[PULL 09/17] target/ppc: Zero second doubleword in DFP instructions

[Daniel Henrique Barboza]

From: Víctor Colombo <victor.colombo@eldorado.org.br>

Starting at PowerISA v3.1, the second doubleword of the registers
used to store results in DFP instructions are supposed to be zeroed.

>From the ISA, chapter 7.2.1.1 Floating-Point Registers:
"""
Chapter 4. Floating-Point Facility provides 32 64-bit
FPRs. Chapter 5. Decimal Floating-Point also employs
FPRs in decimal floating-point (DFP) operations. When
VSX is implemented, the 32 FPRs are mapped to
doubleword 0 of VSRs 0-31. (...)
All instructions that operate on an FPR are redefined
to operate on doubleword element 0 of the
corresponding VSR. (...)
and the contents of doubleword element 1 of the
VSR corresponding to the target FPR or FPR pair for these
instructions are set to 0.
"""

Before, the result stored at doubleword 1 was said to be undefined.

With that, this patch changes the DFP facility to zero doubleword 1
when using set_dfp64 and set_dfp128. This fixes the behavior for ISA
3.1 while keeping the behavior correct for previous ones.

Signed-off-by: Víctor Colombo <victor.colombo@eldorado.org.br>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20220906125523.38765-4-victor.colombo@eldorado.org.br>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
---
 target/ppc/dfp_helper.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/target/ppc/dfp_helper.c b/target/ppc/dfp_helper.c
index 5ba74b2124..be7aa5357a 100644
--- a/target/ppc/dfp_helper.c
+++ b/target/ppc/dfp_helper.c
@@ -42,13 +42,16 @@ static void get_dfp128(ppc_vsr_t *dst, ppc_fprp_t *dfp)
 
 static void set_dfp64(ppc_fprp_t *dfp, ppc_vsr_t *src)
 {
-    dfp->VsrD(0) = src->VsrD(1);
+    dfp[0].VsrD(0) = src->VsrD(1);
+    dfp[0].VsrD(1) = 0ULL;
 }
 
 static void set_dfp128(ppc_fprp_t *dfp, ppc_vsr_t *src)
 {
     dfp[0].VsrD(0) = src->VsrD(0);
     dfp[1].VsrD(0) = src->VsrD(1);
+    dfp[0].VsrD(1) = 0ULL;
+    dfp[1].VsrD(1) = 0ULL;
 }
 
 static void set_dfp128_to_avr(ppc_avr_t *dst, ppc_vsr_t *src)
-- 
2.37.3