[Qemu-devel] [PATCH v2] target-mips: fix mipsdsp_trunc16_sat16_round

[Petar Jovanovic]

From: Petar Jovanovic <address@hidden>

This change corrects rounding and saturation of Q31 fractional value in
mipsdsp_trunc16_sat16_round(). Overflow detection was incorrect for the
corner case for PRECRQ_RS.PH, and this test case is also part of the change.

Signed-off-by: Petar Jovanovic <address@hidden>
---

 v2:

 - added comments to the code

 target-mips/dsp_helper.c                   |   16 +++++++++++-----
 tests/tcg/mips/mips32-dsp/precrq_rs_ph_w.c |   24 ++++++++++++++++++++----
 2 files changed, 31 insertions(+), 9 deletions(-)

diff --git a/target-mips/dsp_helper.c b/target-mips/dsp_helper.c
index 4116de9..85950b3 100644
--- a/target-mips/dsp_helper.c
+++ b/target-mips/dsp_helper.c
@@ -648,16 +648,22 @@ static inline int32_t mipsdsp_sat16_mul_q15_q15(uint16_t 
a, uint16_t b,
 static inline uint16_t mipsdsp_trunc16_sat16_round(int32_t a,
                                                    CPUMIPSState *env)
 {
-    int64_t temp;
+    uint16_t temp;
 
-    temp = (int32_t)a + 0x00008000;
 
-    if (a > (int)0x7fff8000) {
-        temp = 0x7FFFFFFF;
+    /*
+     * The value 0x00008000 will be added to the input Q31 value, and the code
+     * needs to check if the addition causes an overflow. Since a positive 
value
+     * is added, overflow can happen in one direction only.
+     */
+    if (a > 0x7FFF7FFF) {
+        temp = 0x7FFF;
         set_DSPControl_overflow_flag(1, 22, env);
+    } else {
+        temp = ((a + 0x8000) >> 16) & 0xFFFF;
     }
 
-    return (temp >> 16) & 0xFFFF;
+    return temp;
 }
 
 static inline uint8_t mipsdsp_sat8_reduce_precision(uint16_t a,
diff --git a/tests/tcg/mips/mips32-dsp/precrq_rs_ph_w.c 
b/tests/tcg/mips/mips32-dsp/precrq_rs_ph_w.c
index 3535b37..da6845b 100644
--- a/tests/tcg/mips/mips32-dsp/precrq_rs_ph_w.c
+++ b/tests/tcg/mips/mips32-dsp/precrq_rs_ph_w.c
@@ -12,18 +12,34 @@ int main()
     result = 0x12348765;
 
     __asm
-        ("precrq_rs.ph.w %0, %1, %2\n\t"
+        ("wrdsp $0\n\t"
+         "precrq_rs.ph.w %0, %1, %2\n\t"
          : "=r"(rd)
          : "r"(rs), "r"(rt)
         );
     assert(result == rd);
 
-    rs = 0x7fffC678;
+    rs = 0x7FFFC678;
     rt = 0x865432A0;
-    result = 0x7fff8654;
+    result = 0x7FFF8654;
 
     __asm
-        ("precrq_rs.ph.w %0, %2, %3\n\t"
+        ("wrdsp $0\n\t"
+         "precrq_rs.ph.w %0, %2, %3\n\t"
+         "rddsp %1\n\t"
+         : "=r"(rd), "=r"(dsp)
+         : "r"(rs), "r"(rt)
+        );
+    assert(((dsp >> 22) & 0x01) == 1);
+    assert(result == rd);
+
+    rs = 0xBEEFFEED;
+    rt = 0x7FFF8000;
+    result = 0xBEF07FFF;
+
+    __asm
+        ("wrdsp $0\n\t"
+         "precrq_rs.ph.w %0, %2, %3\n\t"
          "rddsp %1\n\t"
          : "=r"(rd), "=r"(dsp)
          : "r"(rs), "r"(rt)
-- 
1.7.9.5