SDL: Add and use `SDL_FALLTHROUGH` for fallthroughs (3bf79)

From 3bf7994fe27447800102e30888a05bb01396a27c Mon Sep 17 00:00:00 2001
From: Misa <[EMAIL REDACTED]>
Date: Mon, 27 Sep 2021 14:38:12 -0700
Subject: [PATCH] Add and use `SDL_FALLTHROUGH` for fallthroughs

Case fallthrough warnings can be suppressed using the __fallthrough__
compiler attribute. Unfortunately, not all compilers have this
attribute, or even have __has_attribute to check if they have the
__fallthrough__ attribute. [[fallthrough]] is also available in C++17
and the next C2x, but not everyone uses C++17 or C2x.

So define the SDL_FALLTHROUGH macro to deal with those problems - if we
are using C++17 or C2x, it expands to [[fallthrough]]; else if the
compiler has __has_attribute and has the __fallthrough__ attribute, then
it expands to __attribute__((__fallthrough__)); else it expands to an
empty statement, with a /* fallthrough */ comment (it's a do {} while
(0) statement, because users of this macro need to use a semicolon,
because [[fallthrough]] and __attribute__((__fallthrough__)) require a
semicolon).

Clang before Clang 10 and GCC before GCC 7 have problems with using
__attribute__ as a sole statement and warn about a "declaration not
declaring anything", so fall back to using the /* fallthrough */ comment
if we are using those older compiler versions.

Applications using SDL are also free to use this macro (because it is
defined in begin_code.h).

All existing /* fallthrough */ comments have been replaced with this
macro. Some of them were unnecessary because they were the last case in
a switch; using SDL_FALLTHROUGH in those cases would result in a compile
error on compilers that support __fallthrough__, for having a
__attribute__((__fallthrough__)) statement that didn't immediately
precede a case label.
---
 include/SDL_stdinc.h                     | 20 ++++----------------
 include/begin_code.h                     | 21 +++++++++++++++++++++
 src/audio/SDL_wave.c                     |  4 ++--
 src/render/software/SDL_draw.h           |  8 ++++----
 src/stdlib/SDL_iconv.c                   |  4 ++--
 src/stdlib/SDL_string.c                  | 12 ++++++------
 src/video/SDL_blit.h                     | 24 ++++++++++++------------
 src/video/SDL_fillrect.c                 | 12 ++++++------
 src/video/SDL_pixels.c                   |  2 +-
 src/video/directfb/SDL_DirectFB_WM.c     |  2 +-
 src/video/directfb/SDL_DirectFB_events.c |  2 +-
 src/video/windows/SDL_windowsevents.c    |  1 +
 test/controllermap.c                     |  4 ++--
 test/testgamecontroller.c                |  2 +-
 test/testjoystick.c                      |  2 +-
 15 files changed, 65 insertions(+), 55 deletions(-)

diff --git a/include/SDL_stdinc.h b/include/SDL_stdinc.h
index c00374d121..15a6354436 100644
--- a/include/SDL_stdinc.h
+++ b/include/SDL_stdinc.h
@@ -496,25 +496,13 @@ SDL_FORCE_INLINE void SDL_memset4(void *dst, Uint32 val, size_t dwords)
     if (dwords == 0) {
         return;
     }
-
-    /* !!! FIXME: there are better ways to do this, but this is just to clean this up for now. */
-    #ifdef __clang__
-    #pragma clang diagnostic push
-    #pragma clang diagnostic ignored "-Wimplicit-fallthrough"
-    #endif
-
     switch (dwords % 4) {
-        case 0: do {    *_p++ = _val;   /* fallthrough */
-        case 3:         *_p++ = _val;   /* fallthrough */
-        case 2:         *_p++ = _val;   /* fallthrough */
-        case 1:         *_p++ = _val;   /* fallthrough */
+        case 0: do {    *_p++ = _val;   SDL_FALLTHROUGH;
+        case 3:         *_p++ = _val;   SDL_FALLTHROUGH;
+        case 2:         *_p++ = _val;   SDL_FALLTHROUGH;
+        case 1:         *_p++ = _val;
         } while ( --_n );
     }
-
-    #ifdef __clang__
-    #pragma clang diagnostic pop
-    #endif
-
 #endif
 }
 
diff --git a/include/begin_code.h b/include/begin_code.h
index 37bf9750f4..9dbd89dc49 100644
--- a/include/begin_code.h
+++ b/include/begin_code.h
@@ -164,3 +164,24 @@
 #endif
 #endif /* NULL */
 #endif /* ! Mac OS X - breaks precompiled headers */
+
+#ifndef SDL_FALLTHROUGH
+#if (defined(__cplusplus) && __cplusplus >= 201703L) || \
+    (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 202000L)
+#define SDL_FALLTHROUGH [[fallthrough]]
+#else
+#if defined(__has_attribute)
+#define _HAS_FALLTHROUGH __has_attribute(__fallthrough__)
+#else
+#define _HAS_FALLTHROUGH 0
+#endif /* __has_attribute */
+#if _HAS_FALLTHROUGH && \
+    ((defined(__GNUC__) && __GNUC__ >= 7) || \
+    (defined(__clang_major__) && __clang_major__ >= 10))
+#define SDL_FALLTHROUGH __attribute__((__fallthrough__))
+#else
+#define SDL_FALLTHROUGH do {} while (0) /* fallthrough */
+#endif /* _HAS_FALLTHROUGH */
+#undef _HAS_FALLTHROUGH
+#endif /* C++17 or C2x */
+#endif /* SDL_FALLTHROUGH not defined */
diff --git a/src/audio/SDL_wave.c b/src/audio/SDL_wave.c
index 88204fdbe5..bf9fb542bf 100644
--- a/src/audio/SDL_wave.c
+++ b/src/audio/SDL_wave.c
@@ -1715,7 +1715,7 @@ WaveCheckFormat(WaveFile *file, size_t datalength)
         if (file->facthint == FactStrict && file->fact.status <= 0) {
             return SDL_SetError("Missing fact chunk in WAVE file");
         }
-        /* fallthrough */
+        SDL_FALLTHROUGH;
     case PCM_CODE:
         /* All supported formats require a non-zero bit depth. */
         if (file->chunk.size < 16) {
@@ -1854,7 +1854,7 @@ WaveLoad(SDL_RWops *src, WaveFile *file, SDL_AudioSpec *spec, Uint8 **audio_buf,
             RIFFend = RIFFchunk.position + SDL_MAX_UINT32;
             break;
         }
-        /* fallthrough */
+        SDL_FALLTHROUGH;
     case RiffSizeForce:
         RIFFend = RIFFchunk.position + RIFFchunk.length;
         RIFFlengthknown = SDL_TRUE;
diff --git a/src/render/software/SDL_draw.h b/src/render/software/SDL_draw.h
index d9abb940bf..5c63954f60 100644
--- a/src/render/software/SDL_draw.h
+++ b/src/render/software/SDL_draw.h
@@ -618,10 +618,10 @@ do { \
     while (height--) { \
         { int n = (width+3)/4; \
             switch (width & 3) { \
-            case 0: do {   op; pixel++; /* fallthrough */ \
-            case 3:        op; pixel++; /* fallthrough */ \
-            case 2:        op; pixel++; /* fallthrough */ \
-            case 1:        op; pixel++; /* fallthrough */ \
+            case 0: do {   op; pixel++; SDL_FALLTHROUGH; \
+            case 3:        op; pixel++; SDL_FALLTHROUGH; \
+            case 2:        op; pixel++; SDL_FALLTHROUGH; \
+            case 1:        op; pixel++; \
                     } while ( --n > 0 ); \
             } \
         } \
diff --git a/src/stdlib/SDL_iconv.c b/src/stdlib/SDL_iconv.c
index e235541f31..34246bc4f9 100644
--- a/src/stdlib/SDL_iconv.c
+++ b/src/stdlib/SDL_iconv.c
@@ -758,7 +758,7 @@ SDL_iconv(SDL_iconv_t cd,
             if (ch > 0x10FFFF) {
                 ch = UNKNOWN_UNICODE;
             }
-            /* fallthrough */
+            SDL_FALLTHROUGH;
         case ENCODING_UCS4BE:
             if (ch > 0x7FFFFFFF) {
                 ch = UNKNOWN_UNICODE;
@@ -780,7 +780,7 @@ SDL_iconv(SDL_iconv_t cd,
             if (ch > 0x10FFFF) {
                 ch = UNKNOWN_UNICODE;
             }
-            /* fallthrough */
+            SDL_FALLTHROUGH;
         case ENCODING_UCS4LE:
             if (ch > 0x7FFFFFFF) {
                 ch = UNKNOWN_UNICODE;
diff --git a/src/stdlib/SDL_string.c b/src/stdlib/SDL_string.c
index 6922a24a9f..13d6c542ef 100644
--- a/src/stdlib/SDL_string.c
+++ b/src/stdlib/SDL_string.c
@@ -1275,7 +1275,7 @@ SDL_vsscanf(const char *text, const char *fmt, va_list ap)
                             }
                         }
                     }
-                    /* Fall through to %d handling */
+                    SDL_FALLTHROUGH;
                 case 'd':
                     if (inttype == DO_LONGLONG) {
                         Sint64 value;
@@ -1323,13 +1323,13 @@ SDL_vsscanf(const char *text, const char *fmt, va_list ap)
                     if (radix == 10) {
                         radix = 8;
                     }
-                    /* Fall through to unsigned handling */
+                    SDL_FALLTHROUGH;
                 case 'x':
                 case 'X':
                     if (radix == 10) {
                         radix = 16;
                     }
-                    /* Fall through to unsigned handling */
+                    SDL_FALLTHROUGH;
                 case 'u':
                     if (inttype == DO_LONGLONG) {
                         Uint64 value = 0;
@@ -1815,7 +1815,7 @@ SDL_vsnprintf(SDL_OUT_Z_CAP(maxlen) char *text, size_t maxlen, const char *fmt,
                 case 'p':
                 case 'x':
                     info.force_case = SDL_CASE_LOWER;
-                    /* Fall through to 'X' handling */
+                    SDL_FALLTHROUGH;
                 case 'X':
                     if (info.force_case == SDL_CASE_NOCHANGE) {
                         info.force_case = SDL_CASE_UPPER;
@@ -1826,12 +1826,12 @@ SDL_vsnprintf(SDL_OUT_Z_CAP(maxlen) char *text, size_t maxlen, const char *fmt,
                     if (*fmt == 'p') {
                         inttype = DO_LONG;
                     }
-                    /* Fall through to unsigned handling */
+                    SDL_FALLTHROUGH;
                 case 'o':
                     if (info.radix == 10) {
                         info.radix = 8;
                     }
-                    /* Fall through to unsigned handling */
+                    SDL_FALLTHROUGH;
                 case 'u':
                     info.force_sign = SDL_FALSE;
                     if (info.precision >= 0) {
diff --git a/src/video/SDL_blit.h b/src/video/SDL_blit.h
index 4b50c7dc38..8b28138e9b 100644
--- a/src/video/SDL_blit.h
+++ b/src/video/SDL_blit.h
@@ -479,14 +479,14 @@ do {                                                                    \
 #define DUFFS_LOOP8(pixel_copy_increment, width)                        \
 { int n = (width+7)/8;                                                  \
     switch (width & 7) {                                                \
-    case 0: do {    pixel_copy_increment; /* fallthrough */             \
-    case 7:     pixel_copy_increment;     /* fallthrough */             \
-    case 6:     pixel_copy_increment;     /* fallthrough */             \
-    case 5:     pixel_copy_increment;     /* fallthrough */             \
-    case 4:     pixel_copy_increment;     /* fallthrough */             \
-    case 3:     pixel_copy_increment;     /* fallthrough */             \
-    case 2:     pixel_copy_increment;     /* fallthrough */             \
-    case 1:     pixel_copy_increment;     /* fallthrough */             \
+    case 0: do {    pixel_copy_increment; SDL_FALLTHROUGH;              \
+    case 7:     pixel_copy_increment;     SDL_FALLTHROUGH;              \
+    case 6:     pixel_copy_increment;     SDL_FALLTHROUGH;              \
+    case 5:     pixel_copy_increment;     SDL_FALLTHROUGH;              \
+    case 4:     pixel_copy_increment;     SDL_FALLTHROUGH;              \
+    case 3:     pixel_copy_increment;     SDL_FALLTHROUGH;              \
+    case 2:     pixel_copy_increment;     SDL_FALLTHROUGH;              \
+    case 1:     pixel_copy_increment;                                   \
         } while ( --n > 0 );                                            \
     }                                                                   \
 }
@@ -495,10 +495,10 @@ do {                                                                    \
 #define DUFFS_LOOP4(pixel_copy_increment, width)                        \
 { int n = (width+3)/4;                                                  \
     switch (width & 3) {                                                \
-    case 0: do {    pixel_copy_increment;   /* fallthrough */           \
-    case 3:     pixel_copy_increment;       /* fallthrough */           \
-    case 2:     pixel_copy_increment;       /* fallthrough */           \
-    case 1:     pixel_copy_increment;       /* fallthrough */           \
+    case 0: do {    pixel_copy_increment;   SDL_FALLTHROUGH;            \
+    case 3:     pixel_copy_increment;       SDL_FALLTHROUGH;            \
+    case 2:     pixel_copy_increment;       SDL_FALLTHROUGH;            \
+    case 1:     pixel_copy_increment;                                   \
         } while (--n > 0);                                              \
     }                                                                   \
 }
diff --git a/src/video/SDL_fillrect.c b/src/video/SDL_fillrect.c
index 6385e3ef8e..59f6761e05 100644
--- a/src/video/SDL_fillrect.c
+++ b/src/video/SDL_fillrect.c
@@ -145,13 +145,13 @@ SDL_FillRect1(Uint8 * pixels, int pitch, Uint32 color, int w, int h)
             switch ((uintptr_t) p & 3) {
             case 1:
                 *p++ = (Uint8) color;
-                --n;                    /* fallthrough */
+                --n;                    SDL_FALLTHROUGH;
             case 2:
                 *p++ = (Uint8) color;
-                --n;                    /* fallthrough */
+                --n;                    SDL_FALLTHROUGH;
             case 3:
                 *p++ = (Uint8) color;
-                --n;                    /* fallthrough */
+                --n;
             }
             SDL_memset4(p, color, (n >> 2));
         }
@@ -159,11 +159,11 @@ SDL_FillRect1(Uint8 * pixels, int pitch, Uint32 color, int w, int h)
             p += (n & ~3);
             switch (n & 3) {
             case 3:
-                *p++ = (Uint8) color;   /* fallthrough */
+                *p++ = (Uint8) color;   SDL_FALLTHROUGH;
             case 2:
-                *p++ = (Uint8) color;   /* fallthrough */
+                *p++ = (Uint8) color;   SDL_FALLTHROUGH;
             case 1:
-                *p++ = (Uint8) color;   /* fallthrough */
+                *p++ = (Uint8) color;
             }
         }
         pixels += pitch;
diff --git a/src/video/SDL_pixels.c b/src/video/SDL_pixels.c
index 8015e02134..1402167203 100644
--- a/src/video/SDL_pixels.c
+++ b/src/video/SDL_pixels.c
@@ -333,7 +333,7 @@ SDL_MasksToPixelFormatEnum(int bpp, Uint32 Rmask, Uint32 Gmask, Uint32 Bmask,
         if (Rmask == 0) {
             return SDL_PIXELFORMAT_RGB555;
         }
-    /* fallthrough */
+        SDL_FALLTHROUGH;
     case 16:
         if (Rmask == 0) {
             return SDL_PIXELFORMAT_RGB565;
diff --git a/src/video/directfb/SDL_DirectFB_WM.c b/src/video/directfb/SDL_DirectFB_WM.c
index 8dd48a38a0..a90f98d662 100644
--- a/src/video/directfb/SDL_DirectFB_WM.c
+++ b/src/video/directfb/SDL_DirectFB_WM.c
@@ -323,7 +323,7 @@ DirectFB_WM_ProcessEvent(_THIS, SDL_Window * window, DFBWindowEvent * evt)
                 }
                 if (window->flags & SDL_WINDOW_MAXIMIZED)
                     return 1;
-                /* fall through */
+                SDL_FALLTHROUGH;
             default:
                 windata->wm_grab = pos;
                 if (grabbed_window != NULL)
diff --git a/src/video/directfb/SDL_DirectFB_events.c b/src/video/directfb/SDL_DirectFB_events.c
index 146bb99ea9..c8c3368a81 100644
--- a/src/video/directfb/SDL_DirectFB_events.c
+++ b/src/video/directfb/SDL_DirectFB_events.c
@@ -261,7 +261,7 @@ ProcessWindowEvent(_THIS, SDL_Window *sdlwin, DFBWindowEvent * evt)
                 SDL_SendWindowEvent(sdlwin, SDL_WINDOWEVENT_MOVED,
                                     evt->x, evt->y);
             }
-            /* fall throught */
+            SDL_FALLTHROUGH;
         case DWET_SIZE:
             /* FIXME: what about < 0 */
             evt->w -= (windata->theme.right_size + windata->theme.left_size);
diff --git a/src/video/windows/SDL_windowsevents.c b/src/video/windows/SDL_windowsevents.c
index d5ffaecd97..2c74265777 100644
--- a/src/video/windows/SDL_windowsevents.c
+++ b/src/video/windows/SDL_windowsevents.c
@@ -729,6 +729,7 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
             }
         }
         /* don't break here, fall through to check the wParam like the button presses */
+        SDL_FALLTHROUGH;
     case WM_LBUTTONUP:
     case WM_RBUTTONUP:
     case WM_MBUTTONUP:
diff --git a/test/controllermap.c b/test/controllermap.c
index 144c60344a..a70764ab97 100644
--- a/test/controllermap.c
+++ b/test/controllermap.c
@@ -562,7 +562,7 @@ WatchJoystick(SDL_Joystick * joystick)
                 if ((event.key.keysym.sym != SDLK_ESCAPE)) {
                     break;
                 }
-                /* Fall through to signal quit */
+                SDL_FALLTHROUGH;
             case SDL_QUIT:
                 done = SDL_TRUE;
                 break;
@@ -755,7 +755,7 @@ main(int argc, char *argv[])
                 if ((event.key.keysym.sym != SDLK_ESCAPE)) {
                     break;
                 }
-                /* Fall through to signal quit */
+                SDL_FALLTHROUGH;
             case SDL_QUIT:
                 done = SDL_TRUE;
                 break;
diff --git a/test/testgamecontroller.c b/test/testgamecontroller.c
index 9cfc61e183..acd6382f7c 100644
--- a/test/testgamecontroller.c
+++ b/test/testgamecontroller.c
@@ -394,7 +394,7 @@ loop(void *arg)
             if (event.key.keysym.sym != SDLK_ESCAPE) {
                 break;
             }
-            /* Fall through to signal quit */
+            SDL_FALLTHROUGH;
         case SDL_QUIT:
             done = SDL_TRUE;
             break;
diff --git a/test/testjoystick.c b/test/testjoystick.c
index 658e111465..fc8389ff5a 100644
--- a/test/testjoystick.c
+++ b/test/testjoystick.c
@@ -186,7 +186,7 @@ loop(void *arg)
                 (event.key.keysym.sym != SDLK_AC_BACK)) {
                 break;
             }
-            /* Fall through to signal quit */
+            SDL_FALLTHROUGH;
         case SDL_FINGERDOWN:
         case SDL_MOUSEBUTTONDOWN:
         case SDL_QUIT: