Register count for feature transformer

Sopel97 · snicolet · commit ce4c523ad3de · 2021-06-13T13:10:56.000+02:00
Compute optimal register count for feature transformer accumulation dynamically. This also introduces a change where AVX512 would only use 8 registers instead of 16 (now possible due to a 2x increase in feature transformer size). closes #3543 No functional change
diff --git a/src/nnue/nnue_feature_transformer.h b/src/nnue/nnue_feature_transformer.h
@@ -28,12 +28,17 @@
 
 namespace Stockfish::Eval::NNUE {
 
+  using BiasType       = std::int16_t;
+  using WeightType     = std::int16_t;
+  using PSQTWeightType = std::int32_t;
+
   // If vector instructions are enabled, we update and refresh the
   // accumulator tile by tile such that each tile fits in the CPU's
   // vector registers.
   #define VECTOR
 
-  static_assert(PSQTBuckets == 8, "Assumed by the current choice of constants.");
+  static_assert(PSQTBuckets % 8 == 0,
+    "Per feature PSQT values cannot be processed at granularity lower than 8 at a time.");
 
   #ifdef USE_AVX512
   typedef __m512i vec_t;
@@ -47,8 +52,7 @@ namespace Stockfish::Eval::NNUE {
   #define vec_add_psqt_32(a,b) _mm256_add_epi32(a,b)
   #define vec_sub_psqt_32(a,b) _mm256_sub_epi32(a,b)
   #define vec_zero_psqt() _mm256_setzero_si256()
-  static constexpr IndexType NumRegs = 8; // only 8 are needed
-  static constexpr IndexType NumPsqtRegs = 1;
+  #define NumRegistersSIMD 32
 
   #elif USE_AVX2
   typedef __m256i vec_t;
@@ -62,8 +66,7 @@ namespace Stockfish::Eval::NNUE {
   #define vec_add_psqt_32(a,b) _mm256_add_epi32(a,b)
   #define vec_sub_psqt_32(a,b) _mm256_sub_epi32(a,b)
   #define vec_zero_psqt() _mm256_setzero_si256()
-  static constexpr IndexType NumRegs = 16;
-  static constexpr IndexType NumPsqtRegs = 1;
+  #define NumRegistersSIMD 16
 
   #elif USE_SSE2
   typedef __m128i vec_t;
@@ -77,8 +80,7 @@ namespace Stockfish::Eval::NNUE {
   #define vec_add_psqt_32(a,b) _mm_add_epi32(a,b)
   #define vec_sub_psqt_32(a,b) _mm_sub_epi32(a,b)
   #define vec_zero_psqt() _mm_setzero_si128()
-  static constexpr IndexType NumRegs = Is64Bit ? 16 : 8;
-  static constexpr IndexType NumPsqtRegs = 2;
+  #define NumRegistersSIMD (Is64Bit ? 16 : 8)
 
   #elif USE_MMX
   typedef __m64 vec_t;
@@ -92,8 +94,7 @@ namespace Stockfish::Eval::NNUE {
   #define vec_add_psqt_32(a,b) _mm_add_pi32(a,b)
   #define vec_sub_psqt_32(a,b) _mm_sub_pi32(a,b)
   #define vec_zero_psqt() _mm_setzero_si64()
-  static constexpr IndexType NumRegs = 8;
-  static constexpr IndexType NumPsqtRegs = 4;
+  #define NumRegistersSIMD 8
 
   #elif USE_NEON
   typedef int16x8_t vec_t;
@@ -107,14 +108,61 @@ namespace Stockfish::Eval::NNUE {
   #define vec_add_psqt_32(a,b) vaddq_s32(a,b)
   #define vec_sub_psqt_32(a,b) vsubq_s32(a,b)
   #define vec_zero_psqt() psqt_vec_t{0}
-  static constexpr IndexType NumRegs = 16;
-  static constexpr IndexType NumPsqtRegs = 2;
+  #define NumRegistersSIMD 16
 
   #else
   #undef VECTOR
 
   #endif
 
+
+  #ifdef VECTOR
+
+      // Compute optimal SIMD register count for feature transformer accumulation.
+
+      // We use __m* types as template arguments, which causes GCC to emit warnings
+      // about losing some attribute information. This is irrelevant to us as we
+      // only take their size, so the following pragma are harmless.
+      #pragma GCC diagnostic push
+      #pragma GCC diagnostic ignored "-Wignored-attributes"
+
+      template <typename SIMDRegisterType,
+                typename LaneType,
+                int      NumLanes,
+                int      MaxRegisters>
+      static constexpr int BestRegisterCount()
+      {
+          #define RegisterSize  sizeof(SIMDRegisterType)
+          #define LaneSize      sizeof(LaneType)
+
+          static_assert(RegisterSize >= LaneSize);
+          static_assert(MaxRegisters <= NumRegistersSIMD);
+          static_assert(MaxRegisters > 0);
+          static_assert(NumRegistersSIMD > 0);
+          static_assert(RegisterSize % LaneSize == 0);
+          static_assert((NumLanes * LaneSize) % RegisterSize == 0);
+
+          const int ideal = (NumLanes * LaneSize) / RegisterSize;
+          if (ideal <= MaxRegisters)
+            return ideal;
+
+          // Look for the largest divisor of the ideal register count that is smaller than MaxRegisters
+          for (int divisor = MaxRegisters; divisor > 1; --divisor)
+            if (ideal % divisor == 0)
+              return divisor;
+
+          return 1;
+      }
+
+      static constexpr int NumRegs     = BestRegisterCount<vec_t, WeightType, TransformedFeatureDimensions, NumRegistersSIMD>();
+      static constexpr int NumPsqtRegs = BestRegisterCount<psqt_vec_t, PSQTWeightType, PSQTBuckets, NumRegistersSIMD>();
+
+      #pragma GCC diagnostic pop
+
+  #endif
+
+
+
   // Input feature converter
   class FeatureTransformer {
 
@@ -557,10 +605,6 @@ namespace Stockfish::Eval::NNUE {
   #endif
     }
 
-    using BiasType = std::int16_t;
-    using WeightType = std::int16_t;
-    using PSQTWeightType = std::int32_t;
-
     alignas(CacheLineSize) BiasType biases[HalfDimensions];
     alignas(CacheLineSize) WeightType weights[HalfDimensions * InputDimensions];
     alignas(CacheLineSize) PSQTWeightType psqtWeights[InputDimensions * PSQTBuckets];
