diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -1357,6 +1357,17 @@
   }
 }
 
+static bool isLaxVectorConversionsDefault(const llvm::Triple &Triple) {
+  switch (Triple.getArch()) {
+  default:
+    return true;
+
+  case llvm::Triple::thumb:
+  case llvm::Triple::thumbeb:
+    return Triple.getSubArch() != llvm::Triple::ARMSubArch_v8_1m_mainline;
+  }
+}
+
 static bool isNoCommonDefault(const llvm::Triple &Triple) {
   switch (Triple.getArch()) {
   default:
@@ -4674,10 +4685,13 @@
   if (TC.SupportsProfiling())
     Args.AddLastArg(CmdArgs, options::OPT_mfentry);
 
-  // -flax-vector-conversions is default.
-  if (!Args.hasFlag(options::OPT_flax_vector_conversions,
-                    options::OPT_fno_lax_vector_conversions))
+  if (const Arg *A = Args.getLastArg(options::OPT_flax_vector_conversions,
+                    options::OPT_fno_lax_vector_conversions)) {
+    if (A->getOption().matches(options::OPT_fno_lax_vector_conversions))
+      CmdArgs.push_back("-fno-lax-vector-conversions");
+  } else if (!isLaxVectorConversionsDefault(Triple)) {
     CmdArgs.push_back("-fno-lax-vector-conversions");
+  }
 
   if (Args.getLastArg(options::OPT_fapple_kext) ||
       (Args.hasArg(options::OPT_mkernel) && types::isCXX(InputType)))