The BLX immediate insn in the Thumb encoding always performs
a switch from Thumb to Arm state. This would be totally useless
in M-profile which has no Arm decoder, and so the instruction
does not exist at all there. Make the encoding UNDEF for M-profile.

(This part of the encoding space is used for the branch-future
and low-overhead-loop insns in v8.1M.)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20201019151301.2046-6-peter.maydell@linaro.org
---
 target/arm/translate.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/target/arm/translate.c b/target/arm/translate.c
index 9f2201ca577..dc3a4031db7 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -7918,6 +7918,14 @@ static bool trans_BLX_i(DisasContext *s, arg_BLX_i *a)
 {
     TCGv_i32 tmp;
 
+    /*
+     * BLX <imm> would be useless on M-profile; the encoding space
+     * is used for other insns from v8.1M onward, and UNDEFs before that.
+     */
+    if (arm_dc_feature(s, ARM_FEATURE_M)) {
+        return false;
+    }
+
     /* For A32, ARM_FEATURE_V5 is checked near the start of the uncond block. */
     if (s->thumb && (a->imm & 2)) {
         return false;
-- 
2.20.1