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GCC/Clang Built-in Functions Documentation

Introduction

GCC and Clang compilers provide a set of built-in functions (__builtin_*) that allow programmers to directly utilize low-level operations, optimize code, and access processor-specific features. These functions are often mapped to hardware instructions or implemented in an efficient manner by the compiler.

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1. Bit Manipulation Functions

These functions operate on bit-level representations of integers.

__builtin_popcount

• Description: Counts the number of set bits (1s) in the binary representation of an unsigned int.
• Parameters:
  • unsigned int x: The input number.
• Return Value: The number of 1s in the binary representation of x.

__builtin_popcountl

• Description: Counts the number of set bits in an unsigned long.
• Parameters:
  • unsigned long x: The input number.
• Return Value: The number of 1s in the binary representation of x.

__builtin_popcountll

• Description: Counts the number of set bits in an unsigned long long.
• Parameters:
  • unsigned long long x: The input number.
• Return Value: The number of 1s in the binary representation of x.

__builtin_ctz

• Description: Counts the number of trailing zeros in an unsigned int.
• Parameters:
  • unsigned int x: The input number.
• Return Value: The number of trailing zeros in the binary representation of x. Returns undefined if x is 0.

__builtin_ctzl

• Description: Counts the number of trailing zeros in an unsigned long.
• Parameters:
  • unsigned long x: The input number.
• Return Value: The number of trailing zeros in the binary representation of x. Returns undefined if x is 0.

__builtin_ctzll

• Description: Counts the number of trailing zeros in an unsigned long long.
• Parameters:
  • unsigned long long x: The input number.
• Return Value: The number of trailing zeros in the binary representation of x. Returns undefined if x is 0.

__builtin_clz

• Description: Counts the number of leading zeros in an unsigned int.
• Parameters:
  • unsigned int x: The input number.
• Return Value: The number of leading zeros in the binary representation of x. Returns undefined if x is 0.

__builtin_clzl

• Description: Counts the number of leading zeros in an unsigned long.
• Parameters:
  • unsigned long x: The input number.
• Return Value: The number of leading zeros in the binary representation of x. Returns undefined if x is 0.

__builtin_clzll

• Description: Counts the number of leading zeros in an unsigned long long.
• Parameters:
  • unsigned long long x: The input number.
• Return Value: The number of leading zeros in the binary representation of x. Returns undefined if x is 0.

__builtin_parity

• Description: Returns the parity of the number of 1s in an unsigned int. Returns 1 if the number of 1s is odd, otherwise 0.
• Parameters:
  • unsigned int x: The input number.
• Return Value: 1 if the number of 1s is odd, 0 otherwise.

__builtin_parityl

• Description: Returns the parity of the number of 1s in an unsigned long.
• Parameters:
  • unsigned long x: The input number.
• Return Value: 1 if the number of 1s is odd, 0 otherwise.

__builtin_parityll

• Description: Returns the parity of the number of 1s in an unsigned long long.
• Parameters:
  • unsigned long long x: The input number.
• Return Value: 1 if the number of 1s is odd, 0 otherwise.

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2. Arithmetic Overflow Functions

These functions detect overflow conditions during arithmetic operations.

__builtin_add_overflow

• Description: Performs addition and checks for overflow.
• Parameters:
  • T x: The first operand.
  • T y: The second operand.
  • T* result: Pointer to store the result.
• Return Value: Returns 1 if overflow occurred, 0 otherwise.

__builtin_sub_overflow

• Description: Performs subtraction and checks for overflow.
• Parameters:
  • T x: The first operand.
  • T y: The second operand.
  • T* result: Pointer to store the result.
• Return Value: Returns 1 if overflow occurred, 0 otherwise.

__builtin_mul_overflow

• Description: Performs multiplication and checks for overflow.
• Parameters:
  • T x: The first operand.
  • T y: The second operand.
  • T* result: Pointer to store the result.
• Return Value: Returns 1 if overflow occurred, 0 otherwise.

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3. Memory Manipulation Functions

These functions provide efficient ways to manipulate memory blocks.

__builtin_memcpy

• Description: Copies a block of memory from the source to the destination.
• Parameters:
  • void* dest: Destination pointer.
  • const void* src: Source pointer.
  • size_t n: Number of bytes to copy.
• Return Value: Returns a pointer to the destination (dest).

__builtin_memset

• Description: Fills a block of memory with a specific value.
• Parameters:
  • void* dest: Pointer to the memory block to fill.
  • int c: The value to fill.
  • size_t n: Number of bytes to fill.
• Return Value: Returns a pointer to the destination (dest).

__builtin_memmove

• Description: Moves a block of memory from the source to the destination. Handles overlapping memory safely.
• Parameters:
  • void* dest: Destination pointer.
  • const void* src: Source pointer.
  • size_t n: Number of bytes to move.
• Return Value: Returns a pointer to the destination (dest).

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4. Control Flow Optimization

These functions provide hints to the compiler for optimizing control flow.

__builtin_expect

• Description: Provides the compiler with branch prediction information.
• Parameters:
  • long exp: The expression to evaluate.
  • long c: The expected value of the expression.
• Return Value: Returns the value of exp. The compiler optimizes for the assumption that exp == c.

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5. Type Checking and Constants

These functions help in type checking and constant evaluation.

__builtin_types_compatible_p

• Description: Checks whether two types are compatible.
• Parameters:
  • T1: The first type.
  • T2: The second type.
• Return Value: Returns 1 if the types are compatible, otherwise 0.

__builtin_constant_p

• Description: Checks if a value is a compile-time constant.
• Parameters:
  • exp: The expression to check.
• Return Value: Returns 1 if the expression is a compile-time constant, otherwise 0.

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6. Math and Utility Functions

These functions provide basic mathematical and utility operations.

__builtin_abs

• Description: Returns the absolute value of an integer.
• Parameters:
  • int x: The input integer.
• Return Value: The absolute value of x.

__builtin_labs

• Description: Returns the absolute value of a long integer.
• Parameters:
  • long x: The input integer.
• Return Value: The absolute value of x.

__builtin_llabs

• Description: Returns the absolute value of a long long integer.
• Parameters:
  • long long x: The input integer.
• Return Value: The absolute value of x.

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Notes

1. These built-in functions are platform-specific and optimized by the compiler.
2. Some functions, such as __builtin_expect, may not provide runtime benefits on all platforms but assist in compiler optimization.
3. Functions like __builtin_popcount may leverage hardware instructions (e.g., POPCNT) if supported by the CPU.