target-mips: add unions to access DSP elements

Instead of playing with bit shifting, add two unions (one for 32-bit
values, one for 64-bit ones) to access all the DSP elements with the
correct type.

This make the code easier to read and less error prone, and allow GCC
to vectorize the code in some cases.
Reviewed-by: NEric Johnson <ericj@mips.com>
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

target-mips/dsp_helper.c

@@ -20,6 +20,28 @@
 #include "cpu.h"
 #include "helper.h"
 
+/* As the byte ordering doesn't matter, i.e. all columns are treated
+   identically, these unions can be used directly.  */
+typedef union {
+    uint8_t  ub[4];
+    int8_t   sb[4];
+    uint16_t uh[2];
+    int16_t  sh[2];
+    uint32_t uw[1];
+    int32_t  sw[1];
+} DSP32Value;
+
+typedef union {
+    uint8_t  ub[8];
+    int8_t   sb[8];
+    uint16_t uh[4];
+    int16_t  sh[4];
+    uint32_t uw[2];
+    int32_t  sw[2];
+    uint64_t ul[1];
+    int64_t  sl[1];
+} DSP64Value;
+
 /*** MIPS DSP internal functions begin ***/
 #define MIPSDSP_ABS(x) (((x) >= 0) ? x : -x)
 #define MIPSDSP_OVERFLOW(a, b, c, d) (!(!((a ^ b ^ -1) & (a ^ c) & d)))