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

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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.)

Backports 920f04fa3ea789f8f85a52cee5395b8887b56cf7
This commit is contained in:
Peter Maydell 2021-03-01 20:23:57 -05:00 committed by Lioncash
parent 5680bc701b
commit 966246d991
1 changed files with 8 additions and 0 deletions
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8
qemu/target/arm/translate.c
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@ -8171,6 +8171,14 @@ static bool trans_BLX_i(DisasContext *s, arg_BLX_i *a)
TCGContext *tcg_ctx = s->uc->tcg_ctx;
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;