From d1ee47361c0b67f9bb5956172e1e6aa91d11a3a8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lo=C3=AFc=20Molinari?= <loic.molinari@collabora.com>
Date: Sat, 18 May 2024 18:07:50 +0200
Subject: [PATCH] helpers: Add FALLTHROUGH macro
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Use gcc and clang's 'fallthrough' attribute instead of a comment to
fall through switch statements. This allows to request fall through
inside a block and prevents issues with preprocessed files.

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>
---
 clients/terminal.c                  |  4 ++--
 desktop-shell/shell.c               |  4 ++--
 libweston/backend-wayland/wayland.c |  2 +-
 shared/helpers.h                    |  7 +++++++
 tests/subsurface-test.c             | 18 +++++++++---------
 5 files changed, 21 insertions(+), 14 deletions(-)

diff --git a/clients/terminal.c b/clients/terminal.c
index c0f222dd..871391ac 100644
--- a/clients/terminal.c
+++ b/clients/terminal.c
@@ -1711,7 +1711,7 @@ handle_non_csi_escape(struct terminal *terminal, char code)
 		break;
 	case 'E':    /* NEL - Newline */
 		terminal->column = 0;
-		// fallthrough
+		FALLTHROUGH;
 	case 'D':    /* IND - Linefeed */
 		terminal->row += 1;
 		if (terminal->row > terminal->margin_bottom) {
@@ -1893,7 +1893,7 @@ handle_special_char(struct terminal *terminal, char c)
 		if (terminal->mode & MODE_LF_NEWLINE) {
 			terminal->column = 0;
 		}
-		/* fallthrough */
+		FALLTHROUGH;
 	case '\v':
 	case '\f':
 		terminal->row++;
diff --git a/desktop-shell/shell.c b/desktop-shell/shell.c
index 8ba3d817..430660f6 100644
--- a/desktop-shell/shell.c
+++ b/desktop-shell/shell.c
@@ -398,13 +398,13 @@ get_output_work_area(struct desktop_shell *shell,
 	switch (shell->panel_position) {
 	case WESTON_DESKTOP_SHELL_PANEL_POSITION_TOP:
 		area->y += sh_output->panel_surface->height;
-		/* fallthrough */
+		FALLTHROUGH;
 	case WESTON_DESKTOP_SHELL_PANEL_POSITION_BOTTOM:
 		area->height -= sh_output->panel_surface->height;
 		break;
 	case WESTON_DESKTOP_SHELL_PANEL_POSITION_LEFT:
 		area->x += sh_output->panel_surface->width;
-		/* fallthrough */
+		FALLTHROUGH;
 	case WESTON_DESKTOP_SHELL_PANEL_POSITION_RIGHT:
 		area->width -= sh_output->panel_surface->width;
 		break;
diff --git a/libweston/backend-wayland/wayland.c b/libweston/backend-wayland/wayland.c
index 0f57c9e4..d79bad21 100644
--- a/libweston/backend-wayland/wayland.c
+++ b/libweston/backend-wayland/wayland.c
@@ -2941,8 +2941,8 @@ wayland_backend_create(struct weston_compositor *compositor,
 		if (renderer != WESTON_RENDERER_PIXMAN)
 			break;
 		weston_log_continue("; falling back to Pixman.\n");
+		FALLTHROUGH;
 	}
-		/* fallthrough */
 	case WESTON_RENDERER_PIXMAN:
 		if (weston_compositor_init_renderer(compositor,
 						    WESTON_RENDERER_PIXMAN,
diff --git a/shared/helpers.h b/shared/helpers.h
index dc203d74..10a9de0e 100644
--- a/shared/helpers.h
+++ b/shared/helpers.h
@@ -235,6 +235,13 @@ do {                        \
 #define unreachable(str) assert(!str)
 #endif
 
+#if __has_attribute(fallthrough)
+/* Supported at least by gcc and clang. */
+#define FALLTHROUGH __attribute__((fallthrough))
+#else
+#define FALLTHROUGH do {} while(0)
+#endif
+
 /**
  * Returns number of bits set in 32-bit value x.
  *
diff --git a/tests/subsurface-test.c b/tests/subsurface-test.c
index 34b95082..9f790ffc 100644
--- a/tests/subsurface-test.c
+++ b/tests/subsurface-test.c
@@ -728,31 +728,31 @@ create_subsurface_tree(struct client *client, struct wl_surface **surfs,
 
 	case 11:
 		SUB_LINK(10, 2);
-		/* fallthrough */
+		FALLTHROUGH;
 	case 10:
 		SUB_LINK(9, 2);
-		/* fallthrough */
+		FALLTHROUGH;
 	case 9:
 		SUB_LINK(8, 6);
-		/* fallthrough */
+		FALLTHROUGH;
 	case 8:
 		SUB_LINK(7, 6);
-		/* fallthrough */
+		FALLTHROUGH;
 	case 7:
 		SUB_LINK(6, 2);
-		/* fallthrough */
+		FALLTHROUGH;
 	case 6:
 		SUB_LINK(5, 1);
-		/* fallthrough */
+		FALLTHROUGH;
 	case 5:
 		SUB_LINK(4, 3);
-		/* fallthrough */
+		FALLTHROUGH;
 	case 4:
 		SUB_LINK(3, 1);
-		/* fallthrough */
+		FALLTHROUGH;
 	case 3:
 		SUB_LINK(2, 0);
-		/* fallthrough */
+		FALLTHROUGH;
 	case 2:
 		SUB_LINK(1, 0);