[ARM64EC] Add softintrin.lib as an implicit dependency to object files.

This copies MSVC behavior, and avoids weird link errors in certain
cases.

Author: efriedma-quic

clang/lib/Driver/ToolChains/Clang.cpp

@@ -4697,7 +4697,7 @@ renderDebugOptions(const ToolChain &TC, const Driver &D, const llvm::Triple &T,
                        Output.getFilename());
 }
 
-static void ProcessVSRuntimeLibrary(const ArgList &Args,
+static void ProcessVSRuntimeLibrary(const ToolChain &TC, const ArgList &Args,
                                     ArgStringList &CmdArgs) {
   unsigned RTOptionID = options::OPT__SLASH_MT;
 
@@ -4760,6 +4760,12 @@ static void ProcessVSRuntimeLibrary(const ArgList &Args,
     // implemented in clang.
     CmdArgs.push_back("--dependent-lib=oldnames");
   }
+
+  // All Arm64EC object files implicitly add softintrin.lib. This is necessary
+  // even if the file doesn't actually refer to any of the routines because
+  // the CRT itself has incomplete dependency markings.
+  if (TC.getTriple().isWindowsArm64EC())
+    CmdArgs.push_back("--dependent-lib=softintrin");
 }
 
 void Clang::ConstructJob(Compilation &C, const JobAction &JA,
@@ -7015,7 +7021,7 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
   if (Triple.isWindowsMSVCEnvironment() && !D.IsCLMode() &&
       Args.hasArg(options::OPT_fms_runtime_lib_EQ))
-    ProcessVSRuntimeLibrary(Args, CmdArgs);
+    ProcessVSRuntimeLibrary(getToolChain(), Args, CmdArgs);
 
   // Handle -fgcc-version, if present.
   VersionTuple GNUCVer;
@@ -8134,7 +8140,7 @@ void Clang::AddClangCLArgs(const ArgList &Args, types::ID InputType,
                            ArgStringList &CmdArgs) const {
   bool isNVPTX = getToolChain().getTriple().isNVPTX();
 
-  ProcessVSRuntimeLibrary(Args, CmdArgs);
+  ProcessVSRuntimeLibrary(getToolChain(), Args, CmdArgs);
 
   if (Arg *ShowIncludes =
           Args.getLastArg(options::OPT__SLASH_showIncludes,

clang/test/Driver/cl-options.c

@@ -790,6 +790,7 @@
 // RUN: %clang_cl -vctoolsdir "" /arm64EC /c -### -- %s 2>&1 | FileCheck --check-prefix=ARM64EC %s 
 // ARM64EC-NOT: /arm64EC has been overridden by specified target
 // ARM64EC: "-triple" "arm64ec-pc-windows-msvc19.33.0"
+// ARM64EC-SAME: "--dependent-lib=softintrin"
 
 // RUN: %clang_cl -vctoolsdir "" /arm64EC /c -target x86_64-pc-windows-msvc  -### -- %s 2>&1 | FileCheck --check-prefix=ARM64EC_OVERRIDE %s
 // ARM64EC_OVERRIDE: warning: /arm64EC has been overridden by specified target: x86_64-pc-windows-msvc; option ignored