Remove the deprecation of IT instructions in the ARM ABI

[mstorsjo]

Upstream ARM has undeprecated the use of IT instructions. There's no formal announcement of the matter in itself, but the previous deprecations are being removed from the official docs.

See https://reviews.llvm.org/D108679#2975603 for a discussion on the matter, CC @dpaoliello who was involved in that discussion.

[ghost]

All CLA requirements met.

[PRMerger16]

@mstorsjo : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[ktoliver]

#label:"aq-pr-triaged"

[colin-home]

@mstorsjo and cc: @dpaoliello
Do you know if this un-deprecation (is that a word?) applies to previous versions of our armasm tool, and to all ARM processors? Just wondering if there's a set of processors or old assemblers where the deprecation still holds, especially if certain code won't work on Windows on ARM.

[dpaoliello]

Do you know if this un-deprecation (is that a word?) applies to previous versions of our armasm tool, and to all ARM processors? Just wondering if there's a set of processors or old assemblers where the deprecation still holds, especially if certain code won't work on Windows on ARM.

@corob-msft The un-deprecation is a change by ARM - there have been no changes that I know of to any Microsoft tools or OSes.

From the Windows side, these instructions were always supported, and I don't believe that we ever supported any hardware that didn't support the instructions.

[mstorsjo]

@mstorsjo and cc: @dpaoliello
Do you know if this un-deprecation (is that a word?) applies to previous versions of our armasm tool, and to all ARM processors? Just wondering if there's a set of processors or old assemblers where the deprecation still holds, especially if certain code won't work on Windows on ARM.

When it comes to actual processors, there hasn't been any issue so far (as it would break backwards binary compatbility), but I've understood it that they (ARM) reserved the right to degrade/break it later (and thus put in the deprecation preemptively) - but now they've decided that they won't do that, so there's no longer any reason to avoid those constructs.

Yeah ideally armasm would also stop warning about these constructs - I guess I could file a public bug report about it, but I'm not sure if it's as effective as if someone of you would start it.

From the Windows side, these instructions were always supported, and I don't believe that we ever supported any hardware that didn't support the instructions.

Yeah I don't think there has been any (armv8-a) processors that didn't support those instructions (and the deprecation was added as part of armv8 to begin with).

[colin-home]

@mstorsjo Thanks for the update and the info. Merging shortly. Docs should be updated tomorrow.