[win][x64] Unwind v2 3/n: Add support for requiring unwind v2 to be used (equivalent to MSVC's /d2epilogunwindrequirev2) (#143577)

#129142 added support for emitting Windows x64 unwind v2 information,
but it was "best effort". If any function didn't follow the requirements
for v2 it was silently downgraded to v1.

There are some parts of Windows (specifically kernel-mode code running
on Xbox) that require v2, hence we need the ability to fail the
compilation if v2 can't be used.

This change also adds a heuristic to check if there might be too many
unwind codes, it's currently conservative (i.e., assumes that certain
prolog instructions will use the maximum number of unwind codes).

Future work: attempting to chain unwind info across multiple tables if
there are too many unwind codes due to epilogs and adding a heuristic to
detect if an epilog will be too far from the end of the function.

Author: dpaoliello

clang/include/clang/Basic/CodeGenOptions.def

@@ -483,8 +483,10 @@ CODEGENOPT(StaticClosure, 1, 0)
 /// Assume that UAVs/SRVs may alias
 CODEGENOPT(ResMayAlias, 1, 0)
 
-/// Enables unwind v2 (epilog) information for x64 Windows.
-CODEGENOPT(WinX64EHUnwindV2, 1, 0)
+/// Controls how unwind v2 (epilog) information should be generated for x64
+/// Windows.
+ENUM_CODEGENOPT(WinX64EHUnwindV2, llvm::WinX64EHUnwindV2Mode,
+                2, llvm::WinX64EHUnwindV2Mode::Disabled)
 
 /// FIXME: Make DebugOptions its own top-level .def file.
 #include "DebugOptions.def"

clang/include/clang/Driver/Options.td

@@ -2167,11 +2167,14 @@ defm assume_nothrow_exception_dtor: BoolFOption<"assume-nothrow-exception-dtor",
   LangOpts<"AssumeNothrowExceptionDtor">, DefaultFalse,
   PosFlag<SetTrue, [], [ClangOption, CC1Option], "Assume that exception objects' destructors are non-throwing">,
   NegFlag<SetFalse>>;
-defm winx64_eh_unwindv2 : BoolFOption<"winx64-eh-unwindv2",
-  CodeGenOpts<"WinX64EHUnwindV2">, DefaultFalse,
-  PosFlag<SetTrue, [], [ClangOption, CC1Option], "Enable">,
-  NegFlag<SetFalse, [], [ClangOption], "Disable">,
-  BothFlags<[], [ClangOption], " unwind v2 (epilog) information for x64 Windows">>;
+def winx64_eh_unwindv2
+    : Joined<["-"], "fwinx64-eh-unwindv2=">, Group<f_Group>,
+    Visibility<[ClangOption, CC1Option]>,
+      HelpText<"Generate unwind v2 (epilog) information for x64 Windows">,
+      Values<"disabled,best-effort,required">,
+      NormalizedValues<["Disabled", "BestEffort", "Required"]>,
+      NormalizedValuesScope<"llvm::WinX64EHUnwindV2Mode">,
+      MarshallingInfoEnum<CodeGenOpts<"WinX64EHUnwindV2">, "Disabled">;
 def fexcess_precision_EQ : Joined<["-"], "fexcess-precision=">, Group<f_Group>,
   Visibility<[ClangOption, CLOption]>,
   HelpText<"Allows control over excess precision on targets where native "
@@ -8972,7 +8975,9 @@ def _SLASH_volatile_Group : OptionGroup<"</volatile group>">,
   Group<cl_compile_Group>;
 
 def _SLASH_d2epilogunwind : CLFlag<"d2epilogunwind">,
-  HelpText<"Enable unwind v2 (epilog) information for x64 Windows">;
+  HelpText<"Best effort generate unwind v2 (epilog) information for x64 Windows">;
+def _SLASH_d2epilogunwindrequirev2 : CLFlag<"d2epilogunwindrequirev2">,
+  HelpText<"Require generation of unwind v2 (epilog) information for x64 Windows">;
 def _SLASH_EH : CLJoined<"EH">, HelpText<"Set exception handling model">;
 def _SLASH_EP : CLFlag<"EP">,
   HelpText<"Disable linemarker output and preprocess to stdout">;

clang/lib/CodeGen/CodeGenModule.cpp

@@ -1319,8 +1319,10 @@ void CodeGenModule::Release() {
                               1);
 
   // Enable unwind v2 (epilog).
-  if (CodeGenOpts.WinX64EHUnwindV2)
-    getModule().addModuleFlag(llvm::Module::Warning, "winx64-eh-unwindv2", 1);
+  if (CodeGenOpts.getWinX64EHUnwindV2() != llvm::WinX64EHUnwindV2Mode::Disabled)
+    getModule().addModuleFlag(
+        llvm::Module::Warning, "winx64-eh-unwindv2",
+        static_cast<unsigned>(CodeGenOpts.getWinX64EHUnwindV2()));
 
   // Indicate whether this Module was compiled with -fopenmp
   if (getLangOpts().OpenMP && !getLangOpts().OpenMPSimd)

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7360,8 +7360,7 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   }
 
   // Unwind v2 (epilog) information for x64 Windows.
-  Args.addOptInFlag(CmdArgs, options::OPT_fwinx64_eh_unwindv2,
-                    options::OPT_fno_winx64_eh_unwindv2);
+  Args.AddLastArg(CmdArgs, options::OPT_winx64_eh_unwindv2);
 
   // C++ "sane" operator new.
   Args.addOptOutFlag(CmdArgs, options::OPT_fassume_sane_operator_new,
@@ -8418,8 +8417,10 @@ void Clang::AddClangCLArgs(const ArgList &Args, types::ID InputType,
     CmdArgs.push_back("-fms-kernel");
 
   // Unwind v2 (epilog) information for x64 Windows.
-  if (Args.hasArg(options::OPT__SLASH_d2epilogunwind))
-    CmdArgs.push_back("-fwinx64-eh-unwindv2");
+  if (Args.hasArg(options::OPT__SLASH_d2epilogunwindrequirev2))
+    CmdArgs.push_back("-fwinx64-eh-unwindv2=required");
+  else if (Args.hasArg(options::OPT__SLASH_d2epilogunwind))
+    CmdArgs.push_back("-fwinx64-eh-unwindv2=best-effort");
 
   for (const Arg *A : Args.filtered(options::OPT__SLASH_guard)) {
     StringRef GuardArgs = A->getValue();

clang/test/CodeGen/epilog-unwind.c

@@ -1,9 +1,11 @@
 // RUN: %clang_cc1 -emit-llvm %s -o - | FileCheck %s -check-prefix=DISABLED
-// RUN: %clang_cc1 -fwinx64-eh-unwindv2 -emit-llvm %s -o - | FileCheck %s -check-prefix=ENABLED
-// RUN: %clang -fwinx64-eh-unwindv2 -S -emit-llvm %s -o - | FileCheck %s -check-prefix=ENABLED
-// RUN: %clang -fno-winx64-eh-unwindv2 -S -emit-llvm %s -o - | FileCheck %s -check-prefix=DISABLED
+// RUN: %clang_cc1 -fwinx64-eh-unwindv2=disabled -emit-llvm %s -o - | FileCheck %s -check-prefix=DISABLED
+// RUN: %clang_cc1 -fwinx64-eh-unwindv2=best-effort -emit-llvm %s -o - | FileCheck %s -check-prefix=BESTEFFORT
+// RUN: %clang_cc1 -fwinx64-eh-unwindv2=required -emit-llvm %s -o - | FileCheck %s -check-prefix=REQUIRED
+// RUN: %clang -fwinx64-eh-unwindv2=best-effort -S -emit-llvm %s -o - | FileCheck %s -check-prefix=BESTEFFORT
 
 void f(void) {}
 
-// ENABLED: !"winx64-eh-unwindv2", i32 1}
+// BESTEFFORT: !"winx64-eh-unwindv2", i32 1}
+// REQUIRED: !"winx64-eh-unwindv2", i32 2}
 // DISABLED-NOT: "winx64-eh-unwindv2"

clang/test/Driver/cl-options.c

@@ -821,7 +821,11 @@
 // ARM64EC_OVERRIDE: warning: /arm64EC has been overridden by specified target: x86_64-pc-windows-msvc; option ignored
 
 // RUN: %clang_cl /d2epilogunwind /c -### -- %s 2>&1 | FileCheck %s --check-prefix=EPILOGUNWIND
-// EPILOGUNWIND: -fwinx64-eh-unwindv2
+// EPILOGUNWIND: -fwinx64-eh-unwindv2=best-effort
+
+// RUN: %clang_cl /d2epilogunwindrequirev2 /c -### -- %s 2>&1 | FileCheck %s --check-prefix=EPILOGUNWINDREQUIREV2
+// RUN: %clang_cl /d2epilogunwindrequirev2 /d2epilogunwind /c -### -- %s 2>&1 | FileCheck %s --check-prefix=EPILOGUNWINDREQUIREV2
+// EPILOGUNWINDREQUIREV2: -fwinx64-eh-unwindv2=require
 
 // RUN: %clang_cl /funcoverride:override_me1 /funcoverride:override_me2 /c -### -- %s 2>&1 | FileCheck %s --check-prefix=FUNCOVERRIDE
 // FUNCOVERRIDE: -loader-replaceable-function=override_me1

llvm/include/llvm/IR/Module.h

@@ -1041,6 +1041,10 @@ class LLVM_ABI Module {
 
   /// Returns target-abi from MDString, null if target-abi is absent.
   StringRef getTargetABIFromMD();
+
+  /// Get how unwind v2 (epilog) information should be generated for x64
+  /// Windows.
+  WinX64EHUnwindV2Mode getWinX64EHUnwindV2Mode() const;
 };
 
 /// Given "llvm.used" or "llvm.compiler.used" as a global name, collect the

llvm/include/llvm/Support/CodeGen.h

@@ -130,6 +130,15 @@ namespace llvm {
     Invalid = 2, ///< Not used.
   };
 
+  enum class WinX64EHUnwindV2Mode {
+    // Don't use unwind v2 (i.e., use v1).
+    Disabled = 0,
+    // Use unwind v2 here possible, otherwise fallback to v1.
+    BestEffort = 1,
+    // Use unwind v2 everywhere, otherwise raise an error.
+    Required = 2,
+  };
+
   } // namespace llvm
 
 #endif

llvm/lib/IR/Module.cpp

@@ -917,3 +917,10 @@ StringRef Module::getTargetABIFromMD() {
     TargetABI = TargetABIMD->getString();
   return TargetABI;
 }
+
+WinX64EHUnwindV2Mode Module::getWinX64EHUnwindV2Mode() const {
+  Metadata *MD = getModuleFlag("winx64-eh-unwindv2");
+  if (auto *CI = mdconst::dyn_extract_or_null<ConstantInt>(MD))
+    return static_cast<WinX64EHUnwindV2Mode>(CI->getZExtValue());
+  return WinX64EHUnwindV2Mode::Disabled;
+}