The software trap instruction HLT that was introduced in Armv8-a is used
as the semihosting trap instruction in AArch64. In order to allow systems
configured to run AArch64 code to also run AArch32 with semihosting it was
decided that AArch32 should also use HLT in the case of the "mixed mode"
environment. This requires that HLT also be backported to all earlier
architectures. The instruction is in the undefined encoding space earlier
architectures but must trigger a semihosting trap [3].
The Arm Architectural Reference Manual [1] doesn't explicitly mention this
however this is an explicit requirement in the Semihosting-v2 protocol [2].
[1] https://developer.arm.com/docs/ddi0487/latest/arm-architecture-reference-manual-armv8-for-armv8-a-architecture-profile
[2] https://developer.arm.com/docs/100863/latest/the-semihosting-interface
[3] 19a6e31c9d
gas/ChangeLog:
* config/tc-arm.c (insns): Redefine THUMB_VARIANT and ARM_VARIANT for
hlt to armv1.
* testsuite/gas/arm/armv8a-automatic-hlt.d: Update TAGs
* testsuite/gas/arm/hlt.d: New test.
* testsuite/gas/arm/hlt.s: New test.
opcodes/ChangeLog:
* arm-dis.c (arm_opcodes): Redefine hlt to armv1.
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