[clang] [AArch64] Bump default CPUs for iOS 26/watchOS 26 to A12/S6. (PR #152235)

[Ahmed Bougacha via cfe-commits]

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@@ -52,6 +52,22 @@ std::string aarch64::getAArch64TargetCPU(const ArgList &Args,
     return "apple-m1";
   }
 
+  if (Triple.getOS() == llvm::Triple::IOS) {
+    assert(!Triple.isSimulatorEnvironment() && "iossim should be mac-like");
+    // iOS 26 only runs on apple-a12 and later CPUs.
+    if (!Triple.isOSVersionLT(26))
+      return "apple-a12";
+  }
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ahmedbougacha wrote:

Ah right, I think of those as a remnant of early LTO support, a default fallback for cases where the frontend doesn't set up the target IR attributes.
The last addition was arm64e/a12, when we still had unattributed functions, in obscure corners of clang IRGen that synthesize thunks and wrappers.  I'd expect those to be thoroughly eliminated now, after the waves of ptrauth attribute variations that will break if missing, and can't rely on this sort of late fallback.

So I'd say we could probably try to remove the LTO logic altogether, at the risk of breaking some library users without a clang-equivalent driver.
There's a case to be made this logic should be an llvm default backing a driver override, which would help other users.  I don't think that's a bad idea, and it seems it'd belong in TargetParser?  But overall I think it's just another on the long list of low-level choices that clang has to do and are hard to separate;  using clang seems unavoidable (also see Jordan and John's old llvm devmtg talk "Skip the FFI" ;)

https://github.com/llvm/llvm-project/pull/152235