[clang] Don't add DWARF debug info when assembling .s with clang-cl /Z7

This fixes a regression from f58330c.

That commit changed the clang-cl options /Zi and /Z7 to be
implemented as aliases of -g rather than having separate handling.

This had the unintended effect, that when assembling .s files
with clang-cl, the /Z7 option (which implies using CodeView debug
info) was treated as a -g option, which causes `ClangAs::ConstructJob`
to pick up the option as part of `Args.getLastArg(options::OPT_g_Group)`,
which sets the `WantDebug` variable.

Within `Clang::ConstructJob`, we check for whether explicit
`-gdwarf` or `-gcodeview` options have been set, and if not, we
pick the default debug format for the current toolchain. However,
in `ClangAs`, if debug info has been enabled, it always adds DWARF
debug info.

Add similar logic in `ClangAs` - check if the user has explicitly
requested either DWARF or CodeView, otherwise look up the toolchain
default. If we (either implicitly or explicitly) should be producing
CodeView, don't enable the default `ClangAs` DWARF generation.

This fixes the issue, where assembling a single `.s` file with
clang-cl, with the /Z7 option, causes the file to contain some DWARF
sections. This causes the output executable to contain DWARF, in
addition to the separate intended main PDB file.

By having the output executable contain DWARF sections, LLDB only
looks at the (very little) DWARF info in the executable, rather than
looking for a separate standalone PDB file. This caused an issue
with LLDB's tests, llvm#101710.

Author: mstorsjo

clang/lib/Driver/ToolChains/Clang.cpp

@@ -8641,6 +8641,32 @@ void ClangAs::ConstructJob(Compilation &C, const JobAction &JA,
     WantDebug = !A->getOption().matches(options::OPT_g0) &&
                 !A->getOption().matches(options::OPT_ggdb0);
 
+  // If a -gdwarf argument appeared, remember it.
+  bool EmitDwarf = false;
+  if (const Arg *A = getDwarfNArg(Args))
+    EmitDwarf = checkDebugInfoOption(A, Args, D, getToolChain());
+
+  bool EmitCodeView = false;
+  if (const Arg *A = Args.getLastArg(options::OPT_gcodeview))
+    EmitCodeView = checkDebugInfoOption(A, Args, D, getToolChain());
+
+  // If the user asked for debug info but did not explicitly specify -gcodeview
+  // or -gdwarf, ask the toolchain for the default format.
+  if (!EmitCodeView && !EmitDwarf && WantDebug) {
+    switch (getToolChain().getDefaultDebugFormat()) {
+    case llvm::codegenoptions::DIF_CodeView:
+      EmitCodeView = true;
+      break;
+    case llvm::codegenoptions::DIF_DWARF:
+      EmitDwarf = true;
+      break;
+    }
+  }
+
+  // If the arguments don't imply DWARF, don't emit any debug info here.
+  if (!EmitDwarf)
+    WantDebug = false;
+
   llvm::codegenoptions::DebugInfoKind DebugInfoKind =
       llvm::codegenoptions::NoDebugInfo;
 

clang/test/Driver/debug-options-as.c

@@ -19,12 +19,27 @@
 // GGDB0-NOT: -debug-info-kind=
 
 // Check to make sure clang with -g on a .s file gets passed.
-// RUN: %clang -### -c -integrated-as -g -x assembler %s 2>&1 \
+// This requires a target that defaults to DWARF.
+// RUN: %clang -### --target=x86_64-linux-gnu -c -integrated-as -g -x assembler %s 2>&1 \
 // RUN:   | FileCheck %s
 //
 // CHECK: "-cc1as"
 // CHECK: "-debug-info-kind=constructor"
 
+// Check that a plain -g, without any -gdwarf, for a MSVC target, doesn't
+// trigger producing DWARF output.
+// RUN: %clang -### --target=x86_64-windows-msvc -c -integrated-as -g -x assembler %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=MSVC %s
+//
+// MSVC: "-cc1as"
+// MSVC-NOT: "-debug-info-kind=constructor"
+
+// Check that clang-cl with the -Z7 option works the same, not triggering
+// any DWARF output.
+//
+// RUN: %clang_cl -### -c -Z7 -x assembler %s 2>&1 \
+// RUN:   | FileCheck -check-prefix=MSVC %s
+
 // Check to make sure clang with -g on a .s file gets passed -dwarf-debug-producer.
 // RUN: %clang -### -c -integrated-as -g -x assembler %s 2>&1 \
 // RUN:   | FileCheck -check-prefix=P %s