on 2021/8/11 下午6:42, Segher Boessenkool wrote:
> Hi Amol,
>
> On Wed, Aug 11, 2021 at 08:25:24AM +0530, Amol via Gcc-help wrote:
>> When compiling the following lines with ppc64le-linux-gnu-gcc, version 11.2.0,
>> an incorrect instruction is assembled.
>>
>> unsigned long val;
>> asm volatile ("mfpir %0" : "=r"(val));
>> // ppc64le-linux-gnu-gcc -mcpu=power9 a.c
>>
>> The output binary contains
>> mfspr r9, 286
>>
>> instead of
>>
>> mfspr r9, 1023
>>
>> ----
>> It turns out that the assembler doesn't actually recognize the mfpir
>> opcode, but,
>> since gcc forces -many on the assembler, it is coaxed into assembling an
>> instruction that isn't the correct one for the power9 arch. The resulting binary
>> misbehaves. Another instance of this problem at [1].
>>
>> Is there a way gcc can be told to not force -many on the assembler,
>> or, otherwise
>> be made to correctly support mfpir?
>
> There is no "mfpir" instruction on Power9, there is an extended mnemonic
> though, and apparently GAS does not implement it yet. Please file a bug
> in the binutils bugzilla for that. (We probably should implement the
> extended mnemonics for many more mfspr/mtspr btw).
>
It looks that Paul had filed one PR for it:
https://sourceware.org/bugzilla/show_bug.cgi?id=27684
It should work with binutils 2.37 at -mcpu=power10. The fix seems to
guard mfpir under Power10 mask while it's introduced since at least Power6,
it looks like a typo, probably just have a test with 2.37 and reopen it if
it doesn't work with -mcpu=power9. :)
BR,
Kewen
> GCC will stop using -many as soon as that is possible to do, GCC 12 if
> that works out, but that will still not solve your problem: it then will
> give an error on mfpir. If GAS *does* implement the Power9 extended
> mnemonic, it will work with as well as without -many.
>
>
> Segher
>
