target/tricore: Clarify special case for FTOUZ insn

this is not something other ISAs do, so clarify it with a comment.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Message-ID: <20230828112651.522058-6-kbastian@mail.uni-paderborn.de>

target/tricore/fpu_helper.c

@@ -475,6 +475,11 @@ uint32_t helper_ftouz(CPUTriCoreState *env, uint32_t arg)
         if (float32_is_any_nan(f_arg)) {
             result = 0;
         }
+    /*
+     * we need to check arg < 0.0 before rounding as TriCore needs to raise
+     * float_flag_invalid as well. For instance, when we have a negative
+     * exponent and sign, softfloat would only raise float_flat_inexact.
+     */
     } else if (float32_lt_quiet(f_arg, 0, &env->fp_status)) {
         flags = float_flag_invalid;
         result = 0;