Re: [PATCH v3 1/2] lib/zstd: add fallback aliases for disabled BMI2 variants

[Dhruva G <goledhruva@gmail.com>]

On 01-09-2026 16:37, Usama Arif wrote:
> When dynamic BMI2 dispatch is disabled, the BMI2-specific functions are
> not compiled and each selector is conditionally compiled to avoid naming
> them.
> 
> The selector-level preprocessor guards will be replaced with a predicate
> that becomes constant false when dynamic BMI2 dispatch is disabled.
> Although the compiler eliminates an `if (0)` branch, it must still parse
> and resolve the BMI2 function referenced by it.
> 
> Add aliases from the unavailable BMI2 function names to their default
> implementations. These aliases make the names valid without emitting
> BMI2-specific code. The selectors remain unchanged in this patch, so the
> aliases are not used yet and there is no code-generation change.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---

Reviewed-by: Dhruva Gole <goledhruva@gmail.com>

>  lib/zstd/common/entropy_common.c            |  4 ++++
>  lib/zstd/common/fse_decompress.c            |  2 ++
>  lib/zstd/compress/zstd_compress_sequences.c |  4 ++++
>  lib/zstd/decompress/huf_decompress.c        |  4 ++++
>  lib/zstd/decompress/zstd_decompress_block.c | 12 ++++++++++++
>  5 files changed, 26 insertions(+)
> 
> diff --git a/lib/zstd/common/entropy_common.c b/lib/zstd/common/entropy_common.c
> index 6cdd82233fb59..15a7c14ae8040 100644
> --- a/lib/zstd/common/entropy_common.c
> +++ b/lib/zstd/common/entropy_common.c
> @@ -202,6 +202,8 @@ BMI2_TARGET_ATTRIBUTE static size_t FSE_readNCount_body_bmi2(
>  {
>      return FSE_readNCount_body(normalizedCounter, maxSVPtr, tableLogPtr, headerBuffer, hbSize);
>  }
> +#else
> +#define FSE_readNCount_body_bmi2 FSE_readNCount_body_default
>  #endif
>  
>  size_t FSE_readNCount_bmi2(
> @@ -323,6 +325,8 @@ static BMI2_TARGET_ATTRIBUTE size_t HUF_readStats_body_bmi2(BYTE* huffWeight, si
>  {
>      return HUF_readStats_body(huffWeight, hwSize, rankStats, nbSymbolsPtr, tableLogPtr, src, srcSize, workSpace, wkspSize, 1);
>  }
> +#else
> +#define HUF_readStats_body_bmi2 HUF_readStats_body_default
>  #endif
>  
>  size_t HUF_readStats_wksp(BYTE* huffWeight, size_t hwSize, U32* rankStats,
> diff --git a/lib/zstd/common/fse_decompress.c b/lib/zstd/common/fse_decompress.c
> index 15081d8dc607c..8d9c43b928833 100644
> --- a/lib/zstd/common/fse_decompress.c
> +++ b/lib/zstd/common/fse_decompress.c
> @@ -300,6 +300,8 @@ BMI2_TARGET_ATTRIBUTE static size_t FSE_decompress_wksp_body_bmi2(void* dst, siz
>  {
>      return FSE_decompress_wksp_body(dst, dstCapacity, cSrc, cSrcSize, maxLog, workSpace, wkspSize, 1);
>  }
> +#else
> +#define FSE_decompress_wksp_body_bmi2 FSE_decompress_wksp_body_default
>  #endif
>  
>  size_t FSE_decompress_wksp_bmi2(void* dst, size_t dstCapacity, const void* cSrc, size_t cSrcSize, unsigned maxLog, void* workSpace, size_t wkspSize, int bmi2)
> diff --git a/lib/zstd/compress/zstd_compress_sequences.c b/lib/zstd/compress/zstd_compress_sequences.c
> index 256980c9d85ad..64abcdf2c5714 100644
> --- a/lib/zstd/compress/zstd_compress_sequences.c
> +++ b/lib/zstd/compress/zstd_compress_sequences.c
> @@ -415,6 +415,10 @@ ZSTD_encodeSequences_bmi2(
>                                      sequences, nbSeq, longOffsets);
>  }
>  
> +#else
> +
> +#define ZSTD_encodeSequences_bmi2 ZSTD_encodeSequences_default
> +
>  #endif
>  
>  size_t ZSTD_encodeSequences(
> diff --git a/lib/zstd/decompress/huf_decompress.c b/lib/zstd/decompress/huf_decompress.c
> index ac8b87f48f847..5663ae524cd83 100644
> --- a/lib/zstd/decompress/huf_decompress.c
> +++ b/lib/zstd/decompress/huf_decompress.c
> @@ -700,6 +700,8 @@ size_t HUF_decompress4X1_usingDTable_internal_bmi2(void* dst, size_t dstSize, vo
>                      size_t cSrcSize, HUF_DTable const* DTable) {
>      return HUF_decompress4X1_usingDTable_internal_body(dst, dstSize, cSrc, cSrcSize, DTable);
>  }
> +#else
> +#define HUF_decompress4X1_usingDTable_internal_bmi2 HUF_decompress4X1_usingDTable_internal_default
>  #endif
>  
>  static
> @@ -1503,6 +1505,8 @@ size_t HUF_decompress4X2_usingDTable_internal_bmi2(void* dst, size_t dstSize, vo
>                      size_t cSrcSize, HUF_DTable const* DTable) {
>      return HUF_decompress4X2_usingDTable_internal_body(dst, dstSize, cSrc, cSrcSize, DTable);
>  }
> +#else
> +#define HUF_decompress4X2_usingDTable_internal_bmi2 HUF_decompress4X2_usingDTable_internal_default
>  #endif
>  
>  static
> diff --git a/lib/zstd/decompress/zstd_decompress_block.c b/lib/zstd/decompress/zstd_decompress_block.c
> index 710eb0ffd5a37..9c4215e435014 100644
> --- a/lib/zstd/decompress/zstd_decompress_block.c
> +++ b/lib/zstd/decompress/zstd_decompress_block.c
> @@ -622,6 +622,8 @@ BMI2_TARGET_ATTRIBUTE static void ZSTD_buildFSETable_body_bmi2(ZSTD_seqSymbol* d
>      ZSTD_buildFSETable_body(dt, normalizedCounter, maxSymbolValue,
>              baseValue, nbAdditionalBits, tableLog, wksp, wkspSize);
>  }
> +#else
> +#define ZSTD_buildFSETable_body_bmi2 ZSTD_buildFSETable_body_default
>  #endif
>  
>  void ZSTD_buildFSETable(ZSTD_seqSymbol* dt,
> @@ -1934,6 +1936,16 @@ ZSTD_decompressSequencesLong_bmi2(ZSTD_DCtx* dctx,
>  }
>  #endif /* ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT */
>  
> +#else
> +
> +#ifndef ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG
> +#define ZSTD_decompressSequences_bmi2 ZSTD_decompressSequences_default
> +#define ZSTD_decompressSequencesSplitLitBuffer_bmi2 ZSTD_decompressSequencesSplitLitBuffer_default
> +#endif
> +#ifndef ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT
> +#define ZSTD_decompressSequencesLong_bmi2 ZSTD_decompressSequencesLong_default
> +#endif
> +
>  #endif /* DYNAMIC_BMI2 */
>  
>  #ifndef ZSTD_FORCE_DECOMPRESS_SEQUENCES_LONG