, `ABSL_CACHELINE_ALIGNED` expands to an `__attribute__`
 // or `__declspec` attribute. For compilers where this is not known to work,
 // the macro expands to nothing.
 //
 // No further guarantees are made here. The result of applying the macro
 // to variables and types is always implementation-defined.
 //
 // WARNING: It is easy to use this attribute incorrectly, even to the point
 // of causing bugs that are difficult to diagnose, crash, etc. It does not
 // of itself guarantee that objects are aligned to a cache line.
 //
 // NOTE: Some compilers are picky about the locations of annotations such as
 // this attribute, so prefer to put it at the beginning of your declaration.
 // For example,
 //
 //   ABSL_CACHELINE_ALIGNED static Foo* foo = ...
 //
 //   class ABSL_CACHELINE_ALIGNED Bar { ...
 //
 // Recommendations:
 //
 // 1) Consult compiler documentation; this comment is not kept in sync as
 //    toolchains evolve.
 // 2) Verify your use has the intended effect. This often requires inspecting
 //    the generated machine code.
 // 3) Prefer applying this attribute to individual variables. Avoid
 //    applying it to types. This tends to localize the effect.
 #if defined(__clang__) || defined(__GNUC__)
 #define ABSL_CACHELINE_ALIGNED __attribute__((aligned(ABSL_CACHELINE_SIZE)))
 #elif defined(_MSC_VER)
 #define ABSL_CACHELINE_ALIGNED __declspec(align(ABSL_CACHELINE_SIZE))
 #else
 #define ABSL_CACHELINE_ALIGNED
 #endif
 
 // ABSL_PREDICT_TRUE, ABSL_PREDICT_FALSE
 //
 // Enables the compiler to prioritize compilation using static analysis for
 // likely paths within a boolean branch.
 //
 // Example:
 //
 //   if (ABSL_PREDICT_TRUE(expression)) {
 //     return result;                        // Faster if more likely
 //   } else {
 //     return 0;
 //   }
 //
 // Compilers can use the information that a certain branch is not likely to be
 // taken (for instance, a CHECK failure) to optimize for the common case in
 // the absence of better information (ie. compiling gcc with `-fprofile-arcs`).
 //
 // Recommendation: Modern CPUs dynamically predict branch execution paths,
 // typically with accuracy greater than 97%. As a result, annotating every
 // branch in a codebase is likely counterproductive; however, annotating
 // specific branches that are both hot and consistently mispredicted is likely
 // to yield performance improvements.
 #if ABSL_HAVE_BUILTIN(__builtin_expect) || \
     (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_PREDICT_FALSE(x) (__builtin_expect(false || (x), false))
 #define ABSL_PREDICT_TRUE(x) (__builtin_expect(false || (x), true))
 #else
 #define ABSL_PREDICT_FALSE(x) (x)
 #define ABSL_PREDICT_TRUE(x) (x)
 #endif
 
 // `ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL()` aborts the program in the fastest
 // possible way, with no attempt at logging. One use is to implement hardening
 // aborts with ABSL_OPTION_HARDENED.  Since this is an internal symbol, it
 // should not be used directly outside of Abseil.
 #if ABSL_HAVE_BUILTIN(__builtin_trap) || \
     (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL() __builtin_trap()
 #else
 #define ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL() abort()
 #endif
 
 // `ABSL_INTERNAL_UNREACHABLE_IMPL()` is the platform specific directive to
 // indicate that a statement is unreachable, and to allow the compiler to
 // optimize accordingly. Clients should use `ABSL_UNREACHABLE()`, which is
 // defined below.
 #if defined(__cpp_lib_unreachable) && __cpp_lib_unreachable >= 202202L
 #define ABSL_INTERNAL_UNREACHABLE_IMPL() std::unreachable()
 #elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)
 #define ABSL_INTERNAL_UNREACHABLE_IMPL() __builtin_unreachable()
 #elif ABSL_HAVE_BUILTIN(__builtin_assume)
 #define ABSL_INTERNAL_UNREACHABLE_IMPL() __builtin_assume(false)
 #elif defined(_MSC_VER)
 #define ABSL_INTERNAL_UNREACHABLE_IMPL() __assume(false)
 #else
 #define ABSL_INTERNAL_UNREACHABLE_IMPL()
 #endif
 
 // `ABSL_UNREACHABLE()` is an unreachable statement.  A program which reaches
 // one has undefined behavior, and the compiler may optimize accordingly.
 #if ABSL_OPTION_HARDENED == 1 && defined(NDEBUG)
 // Abort in hardened mode to avoid dangerous undefined behavior.
 #define ABSL_UNREACHABLE()                \
   do {                                    \
     ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL(); \
     ABSL_INTERNAL_UNREACHABLE_IMPL();     \
   } while (false)
 #else
 // The assert only fires in debug mode to aid in debugging.
 // When NDEBUG is defined, reaching ABSL_UNREACHABLE() is undefined behavior.
 #define ABSL_UNREACHABLE()                       \
   do {                                           \
     /* NOLINTNEXTLINE: misc-static-assert */     \
     assert(false && "ABSL_UNREACHABLE reached"); \
     ABSL_INTERNAL_UNREACHABLE_IMPL();            \
   } while (false)
 #endif
 
 // ABSL_ASSUME(cond)
 //
 // Informs the compiler that a condition is always true and that it can assume
 // it to be true for optimization purposes.
 //
 // WARNING: If the condition is false, the program can produce undefined and
 // potentially dangerous behavior.
 //
 // In !NDEBUG mode, the condition is checked with an assert().
 //
 // NOTE: The expression must not have side effects, as it may only be evaluated
 // in some compilation modes and not others. Some compilers may issue a warning
 // if the compiler cannot prove the expression has no side effects. For example,
 // the expression should not use a function call since the compiler cannot prove
 // that a function call does not have side effects.
 //
 // Example:
 //
 //   int x = ...;
 //   ABSL_ASSUME(x >= 0);
 //   // The compiler can optimize the division to a simple right shift using the
 //   // assumption specified above.
 //   int y = x / 16;
 //
 #if !defined(NDEBUG)
 #define ABSL_ASSUME(cond) assert(cond)
 #elif ABSL_HAVE_BUILTIN(__builtin_assume)
 #define ABSL_ASSUME(cond) __builtin_assume(cond)
 #elif defined(_MSC_VER)
 #define ABSL_ASSUME(cond) __assume(cond)
 #elif defined(__cpp_lib_unreachable) && __cpp_lib_unreachable >= 202202L
 #define ABSL_ASSUME(cond)            \
   do {                               \
     if (!(cond)) std::unreachable(); \
   } while (false)
 #elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)
 #define ABSL_ASSUME(cond)                 \
   do {                                    \
     if (!(cond)) __builtin_unreachable(); \
   } while (false)
 #else
 #define ABSL_ASSUME(cond)               \
   do {                                  \
     static_cast<void>(false && (cond)); \
   } while (false)
 #endif
 
 // ABSL_INTERNAL_UNIQUE_SMALL_NAME(cond)
 // This macro forces small unique name on a static file level symbols like
 // static local variables or static functions. This is intended to be used in
 // macro definitions to optimize the cost of generated code. Do NOT use it on
 // symbols exported from translation unit since it may cause a link time
 // conflict.
 //
 // Example:
 //
 // #define MY_MACRO(txt)
 // namespace {
 //  char VeryVeryLongVarName[] ABSL_INTERNAL_UNIQUE_SMALL_NAME() = txt;
 //  const char* VeryVeryLongFuncName() ABSL_INTERNAL_UNIQUE_SMALL_NAME();
 //  const char* VeryVeryLongFuncName() { return txt; }
 // }
 //
 
 #if defined(__GNUC__)
 #define ABSL_INTERNAL_UNIQUE_SMALL_NAME2(x) #x
 #define ABSL_INTERNAL_UNIQUE_SMALL_NAME1(x) ABSL_INTERNAL_UNIQUE_SMALL_NAME2(x)
 #define ABSL_INTERNAL_UNIQUE_SMALL_NAME() \
   asm(ABSL_INTERNAL_UNIQUE_SMALL_NAME1(.absl.__COUNTER__))
 #else
 #define ABSL_INTERNAL_UNIQUE_SMALL_NAME()
 #endif
 
 #endif  // ABSL_BASE_OPTIMIZATION_H_

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