TargetParser: fix getProcessTriple in universal builds

The bug happens when you build e.g. an x64_64;arm64 JIT with
LLVM_HOST_TRIPLE=x86_64-apple-macos, and then run it on an apple-m1 not under
Rosetta. In that case, sys::getProcessTriple() will return an x86_64 triple,
not an arm64 one.

Differential revision: https://reviews.llvm.org/D138449

llvm/lib/TargetParser/Host.cpp

@@ -1878,10 +1878,44 @@ bool sys::getHostCPUFeatures(StringMap<bool> &Features) {
 bool sys::getHostCPUFeatures(StringMap<bool> &Features) { return false; }
 #endif
 
+#if __APPLE__
+/// \returns the \p triple, but with the Host's arch spliced in.
+static Triple withHostArch(Triple T) {
+#if defined(__arm__)
+  T.setArch(Triple::arm);
+  T.setArchName("arm");
+#elif defined(__arm64e__)
+  T.setArch(Triple::aarch64, Triple::AArch64SubArch_arm64e);
+  T.setArchName("arm64e");
+#elif defined(__aarch64__)
+  T.setArch(Triple::aarch64);
+  T.setArchName("arm64");
+#elif defined(__x86_64h__)
+  T.setArch(Triple::x86_64);
+  T.setArchName("x86_64h");
+#elif defined(__x86_64__)
+  T.setArch(Triple::x86_64);
+  T.setArchName("x86_64");
+#elif defined(__powerpc__)
+  T.setArch(Triple::ppc);
+  T.setArchName("powerpc");
+#else
+#  error "Unimplemented host arch fixup"
+#endif
+  return T;
+}
+#endif
+
 std::string sys::getProcessTriple() {
   std::string TargetTripleString = updateTripleOSVersion(LLVM_HOST_TRIPLE);
   Triple PT(Triple::normalize(TargetTripleString));
 
+#if __APPLE__
+  /// In Universal builds, LLVM_HOST_TRIPLE will have the wrong arch in one of
+  /// the slices. This fixes that up.
+  PT = withHostArch(PT);
+#endif
+
   if (sizeof(void *) == 8 && PT.isArch32Bit())
     PT = PT.get64BitArchVariant();
   if (sizeof(void *) == 4 && PT.isArch64Bit())