From 0ce4dfb2069019f9390fbfbf50be3052443325d5 Mon Sep 17 00:00:00 2001
From: Alibek Zhakubayev <alibek@meta.com>
Date: Thu, 2 Apr 2026 13:30:14 -0700
Subject: [PATCH] Fix AVX512_SPR build failure for 512-bit SIMD types (#5030)
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Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/5030

The `faiss_avx512_spr` CMake target fails to compile because 512-bit SIMD
type aliases (`simd32uint16`, `simd64uint8`, etc.) resolve to empty primary
templates when `SINGLE_SIMD_LEVEL = AVX512_SPR`. Only `AVX512` and `NONE`
have explicit template specializations.

This adds `simd512_level_selector` (analogous to existing
`simd256_level_selector`) that maps `AVX512_SPR -> AVX512` for 512-bit
types. This is correct because AVX512_SPR uses the same 512-bit integer
SIMD instructions as AVX512 — SPR-specific gains come from separate
instruction sets (BF16, FP16, VNNI) used in different code paths.

Changes:
- `simd_levels.h`: Add `simd512_level_selector` and `SINGLE_SIMD_LEVEL_512`
- `simdlib_dispatch.h`: Route 512-bit aliases through `SINGLE_SIMD_LEVEL_512`;
  also check `COMPILE_SIMD_AVX512_SPR` in include guard
- `LookupTableScaler.h`: Use `simd512_level_selector` for 512-bit types in
  `DummyScaler` and `NormTableScaler`

Reviewed By: mnorris11

Differential Revision: D99306837

fbshipit-source-id: 49e71415c155b9328dc76ff0cd492f718060ff1e
---
 faiss/impl/fast_scan/LookupTableScaler.h | 10 ++++++----
 faiss/impl/simdlib/simdlib_dispatch.h    | 13 +++++++------
 faiss/utils/simd_levels.h                | 20 ++++++++++++++++++++
 3 files changed, 33 insertions(+), 10 deletions(-)

diff --git a/faiss/impl/fast_scan/LookupTableScaler.h b/faiss/impl/fast_scan/LookupTableScaler.h
index 1285cfcc1..fa22df53e 100644
--- a/faiss/impl/fast_scan/LookupTableScaler.h
+++ b/faiss/impl/fast_scan/LookupTableScaler.h
@@ -24,10 +24,11 @@ template <SIMDLevel SL = SINGLE_SIMD_LEVEL>
 struct DummyScaler {
     static constexpr int nscale = 0;
     static constexpr SIMDLevel SL256 = simd256_level_selector<SL>::value;
+    static constexpr SIMDLevel SL512 = simd512_level_selector<SL>::value;
     using simd32uint8 = simd32uint8_tpl<SL256>;
     using simd16uint16 = simd16uint16_tpl<SL256>;
-    using simd64uint8 = simd64uint8_tpl<SL>;
-    using simd32uint16 = simd32uint16_tpl<SL>;
+    using simd64uint8 = simd64uint8_tpl<SL512>;
+    using simd32uint16 = simd32uint16_tpl<SL512>;
 
     inline simd32uint8 lookup(const simd32uint8&, const simd32uint8&) const {
         FAISS_THROW_MSG("DummyScaler::lookup should not be called.");
@@ -72,10 +73,11 @@ template <SIMDLevel SL = SINGLE_SIMD_LEVEL>
 struct NormTableScaler {
     static constexpr int nscale = 2;
     static constexpr SIMDLevel SL256 = simd256_level_selector<SL>::value;
+    static constexpr SIMDLevel SL512 = simd512_level_selector<SL>::value;
     using simd32uint8 = simd32uint8_tpl<SL256>;
     using simd16uint16 = simd16uint16_tpl<SL256>;
-    using simd64uint8 = simd64uint8_tpl<SL>;
-    using simd32uint16 = simd32uint16_tpl<SL>;
+    using simd64uint8 = simd64uint8_tpl<SL512>;
+    using simd32uint16 = simd32uint16_tpl<SL512>;
 
     int scale_int;
     simd16uint16 scale_simd;
diff --git a/faiss/impl/simdlib/simdlib_dispatch.h b/faiss/impl/simdlib/simdlib_dispatch.h
index d38bae3c3..5a8a0aaba 100644
--- a/faiss/impl/simdlib/simdlib_dispatch.h
+++ b/faiss/impl/simdlib/simdlib_dispatch.h
@@ -22,7 +22,8 @@
 // Only per-SIMD TUs (compiled with -mavx2 etc.) see the platform
 // specializations. In static mode: only the compiled-in level is available.
 
-#if defined(COMPILE_SIMD_AVX512) && defined(__AVX512F__)
+#if (defined(COMPILE_SIMD_AVX512) || defined(COMPILE_SIMD_AVX512_SPR)) && \
+        defined(__AVX512F__)
 
 // AVX512 includes AVX2 (simdlib_avx512.h includes simdlib_avx2.h)
 #include <faiss/impl/simdlib/simdlib_avx512.h>
@@ -50,10 +51,10 @@ using simd32uint8 = simd32uint8_tpl<SINGLE_SIMD_LEVEL_256>;
 using simd8uint32 = simd8uint32_tpl<SINGLE_SIMD_LEVEL_256>;
 using simd8float32 = simd8float32_tpl<SINGLE_SIMD_LEVEL_256>;
 
-// 512-bit
-using simd512bit = simd512bit_tpl<SINGLE_SIMD_LEVEL>;
-using simd32uint16 = simd32uint16_tpl<SINGLE_SIMD_LEVEL>;
-using simd64uint8 = simd64uint8_tpl<SINGLE_SIMD_LEVEL>;
-using simd16float32 = simd16float32_tpl<SINGLE_SIMD_LEVEL>;
+// 512-bit (AVX512_SPR maps to AVX512 — same 512-bit integer ops)
+using simd512bit = simd512bit_tpl<SINGLE_SIMD_LEVEL_512>;
+using simd32uint16 = simd32uint16_tpl<SINGLE_SIMD_LEVEL_512>;
+using simd64uint8 = simd64uint8_tpl<SINGLE_SIMD_LEVEL_512>;
+using simd16float32 = simd16float32_tpl<SINGLE_SIMD_LEVEL_512>;
 
 } // namespace faiss
diff --git a/faiss/utils/simd_levels.h b/faiss/utils/simd_levels.h
index 52727bf0e..61d84b55f 100644
--- a/faiss/utils/simd_levels.h
+++ b/faiss/utils/simd_levels.h
@@ -86,6 +86,26 @@ struct simd256_level_selector {
 inline constexpr SIMDLevel SINGLE_SIMD_LEVEL_256 =
         simd256_level_selector<SINGLE_SIMD_LEVEL>::value;
 
+/***************************************************************
+ * Helper to select the appropriate 512-bit SIMD level.
+ *
+ * For 512-bit SIMD types (simd32uint16, simd64uint8, etc.), maps:
+ *   AVX512_SPR → AVX512 (512-bit ops share the same instructions)
+ *   AVX512 → AVX512
+ *   NONE → NONE
+ ***************************************************************/
+template <SIMDLevel SL>
+struct simd512_level_selector {
+    static constexpr SIMDLevel value =
+            (SL == SIMDLevel::AVX512_SPR) ? SIMDLevel::AVX512 : SL;
+};
+
+/// SINGLE_SIMD_LEVEL mapped to 512-bit: use this for 512-bit simd types
+/// (simd32uint16, simd64uint8, etc.) which don't have AVX512_SPR
+/// specializations (AVX512_SPR uses the same 512-bit integer ops as AVX512).
+inline constexpr SIMDLevel SINGLE_SIMD_LEVEL_512 =
+        simd512_level_selector<SINGLE_SIMD_LEVEL>::value;
+
 /// Number of float32 lanes for a given SIMD level.
 /// ARM_SVE is variable-width (128–2048 bits); no single constant is correct.
 template <SIMDLevel SL>