`cuda::clamp` by fbusato · Pull Request #5516 · NVIDIA/cccl

fbusato · 2025-08-13T01:12:00Z

Description

cuda::std::clamp is a widely used utility in CCCL and in general code. Unfortunately, it doesn't produce the optimal code for device code for the most common use cases: integer and floating-point types. The main reason is that cuda::std::clamp returns a reference instead of a value, which is not needed for primitive types.

The PR provides cuda::clamp to generate optimal code for integer and floating-point types.

Requires constexpr fmin/fmax.

github-actions · 2025-08-13T02:29:44Z

🟩 CI finished in 1h 16m: Pass: 100%/210 | Total: 1d 17h | Avg: 11m 54s | Max: 51m 47s | Hits: 95%/322859

🟩 cub: Pass: 100%/50 | Total: 11h 39m | Avg: 13m 59s | Max: 41m 40s | Hits: 98%/52978

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🟩 thrust: Pass: 100%/50 | Total: 9h 44m | Avg: 11m 41s | Max: 43m 44s | Hits: 98%/84139

🟩 cmake_options
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  🟩 arm64              Pass: 100%/2   | Total: 12m 08s | Avg:  6m 04s | Max:  7m 01s | Hits:  99%/3827  
🟩 ctk
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  🟩 GCC11              Pass: 100%/2   | Total: 15m 20s | Avg:  7m 40s | Max:  7m 45s | Hits:  99%/3828  
  🟩 GCC12              Pass: 100%/2   | Total: 15m 52s | Avg:  7m 56s | Max:  7m 57s | Hits:  99%/3828  
  🟩 GCC13              Pass: 100%/11  | Total:  1h 50m | Avg: 10m 03s | Max: 43m 44s | Hits:  93%/13398 
  🟩 MSVC14.29          Pass: 100%/2   | Total: 59m 47s | Avg: 29m 53s | Max: 29m 56s | Hits:  99%/3812  
  🟩 MSVC14.43          Pass: 100%/5   | Total:  2h 41m | Avg: 32m 23s | Max: 39m 44s | Hits:  99%/9530  
  🟩 NVHPC25.5          Pass: 100%/2   | Total:  1h 00m | Avg: 30m 01s | Max: 31m 24s | Hits:  99%/3824  
🟩 cxx_family
  🟩 Clang              Pass: 100%/19  | Total:  1h 48m | Avg:  5m 43s | Max:  6m 26s | Hits: 100%/32521 
  🟩 GCC                Pass: 100%/22  | Total:  3h 13m | Avg:  8m 48s | Max: 43m 44s | Hits:  97%/34452 
  🟩 MSVC               Pass: 100%/7   | Total:  3h 41m | Avg: 31m 40s | Max: 39m 44s | Hits:  99%/13342 
  🟩 NVHPC              Pass: 100%/2   | Total:  1h 00m | Avg: 30m 01s | Max: 31m 24s | Hits:  99%/3824  
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  🟩 h100               Pass: 100%/2   | Total: 13m 48s | Avg:  6m 54s | Max:  7m 45s | Hits:  99%/1914  
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  🟩 rtx4090            Pass: 100%/10  | Total:  2h 00m | Avg: 12m 01s | Max: 39m 44s | Hits:  99%/9553  
🟩 jobs
  🟩 Build              Pass: 100%/43  | Total:  8h 32m | Avg: 11m 55s | Max: 43m 44s | Hits:  98%/82233 
  🟩 TestCPU            Pass: 100%/3   | Total: 48m 05s | Avg: 16m 01s | Max: 39m 44s | Hits:  99%/1906  
  🟩 TestGPU            Pass: 100%/4   | Total: 23m 53s | Avg:  5m 58s | Max:  7m 45s
🟩 sm
  🟩 90                 Pass: 100%/2   | Total: 13m 48s | Avg:  6m 54s | Max:  7m 45s | Hits:  99%/1914  
  🟩 90;90a             Pass: 100%/2   | Total: 35m 18s | Avg: 17m 39s | Max: 27m 46s | Hits:  99%/3820  
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🟩 libcudacxx: Pass: 100%/48 | Total: 11h 59m | Avg: 14m 59s | Max: 51m 47s | Hits: 92%/169672

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🟩 ctk
  🟩 12.0               Pass: 100%/5   | Total: 49m 46s | Avg:  9m 57s | Max: 31m 03s | Hits:  99%/18605 
  🟩 12.9               Pass: 100%/43  | Total: 11h 10m | Avg: 15m 34s | Max: 51m 47s | Hits:  91%/151067
🟩 cudacxx
  🟩 ClangCUDA19        Pass: 100%/2   | Total: 54m 55s | Avg: 27m 27s | Max: 28m 54s | Hits:  29%/7551  
  🟩 nvcc12.0           Pass: 100%/5   | Total: 49m 46s | Avg:  9m 57s | Max: 31m 03s | Hits:  99%/18605 
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🟩 cudacxx_family
  🟩 ClangCUDA          Pass: 100%/2   | Total: 54m 55s | Avg: 27m 27s | Max: 28m 54s | Hits:  29%/7551  
  🟩 nvcc               Pass: 100%/46  | Total: 11h 04m | Avg: 14m 27s | Max: 51m 47s | Hits:  95%/162121
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  🟩 GCC13              Pass: 100%/11  | Total:  3h 09m | Avg: 17m 11s | Max: 46m 13s | Hits:  99%/30687 
  🟩 MSVC14.29          Pass: 100%/2   | Total:  1h 09m | Avg: 34m 38s | Max: 38m 14s | Hits:  81%/7225  
  🟩 MSVC14.43          Pass: 100%/4   | Total:  2h 10m | Avg: 32m 38s | Max: 34m 48s | Hits:  99%/14924 
  🟩 NVHPC25.5          Pass: 100%/2   | Total:  1h 38m | Avg: 49m 21s | Max: 51m 47s | Hits:  30%/7533  
🟩 cxx_family
  🟩 Clang              Pass: 100%/18  | Total:  2h 55m | Avg:  9m 43s | Max: 38m 22s | Hits:  91%/67930 
  🟩 GCC                Pass: 100%/22  | Total:  4h 06m | Avg: 11m 11s | Max: 46m 13s | Hits:  99%/72060 
  🟩 MSVC               Pass: 100%/6   | Total:  3h 19m | Avg: 33m 18s | Max: 38m 14s | Hits:  93%/22149 
  🟩 NVHPC              Pass: 100%/2   | Total:  1h 38m | Avg: 49m 21s | Max: 51m 47s | Hits:  30%/7533  
🟩 gpu
  🟩 h100               Pass: 100%/2   | Total: 27m 24s | Avg: 13m 42s | Max: 22m 11s | Hits:  99%/7754  
  🟩 rtx2080            Pass: 100%/46  | Total: 11h 32m | Avg: 15m 03s | Max: 51m 47s | Hits:  91%/161918
🟩 jobs
  🟩 Build              Pass: 100%/42  | Total:  8h 44m | Avg: 12m 29s | Max: 51m 47s | Hits:  91%/158166
  🟩 NVRTC              Pass: 100%/2   | Total:  1h 29m | Avg: 44m 36s | Max: 46m 13s | Hits:  90%/42    
  🟩 Test               Pass: 100%/3   | Total:  1h 44m | Avg: 34m 42s | Max: 43m 35s | Hits:  99%/11464 
  🟩 VerifyCodegen      Pass: 100%/1   | Total:  1m 43s | Avg:  1m 43s | Max:  1m 43s
🟩 sm
  🟩 75                 Pass: 100%/2   | Total:  1h 29m | Avg: 44m 36s | Max: 46m 13s | Hits:  90%/42    
  🟩 90                 Pass: 100%/2   | Total: 27m 24s | Avg: 13m 42s | Max: 22m 11s | Hits:  99%/7754  
  🟩 90;90a             Pass: 100%/2   | Total: 36m 59s | Avg: 18m 29s | Max: 31m 13s | Hits:  99%/7701  
  🟩 100;120            Pass: 100%/2   | Total: 37m 13s | Avg: 18m 36s | Max: 31m 53s | Hits:  99%/7701  
🟩 std
  🟩 17                 Pass: 100%/22  | Total:  5h 03m | Avg: 13m 48s | Max: 46m 56s | Hits:  91%/78377 
  🟩 20                 Pass: 100%/25  | Total:  6h 54m | Avg: 16m 34s | Max: 51m 47s | Hits:  93%/91295

🟩 cudax: Pass: 100%/28 | Total: 2h 58m | Avg: 6m 21s | Max: 28m 04s | Hits: 99%/15390

🟩 cpu
  🟩 amd64              Pass: 100%/24  | Total:  2h 45m | Avg:  6m 54s | Max: 28m 04s | Hits:  99%/13018 
  🟩 arm64              Pass: 100%/4   | Total: 12m 25s | Avg:  3m 06s | Max:  3m 27s | Hits:  99%/2372  
🟩 ctk
  🟩 12.0               Pass: 100%/3   | Total: 19m 46s | Avg:  6m 35s | Max: 13m 27s | Hits:  98%/1474  
  🟩 12.9               Pass: 100%/25  | Total:  2h 38m | Avg:  6m 19s | Max: 28m 04s | Hits:  99%/13916 
🟩 cudacxx
  🟩 nvcc12.0           Pass: 100%/3   | Total: 19m 46s | Avg:  6m 35s | Max: 13m 27s | Hits:  98%/1474  
  🟩 nvcc12.9           Pass: 100%/25  | Total:  2h 38m | Avg:  6m 19s | Max: 28m 04s | Hits:  99%/13916 
🟩 cudacxx_family
  🟩 nvcc               Pass: 100%/28  | Total:  2h 58m | Avg:  6m 21s | Max: 28m 04s | Hits:  99%/15390 
🟩 cxx
  🟩 Clang14            Pass: 100%/2   | Total:  6m 15s | Avg:  3m 07s | Max:  3m 26s | Hits: 100%/1188  
  🟩 Clang15            Pass: 100%/1   | Total:  3m 16s | Avg:  3m 16s | Max:  3m 16s | Hits: 100%/593   
  🟩 Clang16            Pass: 100%/1   | Total:  3m 28s | Avg:  3m 28s | Max:  3m 28s | Hits: 100%/593   
  🟩 Clang17            Pass: 100%/1   | Total:  3m 18s | Avg:  3m 18s | Max:  3m 18s | Hits: 100%/593   
  🟩 Clang18            Pass: 100%/1   | Total:  3m 12s | Avg:  3m 12s | Max:  3m 12s | Hits: 100%/593   
  🟩 Clang19            Pass: 100%/4   | Total: 18m 11s | Avg:  4m 32s | Max:  9m 04s | Hits: 100%/2372  
  🟩 GCC10              Pass: 100%/2   | Total:  7m 09s | Avg:  3m 34s | Max:  3m 39s | Hits:  99%/1188  
  🟩 GCC11              Pass: 100%/1   | Total:  3m 46s | Avg:  3m 46s | Max:  3m 46s | Hits:  99%/593   
  🟩 GCC12              Pass: 100%/1   | Total:  3m 59s | Avg:  3m 59s | Max:  3m 59s | Hits:  99%/593   
  🟩 GCC13              Pass: 100%/8   | Total: 56m 23s | Avg:  7m 02s | Max: 28m 04s | Hits:  99%/4744  
  🟩 MSVC14.39          Pass: 100%/1   | Total: 13m 27s | Avg: 13m 27s | Max: 13m 27s | Hits:  95%/288   
  🟩 MSVC14.43          Pass: 100%/3   | Total: 40m 43s | Avg: 13m 34s | Max: 13m 56s | Hits:  94%/870   
  🟩 NVHPC25.5          Pass: 100%/2   | Total: 14m 55s | Avg:  7m 27s | Max:  7m 36s | Hits:  97%/1182  
🟩 cxx_family
  🟩 Clang              Pass: 100%/10  | Total: 37m 40s | Avg:  3m 46s | Max:  9m 04s | Hits: 100%/5932  
  🟩 GCC                Pass: 100%/12  | Total:  1h 11m | Avg:  5m 56s | Max: 28m 04s | Hits:  99%/7118  
  🟩 MSVC               Pass: 100%/4   | Total: 54m 10s | Avg: 13m 32s | Max: 13m 56s | Hits:  94%/1158  
  🟩 NVHPC              Pass: 100%/2   | Total: 14m 55s | Avg:  7m 27s | Max:  7m 36s | Hits:  97%/1182  
🟩 gpu
  🟩 h100               Pass: 100%/2   | Total:  9m 59s | Avg:  4m 59s | Max:  6m 43s | Hits:  99%/1186  
  🟩 rtx2080            Pass: 100%/26  | Total:  2h 48m | Avg:  6m 27s | Max: 28m 04s | Hits:  99%/14204 
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🟩 sm
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  🟩 90;90a             Pass: 100%/2   | Total: 17m 35s | Avg:  8m 47s | Max: 13m 55s | Hits:  97%/883   
  🟩 100;120            Pass: 100%/2   | Total: 16m 36s | Avg:  8m 18s | Max: 12m 52s | Hits:  98%/883   
🟩 std
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  🟩 20                 Pass: 100%/25  | Total:  2h 44m | Avg:  6m 34s | Max: 28m 04s | Hits:  99%/13613

🟩 python: Pass: 100%/22 | Total: 3h 46m | Avg: 10m 17s | Max: 21m 59s

🟩 cpu
  🟩 amd64              Pass: 100%/22  | Total:  3h 46m | Avg: 10m 17s | Max: 21m 59s
🟩 ctk
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  🟩 12.8               Pass: 100%/2   | Total: 36m 52s | Avg: 18m 26s | Max: 18m 29s
  🟩 12.9               Pass: 100%/14  | Total:  2h 25m | Avg: 10m 21s | Max: 21m 59s
🟩 cudacxx
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  🟩 nvcc12.8           Pass: 100%/2   | Total: 36m 52s | Avg: 18m 26s | Max: 18m 29s
  🟩 nvcc12.9           Pass: 100%/14  | Total:  2h 25m | Avg: 10m 21s | Max: 21m 59s
🟩 cudacxx_family
  🟩 nvcc               Pass: 100%/22  | Total:  3h 46m | Avg: 10m 17s | Max: 21m 59s
🟩 cxx
  🟩 GCC13              Pass: 100%/22  | Total:  3h 46m | Avg: 10m 17s | Max: 21m 59s
🟩 cxx_family
  🟩 GCC                Pass: 100%/22  | Total:  3h 46m | Avg: 10m 17s | Max: 21m 59s
🟩 gpu
  🟩 h100               Pass: 100%/4   | Total: 43m 30s | Avg: 10m 52s | Max: 21m 59s
  🟩 l4                 Pass: 100%/18  | Total:  3h 02m | Avg: 10m 09s | Max: 19m 10s
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🟩 cccl_c_parallel: Pass: 100%/4 | Total: 48m 21s | Avg: 12m 05s | Max: 17m 02s | Hits: 98%/680

🟩 cpu
  🟩 amd64              Pass: 100%/4   | Total: 48m 21s | Avg: 12m 05s | Max: 17m 02s | Hits:  98%/680   
🟩 ctk
  🟩 12.9               Pass: 100%/4   | Total: 48m 21s | Avg: 12m 05s | Max: 17m 02s | Hits:  98%/680   
🟩 cudacxx
  🟩 nvcc12.9           Pass: 100%/4   | Total: 48m 21s | Avg: 12m 05s | Max: 17m 02s | Hits:  98%/680   
🟩 cudacxx_family
  🟩 nvcc               Pass: 100%/4   | Total: 48m 21s | Avg: 12m 05s | Max: 17m 02s | Hits:  98%/680   
🟩 cxx
  🟩 GCC13              Pass: 100%/4   | Total: 48m 21s | Avg: 12m 05s | Max: 17m 02s | Hits:  98%/680   
🟩 cxx_family
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🟩 gpu
  🟩 h100               Pass: 100%/1   | Total: 15m 53s | Avg: 15m 53s | Max: 15m 53s | Hits:  98%/170   
  🟩 l4                 Pass: 100%/1   | Total: 17m 02s | Avg: 17m 02s | Max: 17m 02s | Hits:  98%/170   
  🟩 rtx2080            Pass: 100%/2   | Total: 15m 26s | Avg:  7m 43s | Max: 13m 22s | Hits:  98%/340   
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🟩 packaging: Pass: 100%/4 | Total: 28m 51s | Avg: 7m 12s | Max: 8m 50s

🟩 cpu
  🟩 amd64              Pass: 100%/4   | Total: 28m 51s | Avg:  7m 12s | Max:  8m 50s
🟩 ctk
  🟩 12.0               Pass: 100%/2   | Total: 11m 12s | Avg:  5m 36s | Max:  8m 48s
  🟩 12.9               Pass: 100%/2   | Total: 17m 39s | Avg:  8m 49s | Max:  8m 50s
🟩 cudacxx
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  🟩 GCC12              Pass: 100%/1   | Total:  8m 48s | Avg:  8m 48s | Max:  8m 48s
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🟩 gpu
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🟩 jobs
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🟩 stdpar: Pass: 100%/4 | Total: 15m 51s | Avg: 3m 57s | Max: 4m 02s

🟩 cpu
  🟩 amd64              Pass: 100%/2   | Total:  8m 03s | Avg:  4m 01s | Max:  4m 02s
  🟩 arm64              Pass: 100%/2   | Total:  7m 48s | Avg:  3m 54s | Max:  4m 02s
🟩 ctk
  🟩 12.9               Pass: 100%/4   | Total: 15m 51s | Avg:  3m 57s | Max:  4m 02s
🟩 cudacxx
  🟩 nvcc12.9           Pass: 100%/4   | Total: 15m 51s | Avg:  3m 57s | Max:  4m 02s
🟩 cudacxx_family
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miscco · 2025-08-13T06:53:25Z

+  {
+    if (!_CUDA_VSTD::__cccl_default_is_constant_evaluated())
+    {
+      NV_IF_TARGET(NV_IS_DEVICE, (return ::min(::max(__v, __lo), __hi);))


I am wondering how this is an improvement at runtime?

Honestly this is more of an compiler bug if it generates bad code

@fbusato do you have a reproducer where bad code was produced? I think it would be worthwhile to file this to the compiler team.

I wrote a reproducer here: https://godbolt.org/z/GTEqznbTa

I was able to achieve the min/max implementation by introducing a temporary:

const auto __tmp = (__v < __lo) ? __lo : __v; return (__hi < __tmp) ? __hi : __tmp;

Unfortunately the min/max instructions are not emited when T is a floating point.

I wrote a reproducer here: https://godbolt.org/z/WGbrzM9TT

Thanks! This is really unfortunate. I reported this to the team in charge of the compiler optimizer.

thanks @bernhardmgruber for following up with the compiler team, and thanks @davebayer for further investigating the issue.

Unfortunately the min/max instructions are not emited when T is a floating point.

the problem with floating point is with NaN, but we have a solution for that 🙂 #5034

miscco · 2025-08-13T06:56:01Z

+    {
+      NV_IF_TARGET(NV_IS_DEVICE, (return ::min(::max(__v, __lo), __hi);))
+    }
+    return __v < __lo ? __lo : __hi < __v ? __hi : __v;


Note, we once saw that suboptimal code is generated because the compiler does not see through the nested ternary.

Could you please verify that there is change when writing this as:

Suggested change

return __v < __lo ? __lo : __hi < __v ? __hi : __v;

if (__v < __lo)

{

return __lo;

}

else if(__hi < __v)

{

return __hi;

}

else

{

return __v;

}

miscco · 2025-08-13T06:57:04Z

+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _CUDA___ALGORITHM_CLAMP_H


The file needs to be included in <cuda/algorithm>

bernhardmgruber · 2025-08-13T08:15:30Z

The biggest issue I see with this new API is that users will be confused when to use cuda::std::clamp or cuda::clamp and what the difference and implications are. I would feel better if this API was called T cuda::clamp_value(T value, T lower, T upper), because then it sets itself apart from std::clamp.

fbusato · 2025-08-13T15:53:02Z

@bernhardmgruber entirely agree. Let see if we are able to fix the issue from the compiler side. If yes, I will close the PR

fbusato · 2026-01-22T20:37:51Z

closing in favor of #7325

cuda clamp

b88a918

fbusato self-assigned this Aug 13, 2025

fbusato added the 3.2.0 label Aug 13, 2025

fbusato requested a review from a team as a code owner August 13, 2025 01:12

fbusato added this to CCCL Aug 13, 2025

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github-project-automation Bot moved this to Todo in CCCL Aug 13, 2025

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fbusato moved this from In Review to Blocked in CCCL Aug 13, 2025

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fbusato mentioned this pull request Jan 22, 2026

Implement internal min/max/clamp returning value #7325

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fbusato closed this Jan 22, 2026

github-project-automation Bot moved this from Blocked to Done in CCCL Jan 22, 2026

bernhardmgruber mentioned this pull request Nov 9, 2026

[libcu++] Add cuda::min, cuda::max and cuda::clamp #12512

Open

2 tasks done

-    return __v < __lo ? __lo : __hi < __v ? __hi : __v;
+    if (__v < __lo)
+    {
+      return __lo;
+    }
+    else if(__hi < __v)
+    {
+      return __hi;
+    }
+    else
+    {
+      return __v;
+    }

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fbusato commented Aug 13, 2025

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davebayer Aug 13, 2025 •

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