<cuda/std/bit>#

2026-10-01

3 min read time

Applies to Linux and Windows

This page documents cuda::std::bit in libhipcxx, covering bit manipulation functions such as popcount, rotl, rotr, and bit_width, with notes on GPU SASS instruction mappings.

CUDA Performance Considerations#

Given an unsigned integer with N bits and N <= 32, the <bit> functions translate into the following SASS instructions. For some functions, the result is decorated with a compile-time assumption to restrict its range and allow further optimizations.

  • bit_width() translates into a single FLO SASS instruction. The result is assumed to be in the range [0, N].

  • bit_ceil() translates into ADD, FLO, SHL, IMINMAX SASS instructions. The result is assumed to be greater than or equal to the input.

  • bit_floor() translates into FLO, SHL SASS instructions. The result is assumed to be less than or equal to the input.

  • byteswap() translates into a single PRMT SASS instruction.

  • popcount() translates into a single POPC SASS instruction. The result is assumed to be in the range [0, N].

  • has_single_bit() translates into POPC + ISETP SASS instructions.

  • rotl()/rotr() translate into a single SHF (funnel shift) SASS instruction.

  • countl_zero() translates into FLO, IMINMAX SASS instructions. The result is assumed to be in the range [0, N].

  • countl_one() translates into LOP3, FLO, IMINMAX SASS instructions. The result is assumed to be in the range [0, N].

  • countr_zero() translates into BREV, FLO, IMINMAX SASS instructions. The result is assumed to be in the range [0, N].

  • countr_one() translates into LOP3, BREV, FLO, IMINMAX SASS instructions. The result is assumed to be in the range [0, N].

Additional Notes#

  • All functions are marked [[nodiscard]] and noexcept.

  • All functions support 128-bit integer types.

  • bit_ceil() checks for overflow in debug mode.

  • rotl()/rotr() checks for invalid count value (INT_MIN) in debug mode.

Note

When the input values are run-time values that the compiler can resolve at compile-time, e.g. an index of a loop with a fixed number of iterations, using the functions could not be optimal.

Note

GCC <= 8 uses a slow path with more instructions even in CUDA.

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