[llvm] [clang] New calling convention preserve_none (PR #76868)

[via cfe-commits]

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@@ -1056,6 +1056,22 @@ def CC_Intel_OCL_BI : CallingConv<[
   CCDelegateTo<CC_X86_32_C>
 ]>;
 
+def CC_X86_64_Preserve_None : CallingConv<[
+  // We don't preserve general registers, so all of them can be used to pass
+  // arguments except
+  //   - RBP  frame pointer
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weiguozhi wrote:

Added a comment to mention that R16-R31 are not used to pass arguments because they are not universally available.

https://github.com/llvm/llvm-project/pull/76868