[clang][Driver] Support passing arbitrary args to -cc1as with -Xclangas. (#100714)

Unlike the `-Xassembler` idea mentioned in #97517, this one comes with
no GCC compatibility concerns, and simply mirrors what `-Xclang` does
for `-cc1`.

This is useful for mostly the same reasons that `-Xclang` is. The
motivating use case is `zig cc`, however, where we use `-Xclang
-target-feature` to pass the exhaustive list of target features to Clang
for C-family files. Before this commit, there was no way to do the same
for assembly files.

For context, Zig bases all of its target feature info directly on the
info in LLVM's backends, including all the dependency relationships
therein. So it just makes more sense for Zig to be able to directly pass
all this info to the assembler, rather than having to manually
reconstruct the corresponding frontend command line flags for every
target.

Closes #97517.

Author: alexrp

clang/docs/UsersManual.rst

@@ -5146,6 +5146,7 @@ Execute ``clang-cl /?`` to see a list of supported options:
       -v                      Show commands to run and use verbose output
       -W<warning>             Enable the specified warning
       -Xclang <arg>           Pass <arg> to the clang compiler
+      -Xclangas <arg>         Pass <arg> to the clang assembler
 
 The /clang: Option
 ^^^^^^^^^^^^^^^^^^

clang/include/clang/Driver/Options.td

@@ -958,10 +958,18 @@ def Xclang : Separate<["-"], "Xclang">,
   HelpText<"Pass <arg> to clang -cc1">, MetaVarName<"<arg>">,
   Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, DXCOption]>,
   Group<CompileOnly_Group>;
+def Xclangas : Separate<["-"], "Xclangas">,
+  HelpText<"Pass <arg> to clang -cc1as">, MetaVarName<"<arg>">,
+  Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, DXCOption]>,
+  Group<CompileOnly_Group>;
 def : Joined<["-"], "Xclang=">, Group<CompileOnly_Group>,
   Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, DXCOption]>,
   Alias<Xclang>,
   HelpText<"Alias for -Xclang">, MetaVarName<"<arg>">;
+def : Joined<["-"], "Xclangas=">, Group<CompileOnly_Group>,
+  Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, DXCOption]>,
+  Alias<Xclangas>,
+  HelpText<"Alias for -Xclangas">, MetaVarName<"<arg>">;
 def Xcuda_fatbinary : Separate<["-"], "Xcuda-fatbinary">,
   HelpText<"Pass <arg> to fatbinary invocation">, MetaVarName<"<arg>">;
 def Xcuda_ptxas : Separate<["-"], "Xcuda-ptxas">,

clang/lib/Driver/ToolChains/Clang.cpp

@@ -8895,6 +8895,12 @@ void ClangAs::ConstructJob(Compilation &C, const JobAction &JA,
   CollectArgsForIntegratedAssembler(C, Args, CmdArgs,
                                     getToolChain().getDriver());
 
+  // Forward -Xclangas arguments to -cc1as
+  for (auto Arg : Args.filtered(options::OPT_Xclangas)) {
+    Arg->claim();
+    CmdArgs.push_back(Arg->getValue());
+  }
+
   Args.AddAllArgs(CmdArgs, options::OPT_mllvm);
 
   if (DebugInfoKind > llvm::codegenoptions::NoDebugInfo && Output.isFilename())

clang/test/Driver/Xclangas.s

@@ -0,0 +1,4 @@
+/// Check that -Xclangas/-Xclangas= are passed to -cc1as.
+// RUN: %clang -### -Werror -Xclangas -target-feature -Xclangas=+v5t %s 2>&1 | FileCheck %s
+// CHECK: -cc1as
+// CHECK-SAME: "-target-feature" "+v5t"