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target/arm: Don't allow BLX imm for M-profile 

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
This commit is contained in:
Peter Maydell 2020-10-19 16:12:56 +01:00
parent 45f11876ae
commit 920f04fa3e
1 changed files with 8 additions and 0 deletions
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8
target/arm/translate.c
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@ -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;