[llvm] [win][aarch64] Add support for detecting the Host CPU on Arm64 Windows (PR #151596)

[Eli Friedman via llvm-commits]

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@@ -1450,6 +1462,48 @@ StringRef sys::getHostCPUName() {
   return "generic";
 }
 
+#elif defined(_M_ARM64) || defined(_M_ARM64EC)
+
+union MIDR_EL1 {
+  uint64_t Raw;
+  struct _Parts {
+    uint64_t Revision : 4;
+    uint64_t Partnum : 12;
+    uint64_t Architecture : 4;
+    uint64_t Variant : 4;
+    uint64_t Implementer : 8;
+    uint64_t Reserved : 32;
+  } Parts;
+};
+
+StringRef sys::getHostCPUName() {
+  StringRef CPU = "generic";
+
+  // The "CP 4000" registry key contains a cached copy of the MIDR_EL1 register.
+  HKEY Key;
+  if (RegOpenKeyExA(HKEY_LOCAL_MACHINE,
+                    "HARDWARE\\DESCRIPTION\\System\\CentralProcessor\\0", 0,
+                    KEY_READ, &Key) == ERROR_SUCCESS) {
+    MIDR_EL1 RegValue;
+    DWORD ActualType;
+    DWORD RegValueSize = sizeof(RegValue);
+    if ((RegQueryValueExA(Key, "CP 4000", nullptr, &ActualType,
+                          (PBYTE)&RegValue, &RegValueSize) == ERROR_SUCCESS) &&
+        (ActualType == REG_QWORD) && RegValueSize == sizeof(RegValue)) {
+      auto Part = "0x" + utohexstr(RegValue.Parts.Partnum, /*LowerCase*/ true,
+                                   /*Width*/ 3);
+      CPU = detail::getHostCPUNameForARM(
+          "0x" + utohexstr(RegValue.Parts.Implementer, /*LowerCase*/ true,
----------------
efriedma-quic wrote:

On an 8cx Gen 3, CPU 0-3 have "CP 4000" = 0x410fd4b0, CPU 4-7 have "CP 4000" = 0x410fd4c0.

I think Windows enumerates cores essentially the same way as Linux.  So I think we should pick the "primary" part the same way as Linux, which would be the highest numbered part, not the lowest-numbered part.  Doing something different on Linux vs. Windows is more confusing than helpful.

That's likely not 100% reliable, but we can leave OS-independent improvements for a followup.

https://github.com/llvm/llvm-project/pull/151596