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[Backport 3.1]: [CUB] Replace several direct uses of __clz (#6099) #6202

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Merged
merged 4 commits into from
Oct 14, 2025
Merged

[Backport 3.1]: [CUB] Replace several direct uses of __clz (#6099) #6202

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bc511b7
[CUB] Replace several direct uses of `__clz` (#6099)
wmaxey Oct 2, 2025
30de744
Merge branch 'branch/3.1.x' into backport_6099
miscco Oct 13, 2025
22e28c0
Fix formatting
miscco Oct 13, 2025
bbcc334
Merge branch 'branch/3.1.x' into backport_6099
davebayer Oct 13, 2025
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12 changes: 8 additions & 4 deletions cub/cub/block/block_radix_rank.cuh
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Expand Up @@ -48,8 +48,12 @@
#include <cub/util_ptx.cuh>
#include <cub/util_type.cuh>

#include <cuda/ptx>
#include <cuda/std/__algorithm_>
#include <cuda/__ptx/instructions/get_sreg.h>
#include <cuda/std/__algorithm/max.h>
#include <cuda/std/__bit/integral.h>
#include <cuda/std/__functional/operations.h>
#include <cuda/std/__type_traits/conditional.h>
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looks unused

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We use cuda::std::_If AFAIK

#include <cuda/std/__type_traits/is_same.h>
#include <cuda/std/cstdint>
#include <cuda/std/limits>
#include <cuda/std/span>
Expand Down Expand Up @@ -1070,7 +1074,7 @@ struct BlockRadixRankMatchEarlyCounts
atomicOr(p_match_mask, lane_mask);
__syncwarp(WARP_MASK);
int bin_mask = *p_match_mask;
int leader = (WARP_THREADS - 1) - __clz(bin_mask);
int leader = ::cuda::std::__bit_log2(static_cast<unsigned>(bin_mask));
int warp_offset = 0;
int popc = __popc(bin_mask & ::cuda::ptx::get_sreg_lanemask_le());
if (lane == leader)
Expand Down Expand Up @@ -1100,7 +1104,7 @@ struct BlockRadixRankMatchEarlyCounts
::cuda::std::uint32_t bin = Digit(keys[u]);
int bin_mask =
detail::warp_in_block_matcher_t<RADIX_BITS, PARTIAL_WARP_THREADS, BLOCK_WARPS - 1>::match_any(bin, warp);
int leader = (WARP_THREADS - 1) - __clz(bin_mask);
int leader = ::cuda::std::__bit_log2(static_cast<unsigned>(bin_mask));
int warp_offset = 0;
int popc = __popc(bin_mask & ::cuda::ptx::get_sreg_lanemask_le());
if (lane == leader)
Expand Down
15 changes: 12 additions & 3 deletions cub/cub/warp/specializations/warp_reduce_shfl.cuh
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Expand Up @@ -48,7 +48,16 @@
#include <cub/util_ptx.cuh>
#include <cub/util_type.cuh>

#include <cuda/ptx>
#include <cuda/__functional/maximum.h>
#include <cuda/__functional/minimum.h>
#include <cuda/__ptx/instructions/get_sreg.h>
#include <cuda/std/__bit/countr.h>
#include <cuda/std/__functional/operations.h>
#include <cuda/std/__type_traits/enable_if.h>
#include <cuda/std/__type_traits/integral_constant.h>
#include <cuda/std/__type_traits/is_integral.h>
#include <cuda/std/__type_traits/is_same.h>
#include <cuda/std/__type_traits/is_unsigned.h>
#include <cuda/std/cstdint>
#include <cuda/std/type_traits>

Expand Down Expand Up @@ -694,7 +703,7 @@ struct WarpReduceShfl
_CCCL_DEVICE _CCCL_FORCEINLINE T SegmentedReduce(T input, FlagT flag, ReductionOp reduction_op)
{
// Get the start flags for each thread in the warp.
int warp_flags = __ballot_sync(member_mask, flag);
unsigned warp_flags = __ballot_sync(member_mask, flag);

// Convert to tail-segmented
if (HEAD_SEGMENTED)
Expand All @@ -715,7 +724,7 @@ struct WarpReduceShfl
warp_flags |= 1u << (LOGICAL_WARP_THREADS - 1);

// Find the next set flag
int last_lane = __clz(__brev(warp_flags));
int last_lane = ::cuda::std::countr_zero(warp_flags);

T output = input;
// Template-iterate reduction steps
Expand Down
8 changes: 5 additions & 3 deletions cub/cub/warp/specializations/warp_reduce_smem.cuh
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Expand Up @@ -48,7 +48,9 @@
#include <cub/util_ptx.cuh>
#include <cub/util_type.cuh>

#include <cuda/ptx>
#include <cuda/__ptx/instructions/get_sreg.h>
#include <cuda/std/__bit/countr.h>
#include <cuda/std/__type_traits/integral_constant.h>

CUB_NAMESPACE_BEGIN
namespace detail
Expand Down Expand Up @@ -213,7 +215,7 @@ struct WarpReduceSmem
SegmentedReduce(T input, FlagT flag, ReductionOp reduction_op, ::cuda::std::true_type /*has_ballot*/)
{
// Get the start flags for each thread in the warp.
int warp_flags = __ballot_sync(member_mask, flag);
unsigned warp_flags = __ballot_sync(member_mask, flag);

if (!HEAD_SEGMENTED)
{
Expand All @@ -230,7 +232,7 @@ struct WarpReduceSmem
}

// Find next flag
int next_flag = __clz(__brev(warp_flags));
int next_flag = ::cuda::std::countr_zero(warp_flags);

// Clip the next segment at the warp boundary if necessary
if (LOGICAL_WARP_THREADS != 32)
Expand Down
13 changes: 10 additions & 3 deletions cub/cub/warp/specializations/warp_scan_shfl.cuh
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Expand Up @@ -48,8 +48,15 @@
#include <cub/util_ptx.cuh>
#include <cub/util_type.cuh>

#include <cuda/ptx>
#include <cuda/std/__algorithm_>
#include <cuda/__ptx/instructions/get_sreg.h>
#include <cuda/std/__algorithm/clamp.h>
#include <cuda/std/__bit/has_single_bit.h>
Comment on lines +52 to +53
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these two headers are never used if I'm not wrong

#include <cuda/std/__bit/integral.h>
#include <cuda/std/__functional/operations.h>
#include <cuda/std/__type_traits/integral_constant.h>
#include <cuda/std/__type_traits/is_integral.h>
#include <cuda/std/__type_traits/is_unsigned.h>
#include <cuda/warp>

CUB_NAMESPACE_BEGIN
namespace detail
Expand Down Expand Up @@ -552,7 +559,7 @@ struct WarpScanShfl
ballot = ballot & ::cuda::ptx::get_sreg_lanemask_le();

// Find index of first set bit
int segment_first_lane = _CUDA_VSTD::max(0, 31 - __clz(ballot));
int segment_first_lane = ::cuda::std::__bit_log2(ballot);

// Iterate scan steps
_CCCL_PRAGMA_UNROLL_FULL()
Expand Down
9 changes: 8 additions & 1 deletion libcudacxx/include/cuda/std/__bit/countl.h
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Expand Up @@ -100,7 +100,14 @@ template <typename _Tp>
template <typename _Tp>
[[nodiscard]] _CCCL_HIDE_FROM_ABI _CCCL_DEVICE int __cccl_countl_zero_impl_device(_Tp __v) noexcept
{
return (sizeof(_Tp) == sizeof(uint32_t)) ? ::__clz(static_cast<int>(__v)) : ::__clzll(static_cast<long long>(__v));
if constexpr (sizeof(_Tp) == sizeof(uint32_t))
{
return static_cast<int>(::__clz(static_cast<int>(__v)));
}
else
{
return static_cast<int>(::__clzll(static_cast<long long>(__v)));
}
}
#endif // _CCCL_CUDA_COMPILATION()

Expand Down
4 changes: 2 additions & 2 deletions libcudacxx/include/cuda/std/__bit/countr.h
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Expand Up @@ -114,11 +114,11 @@ template <typename _Tp>
{
if constexpr (sizeof(_Tp) == sizeof(uint32_t))
{
return ::__clz(static_cast<int>(::__brev(__v)));
return static_cast<int>(::__clz(static_cast<int>(::__brev(__v))));
}
else
{
return ::__clzll(static_cast<long long>(::__brevll(__v)));
return static_cast<int>(::__clzll(static_cast<long long>(::__brevll(__v))));
}
}
#endif // _CCCL_CUDA_COMPILATION()
Expand Down
22 changes: 11 additions & 11 deletions libcudacxx/include/cuda/std/__bit/reference.h
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Expand Up @@ -275,10 +275,10 @@ _CCCL_API constexpr __bit_iterator<_Cp, false> __copy_aligned(
// do first word
if (__first.__ctz_ != 0)
{
unsigned __clz = __bits_per_word - __first.__ctz_;
difference_type __dn = _CUDA_VSTD::min(static_cast<difference_type>(__clz), __n);
unsigned __clz_f = __bits_per_word - __first.__ctz_;
difference_type __dn = ::cuda::std::min(static_cast<difference_type>(__clz_f), __n);
__n -= __dn;
__storage_type __m = (~__storage_type(0) << __first.__ctz_) & (~__storage_type(0) >> (__clz - __dn));
__storage_type __m = (~__storage_type(0) << __first.__ctz_) & (~__storage_type(0) >> (__clz_f - __dn));
__storage_type __b = *__first.__seg_ & __m;
*__result.__seg_ &= ~__m;
*__result.__seg_ |= __b;
Expand Down Expand Up @@ -419,8 +419,8 @@ _CCCL_API constexpr __bit_iterator<_Cp, false> __copy_backward_aligned(
{
difference_type __dn = _CUDA_VSTD::min(static_cast<difference_type>(__last.__ctz_), __n);
__n -= __dn;
unsigned __clz = __bits_per_word - __last.__ctz_;
__storage_type __m = (~__storage_type(0) << (__last.__ctz_ - __dn)) & (~__storage_type(0) >> __clz);
unsigned __clz_f = __bits_per_word - __last.__ctz_;
__storage_type __m = (~__storage_type(0) << (__last.__ctz_ - __dn)) & (~__storage_type(0) >> __clz_f);
__storage_type __b = *__last.__seg_ & __m;
*__result.__seg_ &= ~__m;
*__result.__seg_ |= __b;
Expand Down Expand Up @@ -633,10 +633,10 @@ _CCCL_API inline __bit_iterator<_Cr, false> __swap_ranges_aligned(
// do first word
if (__first.__ctz_ != 0)
{
unsigned __clz = __bits_per_word - __first.__ctz_;
difference_type __dn = _CUDA_VSTD::min(static_cast<difference_type>(__clz), __n);
unsigned __clz_f = __bits_per_word - __first.__ctz_;
difference_type __dn = ::cuda::std::min(static_cast<difference_type>(__clz_f), __n);
__n -= __dn;
__storage_type __m = (~__storage_type(0) << __first.__ctz_) & (~__storage_type(0) >> (__clz - __dn));
__storage_type __m = (~__storage_type(0) << __first.__ctz_) & (~__storage_type(0) >> (__clz_f - __dn));
__storage_type __b1 = *__first.__seg_ & __m;
*__first.__seg_ &= ~__m;
__storage_type __b2 = *__result.__seg_ & __m;
Expand Down Expand Up @@ -986,10 +986,10 @@ _CCCL_API constexpr bool __equal_aligned(
// do first word
if (__first1.__ctz_ != 0)
{
unsigned __clz = __bits_per_word - __first1.__ctz_;
difference_type __dn = _CUDA_VSTD::min(static_cast<difference_type>(__clz), __n);
unsigned __clz_f = __bits_per_word - __first1.__ctz_;
difference_type __dn = ::cuda::std::min(static_cast<difference_type>(__clz_f), __n);
__n -= __dn;
__storage_type __m = (~__storage_type(0) << __first1.__ctz_) & (~__storage_type(0) >> (__clz - __dn));
__storage_type __m = (~__storage_type(0) << __first1.__ctz_) & (~__storage_type(0) >> (__clz_f - __dn));
if ((*__first2.__seg_ & __m) != (*__first1.__seg_ & __m))
{
return false;
Expand Down
23 changes: 3 additions & 20 deletions thrust/thrust/detail/integer_math.h
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Expand Up @@ -27,6 +27,8 @@
#endif // no system header
#include <thrust/detail/type_deduction.h>

#include <cuda/std/__bit/countl.h>
#include <cuda/std/__type_traits/make_unsigned.h>
#include <cuda/std/limits>
#include <cuda/std/type_traits>

Expand All @@ -36,25 +38,6 @@ THRUST_NAMESPACE_BEGIN
namespace detail
{

template <typename Integer>
_CCCL_HOST_DEVICE _CCCL_FORCEINLINE Integer clz(Integer x)
{
Integer result;

NV_IF_TARGET(NV_IS_DEVICE,
(result = ::__clz(x);),
(int num_bits = 8 * sizeof(Integer); int num_bits_minus_one = num_bits - 1; result = num_bits;
for (int i = num_bits_minus_one; i >= 0; --i) {
if ((Integer(1) << i) & x)
{
result = num_bits_minus_one - i;
break;
}
}));

return result;
}

template <typename Integer>
_CCCL_HOST_DEVICE _CCCL_FORCEINLINE bool is_power_of_2(Integer x)
{
Expand Down Expand Up @@ -85,7 +68,7 @@ _CCCL_HOST_DEVICE _CCCL_FORCEINLINE Integer log2(Integer x)
Integer num_bits = 8 * sizeof(Integer);
Integer num_bits_minus_one = num_bits - 1;

return num_bits_minus_one - clz(x);
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thrust::detail::log2 will be deleted in #6188

return num_bits_minus_one - ::cuda::std::countl_zero(::cuda::std::__to_unsigned_like(x));
}

template <typename Integer>
Expand Down
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