[PATCH] Fix treatment of ARM unallocated hint instructions

[Quentin Colombet]

Hi Mihail,

Maybe I’m missing something but you’ve defined a new isImm0_4 method but you’re never using it.
Regarding the test you’ve added (invalid-hint-arm.txt), it would be good if it uses FileCheck instead of grep.

Thanks.

-Quentin

On Apr 16, 2013, at 6:17 AM, Mihail Popa <Mihail.Popa at arm.com> wrote:

> Hello.
> 
> In the current implementation, the ARM "hint space" is modelled as a "hint" instruction taking an 8 bit immediate operand. This is not entirely conforming with the ARM specification.
> 
> Firstly "hint" is not and never was a proper ARM mnemonic on v7 or previous. Any code using "hint #imm" will not assemble using the ARM proprietary assembler.
> 
> Secondly, within the hint space, the reference manual defines only 5 permitted values for the immediate field:
> 1. nop (imm == 0)
> 2. yield (imm == 1)
> 3. wfe (imm == 2)
> 4. wfi (imm == 3)
> 5. sev (imm == 4)
> 
> Thirdly, for the remaining values (i.e. 5 through 255) the manual states that: "These unallocated hint encodings are reserved and software must not use them."
> 
> The attached patch restricts the permitted values for the "hint" instruction to 0 through 4.
> This has the following effects:
> 
> 1. "hint #imm" with #imm in [0..4] will assemble to one of {nop, yield, wfe, wfi, sev}
> 2. "hint #imm" with #imm > 4 will be rejected for ARM instruction set
> 3. a bitpattern from the hint space with #imm in [0..4] will be decoded to one of {nop, yield, wfe, wfi, sev}
> 4. a bitpattern from the hint space with #imm > 5 will not be decoded (invalid instruction encoding)
> 
> Patch also trims some trailing white space.
> 
> Regards,
> Mihai
> 
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