target-mips: fix calculation of overflow for SHLL.PH and SHLL.QB

This change corrects and simplifies how discard is calculated for shift
left logical vector instructions. It is used to detect overflow and set bit
22 in the DSPControl register.

The existing tests (shll_ph.c, shll_qb.c) are extended with the corner cases
that expose incorrectness in the previous implementation.

Signed-off-by: Petar Jovanovic <petar.jovanovic@imgtec.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

1 files changed, 6 insertions, 24 deletions

diff --git a/target-mips/dsp_helper.c b/target-mips/dsp_helper.c
index f975da08c4..805247d252 100644
--- a/target-mips/dsp_helper.c
+++ b/target-mips/dsp_helper.c
@@ -682,49 +682,31 @@ static inline uint8_t mipsdsp_sat8_reduce_precision(uint16_t a,
 
 static inline uint8_t mipsdsp_lshift8(uint8_t a, uint8_t s, CPUMIPSState *env)
 {
-    uint8_t sign;
     uint8_t discard;
 
-    if (s == 0) {
-        return a;
-    } else {
-        sign = (a >> 7) & 0x01;
-        if (sign != 0) {
-            discard = (((0x01 << (8 - s)) - 1) << s) |
-                      ((a >> (6 - (s - 1))) & ((0x01 << s) - 1));
-        } else {
-            discard = a >> (6 - (s - 1));
-        }
+    if (s != 0) {
+        discard = a >> (8 - s);
 
         if (discard != 0x00) {
             set_DSPControl_overflow_flag(1, 22, env);
         }
-        return a << s;
     }
+    return a << s;
 }
 
 static inline uint16_t mipsdsp_lshift16(uint16_t a, uint8_t s,
                                         CPUMIPSState *env)
 {
-    uint8_t  sign;
     uint16_t discard;
 
-    if (s == 0) {
-        return a;
-    } else {
-        sign = (a >> 15) & 0x01;
-        if (sign != 0) {
-            discard = (((0x01 << (16 - s)) - 1) << s) |
-                      ((a >> (14 - (s - 1))) & ((0x01 << s) - 1));
-        } else {
-            discard = a >> (14 - (s - 1));
-        }
+    if (s != 0) {
+        discard = (int16_t)a >> (15 - s);
 
         if ((discard != 0x0000) && (discard != 0xFFFF)) {
             set_DSPControl_overflow_flag(1, 22, env);
         }
-        return a << s;
     }
+    return a << s;
 }