[PATCH] D108679: Restrict ARM IT Blocks on Windows

Mon Aug 30 08:31:50 PDT 2021

kristof.beyls added a comment.

In D108679#2965397 <https://reviews.llvm.org/D108679#2965397>, @dpaoliello wrote:

> In D108679#2964617 <https://reviews.llvm.org/D108679#2964617>, @kristof.beyls wrote:
>
>> The performance deprecation of certain forms of IT blocks has been reverted.
>> The performance deprecation notes are being removed from the Arm ARM.
>> I am expecting that in the near future, code generation for Arm will be adapted to not enable -mrestrict-it by default at all, following what was already done in gcc at https://gcc.gnu.org/legacy-ml/gcc-patches/2019-12/msg00134.html.
>>
>> As such, I'm not sure if this patch is a good thing to implement.
>> Maybe the guidelines in Microsoft's document you point to (https://docs.microsoft.com/en-us/cpp/build/overview-of-arm-abi-conventions?view=msvc-160#it-instructions) should be adapted to take into account the above described changes to the ArmARM?
>
> @kristof.beyls Can you please point me to any official documentation or announcement from ARM about this (I've tried looking myself but couldn't see anything)?
> I'd be happy to abandon this change and get the Microsoft docs updated if these instructions are no longer deprecated.

I have not found a direct public announcement from Arm about this.
The "performance deprecation for certain forms of IT blocks" notes in the Arm Architecture Reference Manual (ArmARM) should have been all removed from the latest version at https://developer.arm.com/documentation/ddi0487/latest
I've found that a few places were missed and raised a defect against the Arm Architecture Reference Manual to get them all removed.
AFAIK, not having these "performance deprecation" notes in the ArmARM may end up being the only documentation about this.

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https://reviews.llvm.org/D108679