Re: [PATCH] ArmPkg/ArmDisassemblerLib: fix check for MSR instruction

[Michael Zimmermann <sigmaepsilon92@gmail.com>]

CC the arm maintainers
On Thu, Jun 7, 2018 at 7:07 AM Michael Zimmermann
<sigmaepsilon92@gmail.com> wrote:
>
> From: M1cha <sigmaepsilon92@gmail.com>
>
> GCC8 reported it with the following warning:
> ArmPkg/Library/ArmDisassemblerLib/ArmDisassembler.c: In function 'DisassembleArmInstruction':
> ArmPkg/Library/ArmDisassemblerLib/ArmDisassembler.c:397:30: error: bitwise
> comparison always evaluates to false [-Werror=tautological-compare]
> if ((OpCode  & 0x0db00000) == 0x03200000) {
>
> This condition trys to be true for both the immediate and the register
> version of the MSR instruction. They get identified inside the if-block
> using the variable I, which contains the value of bit 25.
>
> The problem with the comparison reported by GCC is that the
> bitmask excludes bit 25, while the value requires it to be set to one:
> 0x0db00000: 0000 11011 0 11 00 00 0000 000000000000
> 0x03200000: 0000 00110 0 10 00 00 0000 000000000000
>                    ^
> So the solution is to just don't require that bit to be set, because
> it gets checked later using 'I', which results in the following value:
> 0x01200000: 0000 00010 0 10 00 00 0000 000000000000
>
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Michael Zimmermann <sigmaepsilon92@gmail.com>
> ---
>  ArmPkg/Library/ArmDisassemblerLib/ArmDisassembler.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/ArmPkg/Library/ArmDisassemblerLib/ArmDisassembler.c b/ArmPkg/Library/ArmDisassemblerLib/ArmDisassembler.c
> index 29d9414a78b3..b449a5d3cd83 100644
> --- a/ArmPkg/Library/ArmDisassemblerLib/ArmDisassembler.c
> +++ b/ArmPkg/Library/ArmDisassemblerLib/ArmDisassembler.c
> @@ -394,7 +394,7 @@ DisassembleArmInstruction (
>    }
>
>
> -  if ((OpCode  & 0x0db00000) == 0x03200000) {
> +  if ((OpCode  & 0x0db00000) == 0x01200000) {
>      // A4.1.38 MSR{<cond>} CPSR_<fields>, #<immediate> MSR{<cond>} CPSR_<fields>, <Rm>
>      if (I) {
>        // MSR{<cond>} CPSR_<fields>, #<immediate>
> --
> 2.17.1
>