Expose cache line size in __builtin_get_cpu_cache_line_size
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Authored by zoecarver on Mar 20 2020, 2:00 PM.

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jfb
EricWF
efriedma
jyknight
echristo
__simt__

Summary

This patch creates the __builtin_get_cpu_cache_line_size to wrap the cpu cache line size API from D74918.

Diff Detail

Repository: rG LLVM Github Monorepo

Unit TestsFailed
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	Time	Test
	130 ms	Clang.SemaCXX::Unknown Unit Message ("") Script: -- : 'RUN: at line 1'; /mnt/disks/ssd0/agent/workspace/amd64_debian_testing_clang8/build/bin/clang -mcpu=i368 /mnt/disks/ssd0/agent/workspace/amd64_debian_testing_clang8/clang/test/SemaCXX/builtin-cache-line-size.cpp

Event Timeline

zoecarver created this revision.Mar 20 2020, 2:00 PM

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zoecarver added reviewers: jfb, EricWF, efriedma, jyknight, echristo, __simt__.Mar 20 2020, 2:03 PM

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zoecarver mentioned this in D74918: Add method to TargetInfo to get CPU cache line size.Mar 20 2020, 2:05 PM

Harbormaster failed remote builds in B49952: Diff 251753!Mar 20 2020, 2:40 PM

What should the behavior of the builtin be if the calling function has its own target cpu like attribute((target("arch=sandybridge"))) and that CPU has a different size than the CPU that was passed on the command line?

clang/include/clang/Basic/Builtins.def
1483	Can this be "zv" and drop the "tu" from the second string?

@craig.topper thanks for the comments! I'm going to hold off on updating this until we figure out what's happening with D74918, though.

I don't think there's a particularly good reason to expose this as a builtin, unless it's to be used by the standard library implementation. (Which again I do not think we should implement.)

@jyknight yes, the current plan is to use it in libc++'s implementation. I can put that implementation (which uses this builtin) up for review before this lands. That way we can discuss the implementation before adding a (possibly unneeded) builtin. Sound good?

Fix based on review comments

@craig.topper I'm not sure what should happen. It should probably just use that CPU (although that's not a great solution). Is there a way to detect if an attribute was used to change the target in which case we could error? What do you think should happen?

Diff from master

Harbormaster failed remote builds in B50484: Diff 252741!Mar 25 2020, 9:40 PM

Harbormaster failed remote builds in B50485: Diff 252742!Mar 25 2020, 10:11 PM

In D76525#1942825, @zoecarver wrote:

@craig.topper I'm not sure what should happen. It should probably just use that CPU (although that's not a great solution). Is there a way to detect if an attribute was used to change the target in which case we could error? What do you think should happen?

I'm not sure what should happen as I'm not familiar with the library feature being implemented. As implemented here, the target attribute will be ignored and it will just follow the command line. Why do you say that following the attribute is not a great solution?