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

[Daniel Paoliello via llvm-commits]

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@@ -1450,6 +1503,74 @@ StringRef sys::getHostCPUName() {
   return "generic";
 }
 
+#elif defined(_M_ARM64) || defined(_M_ARM64EC)
+
+StringRef sys::getHostCPUName() {
+  constexpr char CentralProcessorKeyName[] =
+      "HARDWARE\\DESCRIPTION\\System\\CentralProcessor";
+  // Sub keys names are simple numbers ("0", "1", etc.) so 10 chars should be
+  // enough for the slash and name.
+  constexpr size_t SubKeyNameMaxSize = ARRAYSIZE(CentralProcessorKeyName) + 10;
+
+  SmallVector<uint64_t> Values;
+  uint64_t PrimaryCpuInfo;
+  char PrimaryPartKeyName[SubKeyNameMaxSize];
+  DWORD PrimaryPartKeyNameSize = 0;
+  HKEY CentralProcessorKey;
+  if (RegOpenKeyExA(HKEY_LOCAL_MACHINE, CentralProcessorKeyName, 0, KEY_READ,
+                    &CentralProcessorKey) == ERROR_SUCCESS) {
+    for (unsigned Index = 0; Index < UINT32_MAX; ++Index) {
+      char SubKeyName[SubKeyNameMaxSize];
+      DWORD SubKeySize = SubKeyNameMaxSize;
+      HKEY SubKey;
+      if ((RegEnumKeyExA(CentralProcessorKey, Index, SubKeyName, &SubKeySize,
+                         nullptr, nullptr, nullptr,
+                         nullptr) == ERROR_SUCCESS) &&
+          (RegOpenKeyExA(CentralProcessorKey, SubKeyName, 0, KEY_READ,
+                         &SubKey) == ERROR_SUCCESS)) {
+        // The "CP 4000" registry key contains a cached copy of the MIDR_EL1
+        // register.
+        uint64_t RegValue;
+        DWORD ActualType;
+        DWORD RegValueSize = sizeof(RegValue);
+        if ((RegQueryValueExA(SubKey, "CP 4000", nullptr, &ActualType,
+                              (PBYTE)&RegValue,
+                              &RegValueSize) == ERROR_SUCCESS) &&
+            (ActualType == REG_QWORD) && RegValueSize == sizeof(RegValue)) {
+          // Assume that the part with the "highest" reg key name is the primary
+          // part (to match the way that Linux's cpuinfo is written). Win32
+          // makes no guarantees about the order of sub keys, so we have to
+          // compare the names.
+          if (PrimaryPartKeyNameSize < SubKeySize ||
+              (PrimaryPartKeyNameSize == SubKeySize &&
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dpaoliello wrote:

Yep, wow, good catch!

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