[clang] [llvm] [AArch64] set AppleA14 architecture version to v8.4-a (PR #92600)

[Tomas Matheson via llvm-commits]

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@@ -718,12 +718,16 @@ def ProcessorFeatures {
   list<SubtargetFeature> AppleA13 = [HasV8_4aOps, FeatureCrypto, FeatureFPARMv8,
                                      FeatureNEON, FeaturePerfMon, FeatureFullFP16,
                                      FeatureFP16FML, FeatureSHA3];
+  // Apple A14 and M1 chips are based on Armv8.5-a but without BTI. Since there
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tmatheson-arm wrote:

As you point out, the only publicly available documentation is the optimization guide, and lists these cores as "8.5 without BTI". Here in the tablegen file it is written as 8.4 plus all of the features from 8.5 which are supported. A comment explaining the discrepancy seems reasonable to me. It also alerts people that if any new backend feature is introduced that models something in 8.5, they need to manually add it to the list for these cores.

https://github.com/llvm/llvm-project/pull/92600