Teach DTrace that unaligned accesses are allowed on aarch64, not just x86.
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Authored by rwatson on Mar 22 2021, 1:51 AM.

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allanjude
andrew
emaste
gnn

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rG6605a2a1634f: Teach DTrace that unaligned accesses are OK on aarch64, not just x86.
rGba08ba5226ff: Teach DTrace that unaligned accesses are OK on aarch64, not just x86.
rGfb581531c1a0: Teach DTrace that unaligned accesses are OK on aarch64, not just x86.

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rG FreeBSD src repository

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rwatson created this revision.Mar 22 2021, 1:51 AM

Herald added a subscriber: imp. · View Herald TranscriptMar 22 2021, 1:51 AM

rwatson requested review of this revision.Mar 22 2021, 1:51 AM

Harbormaster completed remote builds in B37982: Diff 86120.Mar 22 2021, 1:51 AM

rwatson added subscribers: DTrace, ARM.Mar 22 2021, 1:51 AM

I wonder if we should be checking for __NO_STRICT_ALIGNMENT (and defining it for architectures other than x86)

In D29369#657694, @andrew wrote:

I wonder if we should be checking for __NO_STRICT_ALIGNMENT (and defining it for architectures other than x86)

Hmm. I didn't know that existed.

I think it is the right goal, but I don't think we should aim for it in this patch. Not least, I'm not easily set up to test the various other cases covered by that define currently.

andrew accepted this revision.Mar 22 2021, 1:42 PM

This revision is now accepted and ready to land.Mar 22 2021, 1:42 PM

In D29369#657802, @rwatson wrote:

In D29369#657694, @andrew wrote:

I wonder if we should be checking for __NO_STRICT_ALIGNMENT (and defining it for architectures other than x86)

Hmm. I didn't know that existed.

I think it is the right goal, but I don't think we should aim for it in this patch. Not least, I'm not easily set up to test the various other cases covered by that define currently.

Note that the architecture must be truly totally alignment-agnostic for __NO_STRICT_ALIGNMENT to work. If aarch64 still has restrictions on some instructions (such as ldm/stm) the compiler may generate those for accessing network data in unaligned packet headers (I ran into this on arm32, where ipv6 address accesses got optimized into ldm instructions to load all 16 bytes at once).

Closed by commit rGfb581531c1a0: Teach DTrace that unaligned accesses are OK on aarch64, not just x86. (authored by rwatson). · Explain WhyMar 22 2021, 11:58 PM

This revision was automatically updated to reflect the committed changes.

rwatson added a commit: rGfb581531c1a0: Teach DTrace that unaligned accesses are OK on aarch64, not just x86..

Herald added a reviewer: gnn. · View Herald TranscriptMar 22 2021, 11:58 PM

emaste added a commit: rGba08ba5226ff: Teach DTrace that unaligned accesses are OK on aarch64, not just x86..Mar 25 2021, 1:19 PM

emaste added a commit: rG6605a2a1634f: Teach DTrace that unaligned accesses are OK on aarch64, not just x86..Mar 31 2021, 4:07 PM