diff --git a/hecl/extern/libpng/pngerror.c b/hecl/extern/libpng/pngerror.c
index 0781866a8..a4bcf57be 100644
--- a/hecl/extern/libpng/pngerror.c
+++ b/hecl/extern/libpng/pngerror.c
@@ -163,7 +163,7 @@ png_format_number(png_const_charp start, png_charp end, int format,
          case PNG_NUMBER_FORMAT_02u:
             /* Expects at least 2 digits. */
             mincount = 2;
-            /* FALL THROUGH */
+            FALLTHROUGH;
 
          case PNG_NUMBER_FORMAT_u:
             *--end = digits[number % 10];
@@ -173,7 +173,7 @@ png_format_number(png_const_charp start, png_charp end, int format,
          case PNG_NUMBER_FORMAT_02x:
             /* This format expects at least two digits */
             mincount = 2;
-            /* FALL THROUGH */
+            FALLTHROUGH;
 
          case PNG_NUMBER_FORMAT_x:
             *--end = digits[number & 0xf];
diff --git a/hecl/extern/libpng/pngpriv.h b/hecl/extern/libpng/pngpriv.h
index 46684f305..e533aacbc 100644
--- a/hecl/extern/libpng/pngpriv.h
+++ b/hecl/extern/libpng/pngpriv.h
@@ -1909,6 +1909,12 @@ PNG_INTERNAL_FUNCTION(void, png_init_filter_functions_neon,
    (png_structp png_ptr, unsigned int bpp), PNG_EMPTY);
 #endif
 
+#if __has_attribute(fallthrough)
+#define FALLTHROUGH __attribute__((fallthrough))
+#else
+#define FALLTHROUGH
+#endif
+
 /* Maintainer: Put new private prototypes here ^ */
 
 #include "pngdebug.h"
diff --git a/hecl/extern/libpng/pngread.c b/hecl/extern/libpng/pngread.c
index 48aae8488..47d7b1401 100644
--- a/hecl/extern/libpng/pngread.c
+++ b/hecl/extern/libpng/pngread.c
@@ -1872,7 +1872,7 @@ png_create_colormap_entry(png_image_read_control *display,
          {
             case 4:
                entry[afirst ? 0 : 3] = (png_uint_16)alpha;
-               /* FALL THROUGH */
+               FALLTHROUGH;
 
             case 3:
                if (alpha < 65535)
@@ -1894,7 +1894,7 @@ png_create_colormap_entry(png_image_read_control *display,
 
             case 2:
                entry[1 ^ afirst] = (png_uint_16)alpha;
-               /* FALL THROUGH */
+               FALLTHROUGH;
 
             case 1:
                if (alpha < 65535)
@@ -1923,6 +1923,7 @@ png_create_colormap_entry(png_image_read_control *display,
          {
             case 4:
                entry[afirst ? 0 : 3] = (png_byte)alpha;
+               FALLTHROUGH;
             case 3:
                entry[afirst + (2 ^ bgr)] = (png_byte)blue;
                entry[afirst + 1] = (png_byte)green;
@@ -1931,6 +1932,7 @@ png_create_colormap_entry(png_image_read_control *display,
 
             case 2:
                entry[1 ^ afirst] = (png_byte)alpha;
+               FALLTHROUGH;
             case 1:
                entry[afirst] = (png_byte)green;
                break;
@@ -2851,7 +2853,7 @@ png_image_read_colormap(png_voidp argument)
       case P_sRGB:
          /* Change to 8-bit sRGB */
          png_set_alpha_mode_fixed(png_ptr, PNG_ALPHA_PNG, PNG_GAMMA_sRGB);
-         /* FALL THROUGH */
+         FALLTHROUGH;
 
       case P_FILE:
          if (png_ptr->bit_depth > 8)
@@ -3170,7 +3172,7 @@ png_image_read_colormapped(png_voidp argument)
             break;
 
          /* goto bad_output; */
-         /* FALL THROUGH */
+         FALLTHROUGH;
 
       default:
       bad_output:
diff --git a/hecl/extern/libpng/pngrtran.c b/hecl/extern/libpng/pngrtran.c
index f129ef129..cd7c3499f 100644
--- a/hecl/extern/libpng/pngrtran.c
+++ b/hecl/extern/libpng/pngrtran.c
@@ -49,8 +49,8 @@ png_set_crc_action(png_structrp png_ptr, int crit_action, int ancil_action)
       case PNG_CRC_WARN_DISCARD:    /* Not a valid action for critical data */
          png_warning(png_ptr,
             "Can't discard critical data on CRC error");
+         FALLTHROUGH;
       case PNG_CRC_ERROR_QUIT:                                /* Error/quit */
-
       case PNG_CRC_DEFAULT:
       default:
          png_ptr->flags &= ~PNG_FLAG_CRC_CRITICAL_MASK;
diff --git a/hecl/extern/libpng/pngrutil.c b/hecl/extern/libpng/pngrutil.c
index ee584a8c4..df3a77e52 100644
--- a/hecl/extern/libpng/pngrutil.c
+++ b/hecl/extern/libpng/pngrutil.c
@@ -2938,7 +2938,7 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
          case 2:
             png_ptr->user_chunk_cache_max = 1;
             png_chunk_benign_error(png_ptr, "no space in chunk cache");
-            /* FALL THROUGH */
+            FALLTHROUGH;
          case 1:
             /* NOTE: prior to 1.6.0 this case resulted in an unknown critical
              * chunk being skipped, now there will be a hard error below.
@@ -2947,7 +2947,7 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
 
          default: /* not at limit */
             --(png_ptr->user_chunk_cache_max);
-            /* FALL THROUGH */
+            FALLTHROUGH;
          case 0: /* no limit */
 #  endif /* USER_LIMITS */
             /* Here when the limit isn't reached or when limits are compiled
diff --git a/hecl/extern/libpng/pngwrite.c b/hecl/extern/libpng/pngwrite.c
index 0d4ee9f7a..261ad179f 100644
--- a/hecl/extern/libpng/pngwrite.c
+++ b/hecl/extern/libpng/pngwrite.c
@@ -1007,7 +1007,7 @@ png_set_filter(png_structrp png_ptr, int method, int filters)
          case 5:
          case 6:
          case 7: png_app_error(png_ptr, "Unknown row filter for method 0");
-            /* FALL THROUGH */
+            FALLTHROUGH;
 #endif /* WRITE_FILTER */
          case PNG_FILTER_VALUE_NONE:
             png_ptr->do_filter = PNG_FILTER_NONE; break;
@@ -1870,7 +1870,7 @@ png_image_set_PLTE(png_image_write_control *display)
                tRNS[i] = entry[afirst ? 0 : 3];
                if (tRNS[i] < 255)
                   num_trans = i+1;
-               /* FALL THROUGH */
+               FALLTHROUGH;
             case 3:
                palette[i].blue = entry[afirst + (2 ^ bgr)];
                palette[i].green = entry[afirst + 1];
@@ -1881,7 +1881,7 @@ png_image_set_PLTE(png_image_write_control *display)
                tRNS[i] = entry[1 ^ afirst];
                if (tRNS[i] < 255)
                   num_trans = i+1;
-               /* FALL THROUGH */
+               FALLTHROUGH;
             case 1:
                palette[i].blue = palette[i].red = palette[i].green =
                   entry[afirst];