vc_screen: eliminate ifdefs from vcs_write_buf

Introduce a new inline function called vc_compile_le16 and do the shifts
and ORs there. Depending on LE x BE.

I tried cpu_to_le16, but it ends up with worse assembly on BE for
whatever reason -- the compiler seems to be unable to optimize the swap.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200818085706.12163-9-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
index 10a26fd..36b9678 100644 (file)
--- a/drivers/tty/vt/vc_screen.c
+++ b/drivers/tty/vt/vc_screen.c
@@ -481,6 +481,19 @@ static u16 *vcs_write_buf_noattr(struct vc_data *vc, const char *con_buf,
        return org;
 }
 
+/*
+ * Compilers (gcc 10) are unable to optimize the swap in cpu_to_le16. So do it
+ * the poor man way.
+ */
+static inline u16 vc_compile_le16(u8 hi, u8 lo)
+{
+#ifdef __BIG_ENDIAN
+       return (lo << 8u) | hi;
+#else
+       return (hi << 8u) | lo;
+#endif
+}
+
 static u16 *vcs_write_buf(struct vc_data *vc, const char *con_buf,
                unsigned int pos, unsigned int count, bool viewed, u16 **org0)
 {
@@ -513,13 +526,8 @@ static u16 *vcs_write_buf(struct vc_data *vc, const char *con_buf,
        if (pos & 1) {
                count--;
                c = *con_buf++;
-#ifdef __BIG_ENDIAN
-               vcs_scr_writew(vc, c |
-                    (vcs_scr_readw(vc, org) & 0xff00), org);
-#else
-               vcs_scr_writew(vc, (c << 8) |
-                    (vcs_scr_readw(vc, org) & 0xff), org);
-#endif
+               vcs_scr_writew(vc, vc_compile_le16(c, vcs_scr_readw(vc, org)),
+                               org);
                org++;
                pos++;
                if (++col == maxcol) {
@@ -551,11 +559,8 @@ static u16 *vcs_write_buf(struct vc_data *vc, const char *con_buf,
 
        /* odd pos -- the remaining character */
        c = *con_buf++;
-#ifdef __BIG_ENDIAN
-       vcs_scr_writew(vc, (vcs_scr_readw(vc, org) & 0xff) | (c << 8), org);
-#else
-       vcs_scr_writew(vc, (vcs_scr_readw(vc, org) & 0xff00) | c, org);
-#endif
+       vcs_scr_writew(vc, vc_compile_le16(vcs_scr_readw(vc, org) >> 8, c),
+                               org);
 
        return org;
 }