Speedup the AVX-512 implementation of ggml_vec_dot_q4_0()

Author: dfyz

CMakeLists.txt

@@ -55,6 +55,8 @@ option(LLAMA_SANITIZE_UNDEFINED     "llama: enable undefined sanitizer"
 option(LLAMA_AVX                    "llama: enable AVX"                                     ON)
 option(LLAMA_AVX2                   "llama: enable AVX2"                                    ON)
 option(LLAMA_AVX512                 "llama: enable AVX512"                                  OFF)
+option(LLAMA_AVX512_VBMI            "llama: enable AVX512-VBMI"                             OFF)
+option(LLAMA_AVX512_VNNI            "llama: enable AVX512-VNNI"                             OFF)
 option(LLAMA_FMA                    "llama: enable FMA"                                     ON)
 # in MSVC F16C is implied with AVX2/AVX512
 if (NOT MSVC)
@@ -220,6 +222,16 @@ elseif (${CMAKE_SYSTEM_PROCESSOR} MATCHES "^(x86_64|i686|AMD64)$")
     if (MSVC)
         if (LLAMA_AVX512)
             add_compile_options(/arch:AVX512)
+            # MSVC has no compile-time flags enabling specific
+            # AVX512 extensions, neither it defines the
+            # macros corresponding to the extensions.
+            # Do it manually.
+            if (LLAMA_AVX512_VBMI)
+                add_compile_definitions(__AVX512VBMI__)
+            endif()
+            if (LLAMA_AVX512_VNNI)
+                add_compile_definitions(__AVX512VNNI__)
+            endif()
         elseif (LLAMA_AVX2)
             add_compile_options(/arch:AVX2)
         elseif (LLAMA_AVX)
@@ -240,9 +252,13 @@ elseif (${CMAKE_SYSTEM_PROCESSOR} MATCHES "^(x86_64|i686|AMD64)$")
         endif()
         if (LLAMA_AVX512)
             add_compile_options(-mavx512f)
-            # add_compile_options(-mavx512cd)
-            # add_compile_options(-mavx512dq)
-            # add_compile_options(-mavx512bw)
+            add_compile_options(-mavx512bw)
+        endif()
+        if (LLAMA_AVX512_VBMI)
+            add_compile_options(-mavx512vbmi)
+        endif()
+        if (LLAMA_AVX512_VNNI)
+            add_compile_options(-mavx512vnni)
         endif()
     endif()
 else()

ggml.c

@@ -1977,33 +1977,187 @@ inline static void ggml_vec_dot_f32(const int n, float * restrict s, const float
 }
 
 #if __AVX512F__ && QK4_0 == 32
-static inline __m512 dot_q4_0_oneblock_avx512(
+static inline __m512i bytes_from_q4_0_twoblocks_avx512( const __m512i blocks ) {
+    // The 64 bytes of `blocks` contain two consecutive Q4_0 blocks loaded from memory:
+    // +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    // |63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32|
+    // +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    // |                                                                        :. =_ () [] <> () Zz Yy|
+    // +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    // |31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00|
+    // +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    // |Xx Ww Vv Uu Tt Ss Rr Qq             Pp Oo Nn Mm Ll Kk Jj Ii Hh Gg Ff Ee Dd Cc Bb Aa            |
+    // +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
+    //
+    // Bytes 04..19 (block #0) and 24..39 (block #1) both contain 32 nibbles (4-bit unsigned integers).
+    // We have exactly 64 nibbles, so we want to place each nibble into a separate byte.
+    // Bytes 00..03 and 20..23 contain scales, which are irrelevant to this function.
+    // Bytes 40..63 are masked when loading the data, so they are zeroed out.
+#ifdef __AVX512VBMI__
+    const __m512i byte_perm = _mm512_set_epi8(
+        39, 38, 39, 38, 37, 36, 37, 36, 35, 34, 35, 34, 33, 32, 33, 32,
+        31, 30, 31, 30, 29, 28, 29, 28, 27, 26, 27, 26, 25, 24, 25, 24,
+        19, 18, 19, 18, 17, 16, 17, 16, 15, 14, 15, 14, 13, 12, 13, 12,
+        11, 10, 11, 10,  9,  8,  9,  8,  7,  6,  7,  6,  5,  4,  5,  4
+    );
+    const __m512i permuted = _mm512_permutexvar_epi8( byte_perm, blocks );
+    // After applying VPERMB, `permuted` looks like this:
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+    // |63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32|
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+    // |:. =_ :. =_ () [] () [] <> () <> () Zz Yy Zz Yy Xx Ww Xx Ww Vv Uu Vv Uu Tt Ss Tt Ss Rr Qq Rr Qq|
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+    // |31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00|
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+    // |Pp Oo Pp Oo Nn Mm Nn Mm Ll Kk Ll Kk Jj Ii Jj Ii Hh Gg Hh Gg Ff Ee Ff Ee Dd Cc Dd Cc Bb Aa Bb Aa|
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+#else
+    const __m512i word_perm = _mm512_set_epi16(
+        19, 19, 18, 18, 17, 17, 16, 16, 15, 15, 14, 14, 13, 13, 12, 12,
+         9,  9,  8,  8,  7,  7,  6,  6,  5,  5,  4,  4,  3,  3,  2,  2
+    );
+    const __m512i permuted = _mm512_permutexvar_epi16( word_perm, blocks );
+    // This is the fallback path for CPUs that don't support VPERMB. Since we permute 16-bit groups only,
+    // VPERMB can be replaced with VPERMW. We could always use VPERMW, but at least on Tiger Lake and
+    // Ice Lake VPERMW followed by a right shift is quite noticeably slower than VPERMB.
+#endif
+
+    // Shift every odd-numbered 16-bit group to the right by 4 bits.
+    const __mmask32 shift_mask = 0xaaaaaaaa;
+    const __m512i shifted = _mm512_mask_srai_epi16( permuted, shift_mask, permuted, 4 );
+    // After applying VPSRAW, `shifted` looks like this (the "empty" nibbles are filled with zeroes):
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+    // |63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+    // | : .= :. =_  ( )[ () []  < >( <> ()  Z zY Zz Yy  X xW Xx Ww  V vU Vv Uu  T tS Tt Ss  R rQ Rr Qq
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+    // |31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00|
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+    // | P pO Pp Oo  N nM Nn Mm  L lK Ll Kk  J jI Jj Ii  H hG Hh Gg  F fE Ff Ee  D dC Dd Cc  B bA Bb Aa|
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+
+    // Now we just need to zero out the higher nibble in each byte, and we're done.
+    const __m512i low_nibble_mask = _mm512_set1_epi8( 0xf );
+    return _mm512_and_si512( low_nibble_mask, shifted );
+    // The final result looks like this:
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+    // |63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 32|
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+    // | :  =  .  _  (  [  )  ]  <  (  >  )  Z  Y  z  y  X  W  x  w  V  U  v  u  T  S  t  s  R  Q  r  q|
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+    // |31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00|
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+    // | P  O  p  o  N  M  n  m  L  K  l  k  J  I  j  i  H  G  h  g  F  E  f  e  D  C  d  c  B  A  b  a|
+    // +-----------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
+}
+
+static inline __m512 dot_q4_0_twoblocks_avx512(
     __m512 acc,
     const block_q4_0 * restrict x,
     const block_q4_0 * restrict y,
     int i
 ) {
-    // Compute combined scale for the block
-    __m512 d = _mm512_set1_ps( x[i].d * y[i].d );
-
-    __m256i bx = bytesFromNibbles( x[i].qs );
-    __m256i by = bytesFromNibbles( y[i].qs );
-
-    // Now we have a vector with bytes in [ 0 .. 15 ] interval. Offset them into [ -8 .. +7 ] interval.
-    const __m256i off = _mm256_set1_epi8( 8 );
-    bx = _mm256_sub_epi8( bx, off );
-    by = _mm256_sub_epi8( by, off );
-
-    // Sign-extend 16 signed bytes into int16_t
-    __m512i x32 = _mm512_cvtepi8_epi16( bx );
-    __m512i y32 = _mm512_cvtepi8_epi16( by );
-    // Compute products of int16_t integers, add pairwise
-    __m512i i64 = _mm512_madd_epi16( x32, y32 );
+    // A pair of Q4_0 blocks spans 40 bytes, while an AVX-512 register has 64. The remaining 24 bytes
+    // can potentially be unaddressable, so we make sure to mask them out before the load, even though
+    // we don't use them at all. This might hurt the performance slightly, since the compiler is forced
+    // to use e.g. `VMOVDQU64 REG, MASK, [ADDR] + VPERMB ..., REG` instead of just `VPERMB ..., [ADDR]`.
+    const __mmask8 load_mask = 0x1f;
+    const __m512i blocks_0 = _mm512_maskz_loadu_epi64( load_mask, &x[i] );
+    const __m512i blocks_1 = _mm512_maskz_loadu_epi64( load_mask, &y[i] );
+
+    // We want to multiply the scales, so we interpret both registers as 16 32-bit floats:
+    // +----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+
+    // | 15 | 14 | 13 | 12 | 11 | 10 | 09 | 08 | 07 | 06 | 05 | 04 | 03 | 02 | 01 | 00 |
+    // +----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+
+    // blocks_0_float
+    // +----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+
+    // |    |    |    |    |    |    | xx | xx | xx | xx |  B | xx | xx | xx | xx |  A |
+    // +----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+
+    // blocks_1_float
+    // +----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+
+    // |    |    |    |    |    |    | xx | xx | xx | xx |  D | xx | xx | xx | xx |  C |
+    // +----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+
+    const __m512 blocks_0_float = _mm512_castsi512_ps( blocks_0 );
+    const __m512 blocks_1_float = _mm512_castsi512_ps( blocks_1 );
+    // We absolutely shouldn't touch the floats marked with `xx`: they contain some
+    // random data, which might very well underflow. At least on Intel, this leads
+    // to a huge penalty that can't be ignored (easily 100x or more) unless you
+    // compile your code with something like `-ffast-math` to enable FTZ/DAZ flags.
+    // (and ggml can't assume that you do)...
+    const __mmask16 scale_mul_mask = 0x21;
+#ifdef __clang__
+    // ...however, clang decides to optimize the multiplication mask away:
+    // https://godbolt.org/z/P8PqdsfvW
+    // gcc and MSVC do the sane thing. This horrible workaround forces clang to emit the mask.
+    __m512i scales;
+    __asm__(
+        "vmulps %1, %2, %0%{%3%}"
+        : "=v" ( scales )
+        : "vm" ( blocks_0_float ), "v" ( blocks_1_float ), "Yk" ( scale_mul_mask )
+    );
+#else
+    const __m512 scales = _mm512_maskz_mul_ps( scale_mul_mask, blocks_0_float, blocks_1_float );
+#endif
+    const __m512i scale_perm = _mm512_set_epi32(
+        5, 5, 5, 5, 5, 5, 5, 5,
+        0, 0, 0, 0, 0, 0, 0, 0
+    );
+    const __m512 permuted_scales = _mm512_permutexvar_ps( scale_perm, scales );
+    // After VMULPS and VPERMPS, `permuted_scales` looks like this:
+    // +----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+
+    // | 15 | 14 | 13 | 12 | 11 | 10 | 09 | 08 | 07 | 06 | 05 | 04 | 03 | 02 | 01 | 00 |
+    // +----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+
+    // | B*D| B*D| B*D| B*D| B*D| B*D| B*D| B*D| A*C| A*C| A*C| A*C| A*C| A*C| A*C| A*C|
+    // +----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+
+
+    const __m512i bytes_0 = bytes_from_q4_0_twoblocks_avx512( blocks_0 );
+    const __m512i bytes_1 = bytes_from_q4_0_twoblocks_avx512( blocks_1 );
+
+    // Now we want to compute dot products of 4-element byte vectors and store them in
+    // 32-bit integers. That is (only one 4-element vector is shown for clarity):
+    //     +----+----+----+----+
+    // ... | 03 | 02 | 01 | 00 |
+    //     +----+----+----+----+
+    // bytes_0
+    //     +----+----+----+----+
+    // ... |  D |  C |  B |  A |
+    //     +----+----+----+----+
+    // bytes_1
+    //     +----+----+----+----+
+    // ... |  H |  G |  F |  E |
+    //     +----+----+----+----+
+    // final_res_int
+    //     +----+----+----+----+
+    // ... |  A*E+B*F+C*G+D*H  |
+    //     +----+----+----+----+
+    const __m512i plus_8 = _mm512_set1_epi8( 8 );
+    const __m512i bytes_1_minus_8 = _mm512_sub_epi8( bytes_1, plus_8 );
+
+#ifdef __AVX512VNNI__
+    // We have VPDPBUSDS in AVX512-VNNI, which does exactly what we want, but with a catch:
+    // the *left* operand is supposed to be unsigned, while Q4_0 quantization subtracts 8
+    // from each nibble, so they can be negative. So, instead of `(bytes_0 - 8) * (bytes_1 - 8)`,
+    // we compute `bytes_0 * (bytes_1 - 8) + bytes_1 * (-8) + 64`. VPDPBUSDS uses an accumulator,
+    // which means we only need 2 instructions.
+    const __m512i dot_init = _mm512_set1_epi32( 4 * 64 );
+    const __m512i minus_8 = _mm512_set1_epi8( -8 );
+    const __m512i prod_0 = _mm512_dpbusds_epi32( dot_init, bytes_1, minus_8 );
+    const __m512i final_res_int = _mm512_dpbusds_epi32( prod_0, bytes_0, bytes_1_minus_8 );
+#else
+    // As a fallback, we have VPMADDUBSW in AVX512-BW, which uses 16-bit products instead of 32-bit ones.
+    // It has the same catch as VPDPBUSDS: the left operand should be unsigned.
+    // This is essentially the AVX-512 version of the AVX-2 trick used by GH user Const-me
+    //   ref: https://gist.github.com/Const-me/4d30e1fc767ab314596e16e90f53b6f4#file-matmultest-cpp-L119
+    const __m512i one = _mm512_set1_epi16( 1 );
+    const __m512i prod_0 = _mm512_maddubs_epi16( bytes_0, bytes_1_minus_8 );
+    const __m512i prod_1 = _mm512_maddubs_epi16( plus_8, bytes_1_minus_8 );
+    const __m512i diff = _mm512_sub_epi16( prod_0, prod_1 );
+    const __m512i final_res_int = _mm512_madd_epi16( diff, one );
+#endif
 
-    // Convert int32_t to float
-    __m512 p = _mm512_cvtepi32_ps( i64 );
-    // Apply the scale, and accumulate
-    return _mm512_fmadd_ps( d, p, acc );
+    // Finally, we multiply the permuted scales and the 32-bit dot products, then accumulate.
+    const __m512 final_res_float = _mm512_cvtepi32_ps( final_res_int );
+    return _mm512_fmadd_ps( permuted_scales, final_res_float, acc );
 }
 #endif
 
@@ -2135,25 +2289,26 @@ static void ggml_vec_dot_q4_0(const int n, float * restrict s, const void * rest
     __m512 acc0 = _mm512_setzero_ps();
     __m512 acc1 = _mm512_setzero_ps();
 
-    const int superblock_size = 8;
+    const int superblock_size = 16;
+
     const int superblock_count = nb / superblock_size;
 
     for (int superblock_ix = 0; superblock_ix < superblock_count; superblock_ix += 1) {
         int i = superblock_ix * superblock_size;
 
-        acc0 = dot_q4_0_oneblock_avx512( acc0, x, y, i+0 );
-        acc1 = dot_q4_0_oneblock_avx512( acc1, x, y, i+1 );
-        acc0 = dot_q4_0_oneblock_avx512( acc0, x, y, i+2 );
-        acc1 = dot_q4_0_oneblock_avx512( acc1, x, y, i+3 );
-        acc0 = dot_q4_0_oneblock_avx512( acc0, x, y, i+4 );
-        acc1 = dot_q4_0_oneblock_avx512( acc1, x, y, i+5 );
-        acc0 = dot_q4_0_oneblock_avx512( acc0, x, y, i+6 );
-        acc1 = dot_q4_0_oneblock_avx512( acc1, x, y, i+7 );
+        acc0 = dot_q4_0_twoblocks_avx512( acc0, x, y, i+0 );
+        acc1 = dot_q4_0_twoblocks_avx512( acc1, x, y, i+2 );
+        acc0 = dot_q4_0_twoblocks_avx512( acc0, x, y, i+4 );
+        acc1 = dot_q4_0_twoblocks_avx512( acc1, x, y, i+6 );
+        acc0 = dot_q4_0_twoblocks_avx512( acc0, x, y, i+8 );
+        acc1 = dot_q4_0_twoblocks_avx512( acc1, x, y, i+10 );
+        acc0 = dot_q4_0_twoblocks_avx512( acc0, x, y, i+12 );
+        acc1 = dot_q4_0_twoblocks_avx512( acc1, x, y, i+14 );
     }
 
     // Remainders
-    for (int i = superblock_count * superblock_size; i < nb; ++i) {
-        acc0 = dot_q4_0_oneblock_avx512( acc0, x, y, i );
+    for (int i = superblock_count * superblock_size; i < nb; i += 2) {
+        acc0 = dot_q4_0_twoblocks_avx512( acc0, x, y, i );
     }
 
     // Horizontal sum of all lanes of the accumulator
@@ -11303,6 +11458,22 @@ int ggml_cpu_has_avx512(void) {
 #endif
 }
 
+int ggml_cpu_has_avx512_vbmi(void) {
+#if defined(__AVX512VBMI__)
+    return 1;
+#else
+    return 0;
+#endif
+}
+
+int ggml_cpu_has_avx512_vnni(void) {
+#if defined(__AVX512VNNI__)
+    return 1;
+#else
+    return 0;
+#endif
+}
+
 int ggml_cpu_has_fma(void) {
 #if defined(__FMA__)
     return 1;

ggml.h

@@ -808,6 +808,8 @@ size_t ggml_quantize_q4_1(const float * src, void * dst, int n, int k, int64_t *
 int ggml_cpu_has_avx(void);
 int ggml_cpu_has_avx2(void);
 int ggml_cpu_has_avx512(void);
+int ggml_cpu_has_avx512_vbmi(void);
+int ggml_cpu_has_avx512_vnni(void);
 int ggml_cpu_has_fma(void);
 int ggml_cpu_has_neon(void);
 int ggml_cpu_has_arm_fma(void);

llama.cpp

@@ -1914,18 +1914,20 @@ const char * llama_print_system_info(void) {
     static std::string s;
 
     s  = "";
-    s += "AVX = "       + std::to_string(ggml_cpu_has_avx())       + " | ";
-    s += "AVX2 = "      + std::to_string(ggml_cpu_has_avx2())      + " | ";
-    s += "AVX512 = "    + std::to_string(ggml_cpu_has_avx512())    + " | ";
-    s += "FMA = "       + std::to_string(ggml_cpu_has_fma())       + " | ";
-    s += "NEON = "      + std::to_string(ggml_cpu_has_neon())      + " | ";
-    s += "ARM_FMA = "   + std::to_string(ggml_cpu_has_arm_fma())   + " | ";
-    s += "F16C = "      + std::to_string(ggml_cpu_has_f16c())      + " | ";
-    s += "FP16_VA = "   + std::to_string(ggml_cpu_has_fp16_va())   + " | ";
-    s += "WASM_SIMD = " + std::to_string(ggml_cpu_has_wasm_simd()) + " | ";
-    s += "BLAS = "      + std::to_string(ggml_cpu_has_blas())      + " | ";
-    s += "SSE3 = "      + std::to_string(ggml_cpu_has_sse3())      + " | ";
-    s += "VSX = "       + std::to_string(ggml_cpu_has_vsx())       + " | ";
+    s += "AVX = "         + std::to_string(ggml_cpu_has_avx())         + " | ";
+    s += "AVX2 = "        + std::to_string(ggml_cpu_has_avx2())        + " | ";
+    s += "AVX512 = "      + std::to_string(ggml_cpu_has_avx512())      + " | ";
+    s += "AVX512_VBMI = " + std::to_string(ggml_cpu_has_avx512_vbmi()) + " | ";
+    s += "AVX512_VNNI = " + std::to_string(ggml_cpu_has_avx512_vnni()) + " | ";
+    s += "FMA = "         + std::to_string(ggml_cpu_has_fma())         + " | ";
+    s += "NEON = "        + std::to_string(ggml_cpu_has_neon())        + " | ";
+    s += "ARM_FMA = "     + std::to_string(ggml_cpu_has_arm_fma())     + " | ";
+    s += "F16C = "        + std::to_string(ggml_cpu_has_f16c())        + " | ";
+    s += "FP16_VA = "     + std::to_string(ggml_cpu_has_fp16_va())     + " | ";
+    s += "WASM_SIMD = "   + std::to_string(ggml_cpu_has_wasm_simd())   + " | ";
+    s += "BLAS = "        + std::to_string(ggml_cpu_has_blas())        + " | ";
+    s += "SSE3 = "        + std::to_string(ggml_cpu_has_sse3())        + " | ";
+    s += "VSX = "         + std::to_string(ggml_cpu_has_vsx())         + " | ";
 
     return s.c_str();
 }