target/ppc: Split out VSCR_SAT to a vector field

Change the representation of VSCR_SAT such that it is easy
to set from vector code.
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>
Acked-by: NDavid Gibson <david@gibson.dropbear.id.au>
Message-Id: <20190215100058.20015-16-mark.cave-ayland@ilande.co.uk>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

target/ppc/cpu.h

@@ -1052,10 +1052,12 @@ struct CPUPPCState {
     /* Special purpose registers */
     target_ulong spr[1024];
     ppc_spr_t spr_cb[1024];
-    /* Vector status and control register */
+    /* Vector status and control register, minus VSCR_SAT.  */
     uint32_t vscr;
     /* VSX registers (including FP and AVR) */
     ppc_vsr_t vsr[64] QEMU_ALIGNED(16);
+    /* Non-zero if and only if VSCR_SAT should be set.  */
+    ppc_vsr_t vscr_sat QEMU_ALIGNED(16);
     /* SPE registers */
     uint64_t spe_acc;
     uint32_t spe_fscr;

target/ppc/int_helper.c

@@ -459,18 +459,23 @@ void helper_lvsr(ppc_avr_t *r, target_ulong sh)
 
 void helper_mtvscr(CPUPPCState *env, uint32_t vscr)
 {
-    env->vscr = vscr;
+    env->vscr = vscr & ~(1u << VSCR_SAT);
+    /* Which bit we set is completely arbitrary, but clear the rest.  */
+    env->vscr_sat.u64[0] = vscr & (1u << VSCR_SAT);
+    env->vscr_sat.u64[1] = 0;
     set_flush_to_zero((vscr >> VSCR_NJ) & 1, &env->vec_status);
 }
 
 uint32_t helper_mfvscr(CPUPPCState *env)
 {
-    return env->vscr;
+    uint32_t sat = (env->vscr_sat.u64[0] | env->vscr_sat.u64[1]) != 0;
+    return env->vscr | (sat << VSCR_SAT);
 }
 
 static inline void set_vscr_sat(CPUPPCState *env)
 {
-    env->vscr |= 1 << VSCR_SAT;
+    /* The choice of non-zero value is arbitrary.  */
+    env->vscr_sat.u32[0] = 1;
 }
 
 void helper_vaddcuw(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b)