use faster covnersion from float8_e4m3fn to bfloat16 (#12316)

sgl-kernel/csrc/cpu/vec.h

@@ -121,6 +121,30 @@ inline __m512bh CVT_FP8_TO_BF16(__m256i a) {
 #endif
 }
 
+// faster version of float8_e4m3fn conversion to bfloat16
+//
+// we mapped cuda implementation from below link and vectorized with avx512:
+// https://github.com/thu-pacman/chitu/blob/1ed2078ec26581ebdca05b7306d4385f86edaa7c/csrc/cuda/marlin/marlin_gemm/dequant.h#L387
+//
+inline __attribute__((always_inline)) __m512bh CVT_FP8_TO_BF16_EXT(__m256i a) {
+  const __m512i mask0 = _mm512_set1_epi16(0x80);  // sign bit
+  const __m512i mask1 = _mm512_set1_epi16(0x7F);  // exponent and mantissa
+  const __m512i mask2 = _mm512_set1_epi16(0x4000);
+
+  __m512i x = _mm512_cvtepu8_epi16(a);
+  __m512i vsign = _mm512_and_si512(x, mask0);
+  vsign = _mm512_slli_epi16(vsign, 8);
+
+  __m512i vexp_and_mant = _mm512_and_si512(x, mask1);
+  vexp_and_mant = _mm512_slli_epi16(vexp_and_mant, 4);
+
+  // _MM_TERNLOG_A | _MM_TERNLOG_B | _MM_TERNLOG_C: 0b11111110
+  return (__m512bh)(_mm512_ternarylogic_epi32(vsign, mask2, vexp_and_mant, 0b11111110));
+}
+
+// bias for conversion of fp8 to bf16 1/256 in float32
+#define kFP8_BIAS 0x3b800000
+
 #endif
 
 // vector to scalar reduction