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Clarify documentation of cpu_dispatch/cpu_specific 

There has been some internal confusion lately as to how cpu_dispatch and
cpu_specific dispatch to processors, so this patch clarifies the
documentation to make it more clear that:

1- Unlike ICC, we do not consider the vendor string (that is, an AMD
processor might result in something other than generic)

2- there are some processors that aren't really distinguishable thanks
to the library limitation, so the variant being selected is unspecified.

In reality, I believe the 'stable_sort' makes it so the 1st one that
meets the requirements is the one that is selected (and that matches my
experimented result), but I don't want to limit our implementation.
This commit is contained in:
Erich Keane 2022-02-28 07:03:55 -08:00
parent e4b9640867
commit cd1489bb58
1 changed files with 11 additions and 0 deletions
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clang/include/clang/Basic/AttrDocs.td
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@ -285,6 +285,17 @@ features that are required for the function to be called. The result of this is
that future processors execute the most restrictive version of the function the
new processor can execute.
In addition, unlike the ICC implementation of this feature, the selection of the
version does not consider the manufacturer or microarchitecture of the processor.
It tests solely the list of features that are both supported by the specified
processor and present in the compiler-rt library. This can be surprising at times,
as the runtime processor may be from a completely different manufacturer, as long
as it supports the same feature set.
This can additionally be surprising, as some processors are indistringuishable from
others based on the list of testable features. When this happens, the variant
is selected in an unspecified manner.
Function versions are defined with ``cpu_specific``, which takes one or more CPU
names as a parameter. For example: