MIPS: Use SLL by 0 for 32-bit truncation in `__read_64bit_c0_split'

Optimize `__read_64bit_c0_split' and reduce the instruction count by 1,
observing that a DSLL/DSRA pair by 32, is equivalent to SLL by 0, which
architecturally truncates the value requested to 32 bits on 64-bit MIPS
hardware regardless of whether the input operand is or is not a properly
sign-extended 32-bit value.

Signed-off-by: Maciej W. Rozycki <macro@imgtec.com>
Reviewed-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17399/
Signed-off-by: James Hogan <jhogan@kernel.org>

diff --git a/arch/mips/include/asm/mipsregs.h b/arch/mips/include/asm/mipsregs.h
index a681092..6b1f1ad 100644 (file)
--- a/arch/mips/include/asm/mipsregs.h
+++ b/arch/mips/include/asm/mipsregs.h
@@ -1355,19 +1355,17 @@ do {                                                                    \
        if (sel == 0)                                                   \
                __asm__ __volatile__(                                   \
                        ".set\tmips64\n\t"                              \
-                       "dmfc0\t%M0, " #source "\n\t"                   \
-                       "dsll\t%L0, %M0, 32\n\t"                        \
-                       "dsra\t%M0, %M0, 32\n\t"                        \
-                       "dsra\t%L0, %L0, 32\n\t"                        \
+                       "dmfc0\t%L0, " #source "\n\t"                   \
+                       "dsra\t%M0, %L0, 32\n\t"                        \
+                       "sll\t%L0, %L0, 0\n\t"                          \
                        ".set\tmips0"                                   \
                        : "=r" (__val));                                \
        else                                                            \
                __asm__ __volatile__(                                   \
                        ".set\tmips64\n\t"                              \
-                       "dmfc0\t%M0, " #source ", " #sel "\n\t"         \
-                       "dsll\t%L0, %M0, 32\n\t"                        \
-                       "dsra\t%M0, %M0, 32\n\t"                        \
-                       "dsra\t%L0, %L0, 32\n\t"                        \
+                       "dmfc0\t%L0, " #source ", " #sel "\n\t"         \
+                       "dsra\t%M0, %L0, 32\n\t"                        \
+                       "sll\t%L0, %L0, 0\n\t"                          \
                        ".set\tmips0"                                   \
                        : "=r" (__val));                                \
        local_irq_restore(__flags);                                     \