[llvm] [aarch64][win] Add support for import call optimization (equivalent to MSVC /d2ImportCallOptimization) (PR #121516)

[Eli Friedman via llvm-commits]

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@@ -77,6 +78,11 @@ static cl::opt<PtrauthCheckMode> PtrauthAuthChecks(
     cl::desc("Check pointer authentication auth/resign failures"),
     cl::init(Default));
 
+static cl::opt<bool> EnableImportCallOptimization(
+    "aarch64-win-import-call-optimization", cl::Hidden,
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efriedma-quic wrote:

Oops, one thing I forgot to ask about.  How do we enable this?  An LLVM flag is fine for now, but we don't like to use them as the primary mechanism for enabling code generation options.  It doesn't interact well with LTO, or programmatically enabling the option.

We usually use function attributes for options which make sense to enable on a per-function basis.

You can address this in a followup.

https://github.com/llvm/llvm-project/pull/121516