target/ppc: implement cbcdtd

Implements the Convert Binary Coded Decimal To Declets instruction.
Since libdecnumber doesn't expose the methods for direct conversion
(decDigitsToDPD, BCD2DPD, etc.), the BCD values are converted to
decimal32 format, from which the declets are extracted.

Where the behavior is undefined, we try to match the result observed in
a POWER9 DD2.3.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Signed-off-by: Víctor Colombo <victor.colombo@eldorado.org.br>
Message-Id: <20220629162904.105060-11-victor.colombo@eldorado.org.br>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>

target/ppc/dfp_helper.c

@@ -1391,3 +1391,42 @@ DFP_HELPER_SHIFT(DSCLI, 64, 1)
 DFP_HELPER_SHIFT(DSCLIQ, 128, 1)
 DFP_HELPER_SHIFT(DSCRI, 64, 0)
 DFP_HELPER_SHIFT(DSCRIQ, 128, 0)
+
+target_ulong helper_CBCDTD(target_ulong s)
+{
+    uint64_t res = 0;
+    uint32_t dec32;
+    uint8_t bcd[6];
+    int w, i, offs;
+    decNumber a;
+    decContext context;
+
+    decContextDefault(&context, DEC_INIT_DECIMAL32);
+
+    for (w = 1; w >= 0; w--) {
+        res <<= 32;
+        decNumberZero(&a);
+        /* Extract each BCD field of word "w" */
+        for (i = 5; i >= 0; i--) {
+            offs = 4 * (5 - i) + 32 * w;
+            bcd[i] = extract64(s, offs, 4);
+            if (bcd[i] > 9) {
+                /*
+                 * If the field value is greater than 9, the results are
+                 * undefined. We could use a fixed value like 0 or 9, but
+                 * an and with 9 seems to better match the hardware behavior.
+                 */
+                bcd[i] &= 9;
+            }
+        }
+
+        /* Create a decNumber with the BCD values and convert to decimal32 */
+        decNumberSetBCD(&a, bcd, 6);
+        decimal32FromNumber((decimal32 *)&dec32, &a, &context);
+
+        /* Extract the two declets from the decimal32 value */
+        res |= dec32 & 0xfffff;
+    }
+
+    return res;
+}

target/ppc/helper.h

@@ -54,6 +54,7 @@ DEF_HELPER_3(sraw, tl, env, tl, tl)
 DEF_HELPER_FLAGS_2(CFUGED, TCG_CALL_NO_RWG_SE, i64, i64, i64)
 DEF_HELPER_FLAGS_2(PDEPD, TCG_CALL_NO_RWG_SE, i64, i64, i64)
 DEF_HELPER_FLAGS_2(PEXTD, TCG_CALL_NO_RWG_SE, i64, i64, i64)
+DEF_HELPER_FLAGS_1(CBCDTD, TCG_CALL_NO_RWG_SE, tl, tl)
 #if defined(TARGET_PPC64)
 DEF_HELPER_FLAGS_2(cmpeqb, TCG_CALL_NO_RWG_SE, i32, tl, tl)
 DEF_HELPER_FLAGS_1(popcntw, TCG_CALL_NO_RWG_SE, tl, tl)

target/ppc/insn32.decode

@@ -87,6 +87,9 @@
 &X_rc           rt ra rb rc:bool
 @X_rc           ...... rt:5 ra:5 rb:5 .......... rc:1           &X_rc
 
+&X_sa           rs ra
+@X_sa           ...... rs:5 ra:5 ..... .......... .             &X_sa
+
 %x_frtp         22:4 !function=times_2
 %x_frap         17:4 !function=times_2
 %x_frbp         12:4 !function=times_2
@@ -314,6 +317,7 @@ PEXTD           011111 ..... ..... ..... 0010111100 -   @X
 ## BCD Assist
 
 ADDG6S          011111 ..... ..... ..... - 001001010 -  @X
+CBCDTD          011111 ..... ..... ----- 0100111010 -   @X_sa
 
 ### Float-Point Load Instructions
 

target/ppc/translate/fixedpoint-impl.c.inc

@@ -529,3 +529,10 @@ static bool trans_ADDG6S(DisasContext *ctx, arg_X *a)
 
     return true;
 }
+
+static bool trans_CBCDTD(DisasContext *ctx, arg_X_sa *a)
+{
+    REQUIRE_INSNS_FLAGS2(ctx, BCDA_ISA206);
+    gen_helper_CBCDTD(cpu_gpr[a->ra], cpu_gpr[a->rs]);
+    return true;
+}