target-arm: Add support for 'Standard FPSCR Value' as used by Neon

Add support to the ARM helper routines for a second fp_status value
which should be used for operations which the ARM ARM indicates use
"ARM standard floating-point arithmetic" rather than being controlled
by the rounding/flush/NaN settings in the FPSCR.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

target-arm/cpu.h

@@ -173,7 +173,20 @@ typedef struct CPUARMState {
         /* scratch space when Tn are not sufficient.  */
         uint32_t scratch[8];
 
+        /* fp_status is the "normal" fp status. standard_fp_status retains
+         * values corresponding to the ARM "Standard FPSCR Value", ie
+         * default-NaN, flush-to-zero, round-to-nearest and is used by
+         * any operations (generally Neon) which the architecture defines
+         * as controlled by the standard FPSCR value rather than the FPSCR.
+         *
+         * To avoid having to transfer exception bits around, we simply
+         * say that the FPSCR cumulative exception flags are the logical
+         * OR of the flags in the two fp statuses. This relies on the
+         * only thing which needs to read the exception flags being
+         * an explicit FPSCR read.
+         */
         float_status fp_status;
+        float_status standard_fp_status;
     } vfp;
     uint32_t exclusive_addr;
     uint32_t exclusive_val;

target-arm/helper.c

@@ -237,6 +237,9 @@ void cpu_reset(CPUARMState *env)
     env->vfp.xregs[ARM_VFP_FPEXC] = 0;
     env->cp15.c2_base_mask = 0xffffc000u;
 #endif
+    set_flush_to_zero(1, &env->vfp.standard_fp_status);
+    set_flush_inputs_to_zero(1, &env->vfp.standard_fp_status);
+    set_default_nan_mode(1, &env->vfp.standard_fp_status);
     tlb_flush(env, 1);
 }
 
@@ -2256,6 +2259,7 @@ uint32_t HELPER(vfp_get_fpscr)(CPUState *env)
             | (env->vfp.vec_len << 16)
             | (env->vfp.vec_stride << 20);
     i = get_float_exception_flags(&env->vfp.fp_status);
+    i |= get_float_exception_flags(&env->vfp.standard_fp_status);
     fpscr |= vfp_exceptbits_from_host(i);
     return fpscr;
 }
@@ -2323,6 +2327,7 @@ void HELPER(vfp_set_fpscr)(CPUState *env, uint32_t val)
 
     i = vfp_exceptbits_to_host(val);
     set_float_exception_flags(i, &env->vfp.fp_status);
+    set_float_exception_flags(0, &env->vfp.standard_fp_status);
 }
 
 void vfp_set_fpscr(CPUState *env, uint32_t val)