support: fix getProcessTriple in universal builds

[jroelofs]

Summary

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.

[keith]

I'm not super familiar with the history behind LLVM_HOST_TRIPLE, but one thing that definitely worries me about this specific change is there are other consumers of that variable, but only this 1 gets this fixup. I feel like the core issue is the mismatch between having a single LLVM_HOST_TRIPLE, but having cmake build a fat binary.

[jroelofs]

AFAICT, this is the only one where it really matters, aside from (plausibly) the one in ToolChain::isCrossCompiling().

[keith]

It feels to me like we're defeating the purpose of LLVM_HOST_TRIPLE, but I imagine any other solution to this would require a much bigger hammer, so seems worth a try.

llvm/lib/TargetParser/Host.cpp

bool sys::getHostCPUFeatures(StringMap<bool> &Features) {
   Features["lvz"] = hwcap & (1UL << 9);  // HWCAP_LOONGARCH_LVZ
 
   return true;
 }
 #else
 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())
     PT = PT.get32BitArchVariant();
 
   return PT.str();
 }