[DebugInfo] Add -fuse-ctor-homing cc1 flag so we can turn on constructor homing only if limited debug info is already on.

[akhuang]

Summary

This adds a cc1 flag to enable constructor homing but doesn't turn on debug
info if it wasn't enabled already (which is what using
-debug-info-kind=constructor does). This will be used for testing, and won't
be needed anymore once ctor homing is used as default / merged into =limited.

Bug to enable ctor homing: https://bugs.llvm.org/show_bug.cgi?id=46537

[dblaikie]

Looks good! Could you add a test case for this too?

[akhuang]

Add test case.

[akhuang]

Yep, just added a line to the existing ctor homing test case.

[dblaikie]

If possible, could you test the negative cases too? That -fuse-ctor-homing doesn't override -debug-info-kind=line-tables-only or no -debug-info-kind at all?

[akhuang]

In D85799#2214330, @dblaikie wrote:

If possible, could you test the negative cases too? That -fuse-ctor-homing doesn't override -debug-info-kind=line-tables-only or no -debug-info-kind at all?

Oh, good point.

[akhuang]

Add more extensive check that -fuse-ctor-homing only does something when -debug-info-kind=limited

[dblaikie]

Looks great, thanks!

clang/include/clang/Driver/Options.td

 def mrelocation_model : Separate<["-"], "mrelocation-model">,
   HelpText<"The relocation model to use">, Values<"static,pic,ropi,rwpi,ropi-rwpi,dynamic-no-pic">,
   NormalizedValuesScope<"llvm::Reloc">,
   NormalizedValues<["Static", "PIC_", "ROPI", "RWPI", "ROPI_RWPI", "DynamicNoPIC"]>,
   MarshallingInfoString<"CodeGenOpts.RelocationModel", "PIC_", "Model">,
   AutoNormalizeEnum;
 def fno_math_builtin : Flag<["-"], "fno-math-builtin">,
   HelpText<"Disable implicit builtin knowledge of math functions">;
+def fuse_ctor_homing: Flag<["-"], "fuse-ctor-homing">,
+    HelpText<"Use constructor homing if we are using limited debug info already">;
 }
 
 def disable_llvm_verifier : Flag<["-"], "disable-llvm-verifier">,
   HelpText<"Don't run the LLVM IR verifier pass">;
 def disable_llvm_passes : Flag<["-"], "disable-llvm-passes">,
   HelpText<"Use together with -emit-llvm to get pristine LLVM IR from the "
            "frontend by not running any LLVM passes at all">;
 def disable_llvm_optzns : Flag<["-"], "disable-llvm-optzns">,

clang/lib/Frontend/CompilerInvocation.cpp

unsigned Val =
             .Case("unused-types", codegenoptions::UnusedTypeInfo)
             .Default(~0U);
     if (Val == ~0U)
       Diags.Report(diag::err_drv_invalid_value) << A->getAsString(Args)
                                                 << A->getValue();
     else
       Opts.setDebugInfo(static_cast<codegenoptions::DebugInfoKind>(Val));
   }
+  // If -fuse-ctor-homing is set and limited debug info is already on, then use
+  // constructor homing.
+  if (Arg *A = Args.getLastArg(OPT_fuse_ctor_homing))
+    if (Opts.getDebugInfo() == codegenoptions::LimitedDebugInfo)
+      Opts.setDebugInfo(codegenoptions::DebugInfoConstructor);
+
   if (Arg *A = Args.getLastArg(OPT_debugger_tuning_EQ)) {
     unsigned Val = llvm::StringSwitch<unsigned>(A->getValue())
                        .Case("gdb", unsigned(llvm::DebuggerKind::GDB))
                        .Case("lldb", unsigned(llvm::DebuggerKind::LLDB))
                        .Case("sce", unsigned(llvm::DebuggerKind::SCE))
                        .Default(~0U);
     if (Val == ~0U)
       Diags.Report(diag::err_drv_invalid_value) << A->getAsString(Args)

clang/test/CodeGenCXX/debug-info-ctor-homing-flag.cpp

+// RUN: %clang -cc1 -debug-info-kind=constructor -emit-llvm %s -o - \
+// RUN:        | FileCheck %s -check-prefix=CTOR_HOMING
+// RUN: %clang -cc1 -debug-info-kind=limited -fuse-ctor-homing -emit-llvm %s -o - \
+// RUN:        | FileCheck %s -check-prefix=CTOR_HOMING
+// RUN: %clang -cc1 -debug-info-kind=standalone -fuse-ctor-homing -emit-llvm %s -o - \
+// RUN:        | FileCheck %s -check-prefix=FULL_DEBUG
+// RUN: %clang -cc1 -debug-info-kind=line-tables-only -fuse-ctor-homing -emit-llvm %s -o - \
+// RUN:        | FileCheck %s -check-prefix=NO_DEBUG
+// RUN: %clang -cc1 -fuse-ctor-homing -emit-llvm %s -o - \
+// RUN:        | FileCheck %s -check-prefix=NO_DEBUG
+
+// This tests that the -fuse-ctor-homing is only used if limited debug info would have
+// been used otherwise.
+
+// CTOR_HOMING: !DICompositeType(tag: DW_TAG_structure_type, name: "A"{{.*}}flags: DIFlagFwdDecl
+// FULL_DEBUG: !DICompositeType(tag: DW_TAG_structure_type, name: "A"{{.*}}DIFlagTypePassByValue
+// NO_DEBUG-NOT: !DICompositeType(tag: DW_TAG_structure_type, name: "A"
+struct A {
+  A();
+} TestA;