riscv: Remove unused argument

Signed-off-by: Cosmin Truta <ctruta@gmail.com>

CMakeLists.txt

@@ -322,9 +322,9 @@ if(PNG_HARDWARE_OPTIMIZATIONS)
   # Set definitions and sources for RISC-V.
   if(PNG_TARGET_ARCHITECTURE MATCHES "^(riscv)")
     include(CheckCCompilerFlag)
-    set(PNG_RISCV_RVV_POSSIBLE_VALUES check on off)
+    set(PNG_RISCV_RVV_POSSIBLE_VALUES on off)
     set(PNG_RISCV_RVV "off"
-      CACHE STRING "Enable RISC-V Vector optimizations: check|on|off; off is default")
+      CACHE STRING "Enable RISC-V Vector optimizations: on|off; off is default")
     set_property(CACHE PNG_RISCV_RVV
                 PROPERTY STRINGS ${PNG_RISCV_RVV_POSSIBLE_VALUES})
     list(FIND PNG_RISCV_RVV_POSSIBLE_VALUES ${PNG_RISCV_RVV} index)
@@ -332,9 +332,6 @@ if(PNG_HARDWARE_OPTIMIZATIONS)
       message(FATAL_ERROR "PNG_RISCV_RVV must be one of [${PNG_RISCV_RVV_POSSIBLE_VALUES}]")
     elseif(NOT PNG_RISCV_RVV STREQUAL "off")
 
-      set(_SAVED_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-      set(CMAKE_REQUIRED_FLAGS "-march=rv64gv1p0")
-
       check_c_source_compiles("
         #include <riscv_vector.h>
         #include <asm/hwcap.h>
@@ -349,8 +346,6 @@ if(PNG_HARDWARE_OPTIMIZATIONS)
           return (int)__riscv_vfmv_f_s_f32m1_f32(val);
       }" COMPILER_SUPPORTS_RVV)
 
-      set(CMAKE_REQUIRED_FLAGS ${_SAVED_CMAKE_REQUIRED_FLAGS})
-
       if(NOT COMPILER_SUPPORTS_RVV)
         message(FATAL_ERROR "Compiler does not support RISC-V Vector extension or its unable to detect it")
       endif()
@@ -359,9 +354,8 @@ if(PNG_HARDWARE_OPTIMIZATIONS)
           riscv/riscv_init.c)
       if(PNG_RISCV_RVV STREQUAL "on")
         add_definitions(-DPNG_RISCV_RVV_OPT=2)
-      elseif(PNG_RISCV_RVV STREQUAL "check")
+      else()
-        add_definitions(-DPNG_RISCV_RVV_CHECK_SUPPORTED)
+        add_definitions(-DPNG_RISCV_RVV_OPT=0)
-        add_definitions(-DPNG_RISCV_RVV_OPT=1)
       endif()
     else()
       add_definitions(-DPNG_RISCV_RVV_OPT=0)

configure.ac

@@ -679,8 +679,7 @@ AS_HELP_STRING([[[--enable-riscv-rvv]]],
       [Enable RISC-V Vector optimizations: =no/off, check, api, yes/on:]
       [no/off: disable the optimizations; check: use internal checking code]
       [api: disable by default, enable by a call to png_set_option]
-      [yes/on: turn on unconditionally.]
+      [yes/on: turn on. If not specified: determined by the compiler.]),
-      [If not specified: determined by the compiler.]),
    [case "$enableval" in
       no|off)
          # disable the default enabling on __riscv systems:
@@ -688,20 +687,11 @@ AS_HELP_STRING([[[--enable-riscv-rvv]]],
                    [Disable RISC-V Vector optimizations])
          # Prevent inclusion of the platform-specific files below:
          enable_riscv_rvv=no ;;
-      check)
-         AC_DEFINE([PNG_RISCV_RVV_CHECK_SUPPORTED], [],
-                   [Check for RISC-V Vector support at run-time])
-         AC_MSG_WARN([--enable-riscv-rvv Please check contrib/riscv-rvv/README file]
-            [for the list of supported OSes.]);;
-      api)
-         AC_DEFINE([PNG_RISCV_RVV_API_SUPPORTED], [],
-                   [Turn on RISC-V Vector optimizations at run-time]);;
       yes|on)
          AC_DEFINE([PNG_RISCV_RVV_OPT], [2],
                    [Enable RISC-V Vector optimizations])
-         AC_MSG_WARN([--enable-riscv-rvv: please specify 'check' or 'api', if]
+         AC_MSG_WARN([--enable-riscv-rvv:]
-            [you want the optimizations unconditionally pass e.g. '-march=rv64gv1p0']
+            [if you want the optimizations pass e.g. '-march=rv64gv1p0' to the compiler.]);;
-            [to the compiler.]);;
       *)
          AC_MSG_ERROR([--enable-riscv-rvv=${enable_riscv_rvv}: invalid value])
    esac])
@@ -715,8 +705,6 @@ then
    compiler_support_riscv_rvv=no
    AC_MSG_CHECKING(whether to use RISC-V RVV intrinsics)
 
-   save_CFLAGS=$CFLAGS
-   CFLAGS="$CFLAGS -march=rv64gv1p0"
    AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
 #include <riscv_vector.h>
 #include <asm/hwcap.h>
@@ -737,7 +725,6 @@ int main() {
    else
       AC_MSG_WARN([Compiler does not support riscv rvv.])
    fi
-   CFLAGS=$save_CFLAGS
 fi
 
 # Add RISC-V-specific files to all builds where $host_cpu is riscv ('riscv64')

contrib/riscv-rvv/linux.c

@@ -25,7 +25,7 @@
 #endif
 
 static int
-png_have_rvv(png_structp png_ptr) {
+png_have_rvv() {
 #if defined(__linux__)
    return getauxval (AT_HWCAP) & COMPAT_HWCAP_ISA_V ? 1 : 0;
 #else

riscv/filter_rvv_intrinsics.c

@@ -51,21 +51,22 @@ png_read_filter_row_sub_rvv(size_t len, size_t bpp, unsigned char* row)
     * x .. [v8](e8)
     */
 
-   asm volatile ("vsetvli      zero, %0, e8, m1" : : "r" (bpp));
+   size_t vl = __riscv_vsetvl_e8m1(bpp);
 
    /* a = *row */
-   asm volatile ("vle8.v       v0, (%0)" : : "r" (row));
+   vuint8m1_t a = __riscv_vle8_v_u8m1(row, vl);
    row += bpp;
 
    while (row < rp_end)
    {
       /* x = *row */
-      asm volatile ("vle8.v       v8, (%0)" : : "r" (row));
+      vuint8m1_t x = __riscv_vle8_v_u8m1(row, vl);
+
       /* a = a + x */
-      asm volatile ("vadd.vv      v0, v0, v8");
+      a = __riscv_vadd_vv_u8m1(a, x, vl);
 
       /* *row = a */
-      asm volatile ("vse8.v       v0, (%0)" : : "r" (row));
+      __riscv_vse8_v_u8m1(row, a, vl);
       row += bpp;
    }
 }
@@ -110,44 +111,46 @@ png_read_filter_row_avg_rvv(size_t len, size_t bpp, unsigned char* row,
 
    /* first pixel */
 
-   asm volatile ("vsetvli      zero, %0, e8, m1" : : "r" (bpp));
+   size_t vl = __riscv_vsetvl_e8m1(bpp);
 
    /* b = *prev_row */
-   asm volatile ("vle8.v       v4, (%0)" : : "r" (prev_row));
+   vuint8m1_t b = __riscv_vle8_v_u8m1(prev_row, vl);
    prev_row += bpp;
 
    /* x = *row */
-   asm volatile ("vle8.v       v8, (%0)" : : "r" (row));
+   vuint8m1_t x = __riscv_vle8_v_u8m1(row, vl);
 
    /* b = b / 2 */
-   asm volatile ("vsrl.vi      v4, v4, 1");
+   b = __riscv_vsrl_vx_u8m1(b, 1, vl);
+
    /* a = x + b */
-   asm volatile ("vadd.vv      v2, v4, v8");
+   vuint8m1_t a = __riscv_vadd_vv_u8m1(b, x, vl);
 
    /* *row = a */
-   asm volatile ("vse8.v       v2, (%0)" : : "r" (row));
+   __riscv_vse8_v_u8m1(row, a, vl);
    row += bpp;
 
    /* remaining pixels */
-
    while (row < rp_end)
    {
       /* b = *prev_row */
-      asm volatile ("vle8.v       v4, (%0)" : : "r" (prev_row));
+      b = __riscv_vle8_v_u8m1(prev_row, vl);
       prev_row += bpp;
 
       /* x = *row */
-      asm volatile ("vle8.v       v8, (%0)" : : "r" (row));
+      x = __riscv_vle8_v_u8m1(row, vl);
 
       /* tmp = a + b */
-      asm volatile ("vwaddu.vv    v12, v2, v4"); /* add with widening */
+      vuint16m2_t tmp = __riscv_vwaddu_vv_u16m2(a, b, vl);
+
       /* a = tmp/2 */
-      asm volatile ("vnsrl.wi     v2, v12, 1");  /* divide/shift with narrowing */
+      a = __riscv_vnsrl_wx_u8m1(tmp, 1, vl);
+
       /* a += x */
-      asm volatile ("vadd.vv      v2, v2, v8");
+      a = __riscv_vadd_vv_u8m1(a, x, vl);
 
       /* *row = a */
-      asm volatile ("vse8.v       v2, (%0)" : : "r" (row));
+      __riscv_vse8_v_u8m1(row, a, vl);
       row += bpp;
    }
 }
@@ -205,18 +208,14 @@ static inline vint16m1_t
 abs_diff(vuint16m1_t a, vuint16m1_t b, size_t vl)
 {
    vint16m1_t diff = __riscv_vreinterpret_v_u16m1_i16m1(__riscv_vsub_vv_u16m1(a, b, vl));
-   vint16m1_t neg = __riscv_vneg_v_i16m1(diff, vl);
+   vbool16_t mask = __riscv_vmslt_vx_i16m1_b16(diff, 0, vl);
-
+   return __riscv_vrsub_vx_i16m1_m(mask, diff, 0, vl);
-   return __riscv_vmax_vv_i16m1(diff, neg, vl);
 }
 
 static inline vint16m1_t
 abs_sum(vint16m1_t a, vint16m1_t b, size_t vl)
 {
-   vint16m1_t sum = __riscv_vadd_vv_i16m1(a, b, vl);
+   return __riscv_vadd_vv_i16m1(a, b, vl);
-   vint16m1_t neg = __riscv_vneg_v_i16m1(sum, vl);
-
-   return __riscv_vmax_vv_i16m1(sum, neg, vl);
 }
 
 static inline void
@@ -226,122 +225,109 @@ png_read_filter_row_paeth_rvv(size_t len, size_t bpp, unsigned char* row,
    png_bytep rp_end = row + len;
 
    /*
-    * row:      | a | x |
+    * row:  | a | x |
-    * prev:     | c | b |
+    * prev: | c | b |
     *
-    * mask ..   [v0]
+    * a .. [v2](e8)
-    * a ..      [v2](e8)
+    * b .. [v4](e8)
-    * b ..      [v4](e8)
+    * c .. [v6](e8)
-    * c ..      [v6](e8)
+    * x .. [v8](e8)
-    * x ..      [v8](e8)
+    * p .. [v12-v13](e16)
-    * p ..      [v12-v13](e16)
+    * pa, pb, pc .. [v16-v17, v20-v21, v24-v25](e16)
-    * pa ..     [v16-v17](e16)
-    * pb ..     [v20-v21](e16)
-    * pc ..     [v24-v25](e16)
-    * tmpmask ..[v31]
     */
 
    /* first pixel */
 
-   asm volatile ("vsetvli      zero, %0, e8, m1" : : "r" (bpp));
+   size_t vl = __riscv_vsetvl_e8m1(bpp);
 
-   /* a = *row + *prev_row */
+   /* a = *row */
-   asm volatile ("vle8.v       v2, (%0)" : : "r" (row));
+   vuint8m1_t a = __riscv_vle8_v_u8m1(row, vl);
-   asm volatile ("vle8.v       v6, (%0)" : : "r" (prev));
+
-   prev += bpp;
+   /* c = *prev */
-   asm volatile ("vadd.vv      v2, v2, v6");
+   vuint8m1_t c = __riscv_vle8_v_u8m1(prev, vl);
+
+   /* a += c */
+   a = __riscv_vadd_vv_u8m1(a, c, vl);
 
    /* *row = a */
-   asm volatile ("vse8.v       v2, (%0)" : : "r" (row));
+   __riscv_vse8_v_u8m1(row, a, vl);
    row += bpp;
+   prev += bpp;
 
    /* remaining pixels */
+
    while (row < rp_end)
    {
-      /* b = *prev_row */
+      /* b = *prev */
-      asm volatile ("vle8.v       v4, (%0)" : : "r" (prev));
+      vuint8m1_t b = __riscv_vle8_v_u8m1(prev, vl);
       prev += bpp;
 
       /* x = *row */
-      asm volatile ("vle8.v       v8, (%0)" : : "r" (row));
+      vuint8m1_t x = __riscv_vle8_v_u8m1(row, vl);
 
-      /* sub (widening to 16bit) */
+      /* Calculate p = b - c and pc = a - c using widening subtraction */
-      /* p = b - c */
+      vuint16m2_t p_wide = __riscv_vwsubu_vv_u16m2(b, c, vl);
-      asm volatile ("vwsubu.vv    v12, v4, v6");
+      vuint16m2_t pc_wide = __riscv_vwsubu_vv_u16m2(a, c, vl);
-      /* pc = a - c */
-      asm volatile ("vwsubu.vv    v24, v2, v6");
 
-      /* switch to widened */
+      /* Convert to signed for easier manipulation */
-      asm volatile ("vsetvli      zero, %0, e16, m2" : : "r" (bpp));
+      size_t vl16 = __riscv_vsetvl_e16m2(bpp);
+      vint16m2_t p = __riscv_vreinterpret_v_u16m2_i16m2(p_wide);
+      vint16m2_t pc = __riscv_vreinterpret_v_u16m2_i16m2(pc_wide);
 
-      /* pa = abs(p) -> pa = p < 0 ? -p : p */
+      /* pa = |p| */
-      asm volatile ("vmv.v.v      v16, v12");             /* pa = p */
+      vbool8_t p_neg_mask = __riscv_vmslt_vx_i16m2_b8(p, 0, vl16);
-      asm volatile ("vmslt.vx     v0, v16, zero");        /* set mask[i] if pa[i] < 0 */
+      vint16m2_t pa = __riscv_vrsub_vx_i16m2_m(p_neg_mask, p, 0, vl16);
-      asm volatile ("vrsub.vx     v16, v16, zero, v0.t"); /* invert negative values in pa; vd[i] = 0 - vs2[i] (if mask[i])
-                                                           * could be replaced by vneg in rvv >= 1.0
-                                                           */
 
-      /* pb = abs(p) -> pb = pc < 0 ? -pc : pc */
+      /* pb = |pc| */
-      asm volatile ("vmv.v.v      v20, v24");             /* pb = pc */
+      vbool8_t pc_neg_mask = __riscv_vmslt_vx_i16m2_b8(pc, 0, vl16);
-      asm volatile ("vmslt.vx     v0, v20, zero");        /* set mask[i] if pc[i] < 0 */
+      vint16m2_t pb = __riscv_vrsub_vx_i16m2_m(pc_neg_mask, pc, 0, vl16);
-      asm volatile ("vrsub.vx     v20, v20, zero, v0.t"); /* invert negative values in pb; vd[i] = 0 - vs2[i] (if mask[i])
-                                                           * could be replaced by vneg in rvv >= 1.0
-                                                           */
 
-      /* pc = abs(p + pc) -> pc = (p + pc) < 0 ? -(p + pc) : p + pc */
+      /* pc = |p + pc| */
-      asm volatile ("vadd.vv      v24, v24, v12");        /* pc = p + pc */
+      vint16m2_t p_plus_pc = __riscv_vadd_vv_i16m2(p, pc, vl16);
-      asm volatile ("vmslt.vx     v0, v24, zero");        /* set mask[i] if pc[i] < 0 */
+      vbool8_t p_plus_pc_neg_mask = __riscv_vmslt_vx_i16m2_b8(p_plus_pc, 0, vl16);
-      asm volatile ("vrsub.vx     v24, v24, zero, v0.t"); /* invert negative values in pc; vd[i] = 0 - vs2[i] (if mask[i])
+      pc = __riscv_vrsub_vx_i16m2_m(p_plus_pc_neg_mask, p_plus_pc, 0, vl16);
-                                                           * could be replaced by vneg in rvv >= 1.0
-                                                           */
 
       /*
-       * if (pb < pa)
+       * The key insight is that we want the minimum of pa, pb, pc.
-       * {
+       * - If pa <= pb and pa <= pc, use a
-       *    pa = pb;
+       * - Else if pb <= pc, use b
-       *    a = b;
+       * - Else use c
-       *    // see (*1)
-       * }
        */
-      asm volatile ("vmslt.vv     v0, v20, v16");         /* set mask[i] if pb[i] < pa[i] */
-      asm volatile ("vmerge.vvm   v16, v16, v20, v0");    /* pa[i] = pb[i] (if mask[i]) */
 
-      /*
+      /* Find which predictor to use based on minimum absolute difference */
-       * if (pc < pa)
+      vbool8_t pa_le_pb = __riscv_vmsle_vv_i16m2_b8(pa, pb, vl16);
-       * {
+      vbool8_t pa_le_pc = __riscv_vmsle_vv_i16m2_b8(pa, pc, vl16);
-       *    a = c;
+      vbool8_t pb_le_pc = __riscv_vmsle_vv_i16m2_b8(pb, pc, vl16);
-       *    // see (*2)
-       * }
-       */
-      asm volatile ("vmslt.vv     v31, v24, v16");        /* set tmpmask[i] if pc[i] < pa[i] */
 
-      /* switch to narrow */
+      /* use_a = pa <= pb && pa <= pc */
-      asm volatile ("vsetvli      zero, %0, e8, m1" : : "r" (bpp));
+      vbool8_t use_a = __riscv_vmand_mm_b8(pa_le_pb, pa_le_pc, vl16);
 
-      /* (*1) */
+      /* use_b = !use_a && pb <= pc */
-      asm volatile ("vmerge.vvm   v2, v2, v4, v0");       /* a = b (if mask[i]) */
+      vbool8_t not_use_a = __riscv_vmnot_m_b8(use_a, vl16);
+      vbool8_t use_b = __riscv_vmand_mm_b8(not_use_a, pb_le_pc, vl16);
 
-      /* (*2) */
+      /* Switch back to e8m1 for final operations */
-      asm volatile ("vmand.mm     v0, v31, v31");         /* mask = tmpmask
+      vl = __riscv_vsetvl_e8m1(bpp);
-                                                           * vmand works for rvv 0.7 up to 1.0
-                                                           * could be replaced by vmcpy in 0.7.1/0.8.1
-                                                           * or vmmv.m in 1.0
-                                                           */
-      asm volatile ("vmerge.vvm   v2, v2, v6, v0");       /* a = c (if mask[i]) */
 
-      /* a += x */
+      /* Start with a, then conditionally replace with b or c */
-      asm volatile ("vadd.vv      v2, v2, v8");
+      vuint8m1_t result = a;
+      result = __riscv_vmerge_vvm_u8m1(result, b, use_b, vl);
+
+      /* use_c = !use_a && !use_b */
+      vbool8_t use_c = __riscv_vmnand_mm_b8(__riscv_vmor_mm_b8(use_a, use_b, vl), __riscv_vmor_mm_b8(use_a, use_b, vl), vl);
+      result = __riscv_vmerge_vvm_u8m1(result, c, use_c, vl);
+
+      /* a = result + x */
+      a = __riscv_vadd_vv_u8m1(result, x, vl);
 
       /* *row = a */
-      asm volatile ("vse8.v       v2, (%0)" : : "r" (row));
+      __riscv_vse8_v_u8m1(row, a, vl);
       row += bpp;
 
-      /* prepare next iteration (prev is already in a) */
+      /* c = b for next iteration */
-      /* c = b */
+      c = b;
-      asm volatile ("vmv.v.v      v6, v4");
    }
 }
-
 void
 png_read_filter_row_paeth3_rvv(png_row_infop row_info, png_bytep row,
     png_const_bytep prev_row)
@@ -349,8 +335,6 @@ png_read_filter_row_paeth3_rvv(png_row_infop row_info, png_bytep row,
    size_t len = row_info->rowbytes;
 
    png_read_filter_row_paeth_rvv(len, 3, row, prev_row);
-
-   PNG_UNUSED(prev_row)
 }
 
 void
@@ -360,9 +344,7 @@ png_read_filter_row_paeth4_rvv(png_row_infop row_info, png_bytep row,
    size_t len = row_info->rowbytes;
 
    png_read_filter_row_paeth_rvv(len, 4, row, prev_row);
-
-   PNG_UNUSED(prev_row)
 }
 
-#endif /* PNG_RISCV_RVV_IMPLEMENTATION */
+#endif /* PNG_RISCV_RVV_IMPLEMENTATION == 1 */
-#endif /* READ */
+#endif /* PNG_READ_SUPPORTED */

riscv/riscv_init.c

@@ -16,20 +16,6 @@
 
 #include <riscv_vector.h>
 
-#ifdef PNG_RISCV_RVV_CHECK_SUPPORTED /* Do run-time checks */
-/* WARNING: it is strongly recommended that you do not build libpng with
- * run-time checks for CPU features if at all possible. In the case of the
- * RISC-V Vector instructions there is no processor-specific way of detecting
- * the presence of the required support, therefore run-time detection is
- * extremely OS specific.
- *
- * You may set the macro PNG_RISCV_RVV_FILE to the file name of file containing
- * a fragment of C source code which defines the png_have_rvv function. There
- * are a number of implementations in contrib/riscv-rvv, but the only one that
- * has partial support is contrib/riscv-rvv/linux.c - a generic Linux
- * implementation which reads /proc/cpuinfo.
- */
-
 #include <signal.h>
 
 #ifndef PNG_RISCV_RVV_FILE
@@ -40,11 +26,10 @@
 #  endif
 #endif
 
-static int png_have_rvv(png_structp png_ptr);
+static int png_have_rvv();
 #ifdef PNG_RISCV_RVV_FILE
 #  include PNG_RISCV_RVV_FILE
 #endif
-#endif /* PNG_RISCV_RVV_CHECK_SUPPORTED */
 
 #ifndef PNG_ALIGNED_MEMORY_SUPPORTED
 #  error "ALIGNED_MEMORY is required; set: -DPNG_ALIGNED_MEMORY_SUPPORTED"
@@ -53,59 +38,16 @@ static int png_have_rvv(png_structp png_ptr);
 void
 png_init_filter_functions_rvv(png_structp pp, unsigned int bpp)
 {
-   /* The switch statement is compiled in for RISCV_RVV_API, the call to
-    * png_have_rvv is compiled in for RISCV_RVV_CHECK.  If both are
-    * defined the check is only performed if the API has not set the VECTOR
-    * option on or off explicitly.  In this case the check controls what
-    * happens.
-    */
    png_debug(1, "in png_init_filter_functions_rvv");
-#ifdef PNG_RISCV_RVV_API_SUPPORTED
-   switch ((pp->options >> PNG_RISCV_RVV) & 3)
-   {
-      case PNG_OPTION_UNSET:
-         /* Allow the run-time check to execute if it has been enabled -
-          * thus both API and CHECK can be turned on.  If it isn't supported
-          * this case will fall through to the 'default' below, which just
-          * returns.
-          */
-#endif /* PNG_RISCV_RVV_API_SUPPORTED */
-#ifdef PNG_RISCV_RVV_CHECK_SUPPORTED
-         {
-            static volatile sig_atomic_t no_rvv = -1; /* not checked */
 
-            if (no_rvv < 0)
+   static volatile sig_atomic_t no_rvv = -1; /* not checked */
-               no_rvv = !png_have_rvv(pp);
 
-            if (no_rvv)
+   if (no_rvv < 0)
-               return;
+      no_rvv = !png_have_rvv();
-         }
-#ifdef PNG_RISCV_RVV_API_SUPPORTED
-         break;
-#endif
-#endif /* PNG_RISCV_RVV_CHECK_SUPPORTED */
 
-#ifdef PNG_RISCV_RVV_API_SUPPORTED
+   if (no_rvv)
-      default: /* OFF or INVALID */
+      return;
-         return;
 
-      case PNG_OPTION_ON:
-         /* Option turned on */
-         break;
-   }
-#endif /* PNG_RISCV_RVV_API_SUPPORTED */
-
-   /* IMPORTANT: any new external functions used here must be declared using
-    * PNG_INTERNAL_FUNCTION in ../pngpriv.h.  This is required so that the
-    * 'prefix' option to configure works:
-    *
-    *    ./configure --with-libpng-prefix=foobar_
-    *
-    * Verify you have got this right by running the above command, doing a build
-    * and examining pngprefix.h; it must contain a #define for every external
-    * function you add.  (Notice that this happens automatically for the
-    * initialization function.)
-    */
    pp->read_filter[PNG_FILTER_VALUE_UP-1] = png_read_filter_row_up_rvv;
 
    if (bpp == 3)