[ARM] Allow MOV/MOV.W to accept all possible immediates

[Jiong Wang]

On 05/01/17 10:49, Szabolcs Nagy wrote:
> adding the reporter to cc and rich because it broke musl.
>
> On 05/01/17 10:45, Szabolcs Nagy wrote:
>> On 01/11/16 12:21, Jiong Wang wrote:
>>> Hi,
>>>
>>>    AArch32 MOV Rd, #imm should accept all immediates which can be encoded into
>>> available encoding, A1/A2 for ARM, T1/T2/T3 for Thumb/Thumb2, and MOV is the
>>> preferred disassembly syntax.  See latest ARM Architecture Reference Manual for
>>> ARMv8-A, section F6.1.108 and for ARMv8-M, section C2.4.89.
>>>
>>>    The same for MOV.W under Thumb mode.  It should try all possible 32-bit Thumb
>>> encoding instead of T2 only.
>>>
>>>    This patch let MOV/MOV.W accept more immediate formats while their currently
>>> supported immediate formats are not affected, so there is no backward compatibility
>>> issue, also this patch haven't touched the disassembler.
>>>
>>>    I think this patch brings GAS closer to ARM Architecture Reference Manual.
>>>
>>>    OK for master?
>>>
>>> gas/
>>> 2016-11-01  Jiong Wang  <jiong.wang@arm.com>
>>>                            * config/tc-arm.c (SBIT_SHIFT): New.
>>>          (T2_SBIT_SHIFT): Likewise.
>>>          (t32_insn_ok): Return TRUE for MOV in ARMv8-M Baseline.
>>>          (md_apply_fix): Try UINT16 encoding when ARM/Thumb modified immediate
>>>          encoding failed.
>>>          * testsuite/gas/arm/archv6t2-bad.s: New error case.
>>>          * testsuite/gas/arm/archv6t2-bad.l: New error match.
>>>          * testsuite/gas/arm/archv6t2.s: New testcase.
>>>          * testsuite/gas/arm/archv6t2.d: New expected result.
>>>          * testsuite/gas/arm/archv8m.s: New testcase
>>>          * testsuite/gas/arm/archv8m-base.d: New expected result.
>>>          * testsuite/gas/arm/archv8m-main.d: Likewise.
>>>          * testsuite/gas/arm/archv8m-main-dsp-1.d: Likewise.
>> this caused
>>
>> $ cat bug.s
>> .syntax unified
>> .text
>> .arch armv7-a
>> 	mov r3,#1
>>
>> .arch armv4t
>> .eabi_attribute 6,2
>> 	bx lr
>>
>> $ arm-none-linux-gnueabihf-as -mthumb bug.s
>> bug.s: Assembler messages:
>> bug.s:4: Error: invalid constant (1) after fixup
>>
>> previously this worked, but i'm not sure if the mix
>> of .arch directives and .syntax unified is valid here.

It looks to me the failure is caused by the second ".arch armv4t" 
overrides the
first ".arch armv7-a", therefore the arch feature is lowerd to armv4t in
assembler fixup stage.  While "mov r3, #1" under thumb mode is only allowed
with ARMv6T2.

This patch tightend the check from

       if (fixP->fx_r_type == BFD_RELOC_ARM_T32_IMMEDIATE
           || fixP->fx_r_type == BFD_RELOC_ARM_T32_ADD_IMM)

into:

       if ((fixP->fx_r_type == BFD_RELOC_ARM_T32_IMMEDIATE
            && ARM_CPU_HAS_FEATURE (cpu_variant, arm_ext_v6t2))
           || fixP->fx_r_type == BFD_RELOC_ARM_T32_ADD_IMM)


thus the sequences in bug.s is not allowed.