[llvm] 54971c8 - [llvm] Use llvm::bit_ceil (NFC)

[Nikita Popov via llvm-commits]

On Sun, Jan 29, 2023, at 09:12, Kazu Hirata via llvm-commits wrote:
> 
> Author: Kazu Hirata
> Date: 2023-01-29T00:12:53-08:00
> New Revision: 54971c8a39e8b682e7a042eee33f94a8283d8305
> 
> URL: https://github.com/llvm/llvm-project/commit/54971c8a39e8b682e7a042eee33f94a8283d8305
> DIFF: https://github.com/llvm/llvm-project/commit/54971c8a39e8b682e7a042eee33f94a8283d8305.diff
> 
> LOG: [llvm] Use llvm::bit_ceil (NFC)
> 
> If X is nonzero, NextPowerOf2(X - 1) is equivalent to
> llvm::bit_ceil(X).  In this patch, std::max guarantees that X is
> nonzero.

This causes a measurable compile-time regression: https://llvm-compile-time-tracker.com/compare.php?from=0de61934a37180d6028852740eaa0e9e409d001a&to=54971c8a39e8b682e7a042eee33f94a8283d8305&stat=instructions:u

The DenseMap changes should probably be reverted, as they are performance-critical.

Regards,
Nikita

> 
> Added: 
>     
> 
> Modified: 
>     llvm/include/llvm/ADT/DenseMap.h
>     llvm/lib/Transforms/Utils/SimplifyCFG.cpp
> 
> Removed: 
>     
> 
> 
> ################################################################################
> diff  --git a/llvm/include/llvm/ADT/DenseMap.h b/llvm/include/llvm/ADT/DenseMap.h
> index 7adc6710cfa8..9a6852646268 100644
> --- a/llvm/include/llvm/ADT/DenseMap.h
> +++ b/llvm/include/llvm/ADT/DenseMap.h
> @@ -803,7 +803,7 @@ class DenseMap : public DenseMapBase<DenseMap<KeyT, ValueT, KeyInfoT, BucketT>,
>      unsigned OldNumBuckets = NumBuckets;
>      BucketT *OldBuckets = Buckets;
>  
> -    allocateBuckets(std::max<unsigned>(64, static_cast<unsigned>(NextPowerOf2(AtLeast-1))));
> +    allocateBuckets(llvm::bit_ceil(std::max(64u, AtLeast)));
>      assert(Buckets);
>      if (!OldBuckets) {
>        this->BaseT::initEmpty();
> @@ -1042,7 +1042,7 @@ class SmallDenseMap
>  
>    void grow(unsigned AtLeast) {
>      if (AtLeast > InlineBuckets)
> -      AtLeast = std::max<unsigned>(64, NextPowerOf2(AtLeast-1));
> +      AtLeast = llvm::bit_ceil(std::max(64u, AtLeast));
>  
>      if (Small) {
>        // First move the inline buckets into a temporary storage.
> 
> diff  --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
> index 372a6698ce8a..e6d9b265dab9 100644
> --- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
> +++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
> @@ -6552,7 +6552,7 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
>  
>      // Make the mask's bitwidth at least 8-bit and a power-of-2 to avoid
>      // unnecessary illegal types.
> -    uint64_t TableSizePowOf2 = NextPowerOf2(std::max(7ULL, TableSize - 1ULL));
> +    uint64_t TableSizePowOf2 = llvm::bit_ceil(std::max<uint64_t>(8, TableSize));
>      APInt MaskInt(TableSizePowOf2, 0);
>      APInt One(TableSizePowOf2, 1);
>      // Build bitmask; fill in a 1 bit for every case.
> 
> 
>         
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