Implement -fsemantic-interposition

First attempt at implementing -fsemantic-interposition.

Rely on GlobalValue::isInterposable that already captures most of the expected
behavior.

Rely on a ModuleFlag to state whether we should respect SemanticInterposition or
not. The default remains no.

So this should be a no-op if -fsemantic-interposition isn't used, and if it is,
isInterposable being already used in most optimisation, they should honor it
properly.

Note that it only impacts architecture compiled with -fPIC and no pie.

Differential Revision: https://reviews.llvm.org/D72829

clang/docs/ClangCommandLineReference.rst

@@ -904,6 +904,12 @@ Strip (or keep only, if negative) a given number of path components when emittin
 
 Turn on runtime checks for various forms of undefined or suspicious behavior. See user manual for available checks
 
+.. option:: -fno-semantic-interposition, -fsemantic-interposition
+
+Enable semantic interposition. Semantic interposition allows for the
+interposition of a symbol by another at runtime, thus preventing a range of
+inter-procedural optimisation.
+
 .. option:: -moutline, -mno-outline
 
 Enable function outlining (AArch64 only)

clang/include/clang/Basic/LangOptions.def

@@ -284,6 +284,7 @@ ENUM_LANGOPT(TypeVisibilityMode, Visibility, 3, DefaultVisibility,
              "default visibility for types [-ftype-visibility]")
 LANGOPT(SetVisibilityForExternDecls, 1, 0,
         "apply global symbol visibility to external declarations without an explicit visibility")
+BENIGN_LANGOPT(SemanticInterposition        , 1, 0, "semantic interposition")
 ENUM_LANGOPT(StackProtector, StackProtectorMode, 2, SSPOff,
              "stack protector mode")
 ENUM_LANGOPT(TrivialAutoVarInit, TrivialAutoVarInitKind, 2, TrivialAutoVarInitKind::Uninitialized,

clang/include/clang/Driver/Options.td

@@ -3283,7 +3283,8 @@ defm inline_small_functions : BooleanFFlag<"inline-small-functions">,
 defm ipa_cp : BooleanFFlag<"ipa-cp">,
     Group<clang_ignored_gcc_optimization_f_Group>;
 defm ivopts : BooleanFFlag<"ivopts">, Group<clang_ignored_gcc_optimization_f_Group>;
-def : Flag<["-"], "fno-semantic-interposition">, Group<clang_ignored_f_Group>;
+def fsemantic_interposition : Flag<["-"], "fsemantic-interposition">, Group<f_Group>, Flags<[CC1Option]>;
+def fno_semantic_interposition: Flag<["-"], "fno-semantic-interposition">, Group<f_Group>;
 defm non_call_exceptions : BooleanFFlag<"non-call-exceptions">, Group<clang_ignored_f_Group>;
 defm peel_loops : BooleanFFlag<"peel-loops">, Group<clang_ignored_gcc_optimization_f_Group>;
 defm permissive : BooleanFFlag<"permissive">, Group<clang_ignored_f_Group>;

clang/lib/CodeGen/CodeGenModule.cpp

@@ -483,6 +483,11 @@ void CodeGenModule::Release() {
     getModule().addModuleFlag(llvm::Module::Max, "Dwarf Version",
                               CodeGenOpts.DwarfVersion);
   }
+
+  if (Context.getLangOpts().SemanticInterposition)
+    // Require various optimization to respect semantic interposition.
+    getModule().setSemanticInterposition(1);
+
   if (CodeGenOpts.EmitCodeView) {
     // Indicate that we want CodeView in the metadata.
     getModule().addModuleFlag(llvm::Module::Warning, "CodeView", 1);
@@ -872,7 +877,7 @@ static bool shouldAssumeDSOLocal(const CodeGenModule &CGM,
   if (isa<llvm::Function>(GV) && !CGOpts.NoPLT && RM == llvm::Reloc::Static)
     return true;
 
-  // Otherwise don't assue it is local.
+  // Otherwise don't assume it is local.
   return false;
 }
 

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5037,6 +5037,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                   options::OPT_fno_emulated_tls);
   Args.AddLastArg(CmdArgs, options::OPT_fkeep_static_consts);
 
+  if (Args.hasFlag(options::OPT_fsemantic_interposition,
+                   options::OPT_fno_semantic_interposition, false))
+    CmdArgs.push_back("-fsemantic-interposition");
+
   // AltiVec-like language extensions aren't relevant for assembling.
   if (!isa<PreprocessJobAction>(JA) || Output.getType() != types::TY_PP_Asm)
     Args.AddLastArg(CmdArgs, options::OPT_fzvector);

clang/lib/Frontend/CompilerInvocation.cpp

@@ -3036,6 +3036,10 @@ static void ParseLangArgs(LangOptions &Opts, ArgList &Args, InputKind IK,
       Opts.setDefaultCallingConv(DefaultCC);
   }
 
+  // -fsemantic-interposition
+  Opts.SemanticInterposition =
+      Args.hasArg(OPT_fsemantic_interposition) && Opts.PICLevel && !Opts.PIE;
+
   // -mrtd option
   if (Arg *A = Args.getLastArg(OPT_mrtd)) {
     if (Opts.getDefaultCallingConv() != LangOptions::DCC_None)

clang/test/CodeGen/semantic-interposition.c

@@ -0,0 +1,14 @@
+// Semantic Interposition is active if
+//  -fsemantic-interposition is set,
+// - pic-level > 0
+// - pic-is-pie is not set
+
+// RUN: %clang_cc1 -emit-llvm -fsemantic-interposition -pic-level 0 %s -o - | FileCheck %s -check-prefix=CHECK-NO-INTERPOSITION
+// RUN: %clang_cc1 -emit-llvm -fsemantic-interposition -pic-level 1 %s -o - | FileCheck %s -check-prefix=CHECK-INTERPOSITION
+// RUN: %clang_cc1 -emit-llvm -fsemantic-interposition -pic-level 2 %s -o - | FileCheck %s -check-prefix=CHECK-INTERPOSITION
+// RUN: %clang_cc1 -emit-llvm -fsemantic-interposition -pic-level 0 %s -o - | FileCheck %s -check-prefix=CHECK-NO-INTERPOSITION
+// RUN: %clang_cc1 -emit-llvm -fsemantic-interposition -pic-level 1 -pic-is-pie %s -o - | FileCheck %s -check-prefix=CHECK-NO-INTERPOSITION
+// RUN: %clang_cc1 -emit-llvm -fsemantic-interposition -pic-level 2 -pic-is-pie %s -o - | FileCheck %s -check-prefix=CHECK-NO-INTERPOSITION
+
+// CHECK-NO-INTERPOSITION-NOT: "SemanticInterposition"
+// CHECK-INTERPOSITION: !{{[0-9]+}} = !{i32 1, !"SemanticInterposition", i32 1}

clang/test/Driver/clang_f_opts.c

@@ -252,6 +252,7 @@
 // RUN:     -fivopts -fno-ivopts                                              \
 // RUN:     -fnon-call-exceptions -fno-non-call-exceptions                    \
 // RUN:     -fno-semantic-interposition                                       \
+// RUN:     -fsemantic-interposition                                          \
 // RUN:     -fpermissive -fno-permissive                                      \
 // RUN:     -fdefer-pop -fno-defer-pop                                        \
 // RUN:     -fprefetch-loop-arrays -fno-prefetch-loop-arrays                  \

llvm/include/llvm/IR/GlobalValue.h

@@ -423,10 +423,10 @@ public:
   }
 
   /// Return true if this global's definition can be substituted with an
-  /// *arbitrary* definition at link time.  We cannot do any IPO or inlinining
-  /// across interposable call edges, since the callee can be replaced with
-  /// something arbitrary at link time.
-  bool isInterposable() const { return isInterposableLinkage(getLinkage()); }
+  /// *arbitrary* definition at link time or load time. We cannot do any IPO or
+  /// inlining across interposable call edges, since the callee can be
+  /// replaced with something arbitrary.
+  bool isInterposable() const;
 
   bool hasExternalLinkage() const { return isExternalLinkage(getLinkage()); }
   bool hasAvailableExternallyLinkage() const {

llvm/include/llvm/IR/Module.h

@@ -848,6 +848,12 @@ public:
   Metadata *getProfileSummary(bool IsCS);
   /// @}
 
+  /// Returns whether semantic interposition is to be respected.
+  bool getSemanticInterposition() const;
+
+  /// Set whether semantic interposition is to be respected.
+  void setSemanticInterposition(bool);
+
   /// Returns true if PLT should be avoided for RTLib calls.
   bool getRtLibUseGOT() const;
 

llvm/lib/IR/Globals.cpp

@@ -94,6 +94,13 @@ void GlobalValue::eraseFromParent() {
   llvm_unreachable("not a global");
 }
 
+bool GlobalValue::isInterposable() const {
+  if (isInterposableLinkage(getLinkage()))
+    return true;
+  return getParent() && getParent()->getSemanticInterposition() &&
+         !isDSOLocal();
+}
+
 unsigned GlobalValue::getAlignment() const {
   if (auto *GA = dyn_cast<GlobalAlias>(this)) {
     // In general we cannot compute this at the IR level, but we try.

llvm/lib/IR/Module.cpp

@@ -555,6 +555,20 @@ Metadata *Module::getProfileSummary(bool IsCS) {
                : getModuleFlag("ProfileSummary"));
 }
 
+bool Module::getSemanticInterposition() const {
+  Metadata *MF = getModuleFlag("SemanticInterposition");
+
+  auto *Val = cast_or_null<ConstantAsMetadata>(MF);
+  if (!Val)
+    return false;
+
+  return cast<ConstantInt>(Val->getValue())->getZExtValue();
+}
+
+void Module::setSemanticInterposition(bool SI) {
+  addModuleFlag(ModFlagBehavior::Error, "SemanticInterposition", SI);
+}
+
 void Module::setOwnedMemoryBuffer(std::unique_ptr<MemoryBuffer> MB) {
   OwnedMemoryBuffer = std::move(MB);
 }

llvm/lib/IR/Verifier.cpp

@@ -1476,6 +1476,13 @@ Verifier::visitModuleFlag(const MDNode *Op,
            "'Linker Options' named metadata no longer supported");
   }
 
+  if (ID->getString() == "SemanticInterposition") {
+    ConstantInt *Value =
+        mdconst::dyn_extract_or_null<ConstantInt>(Op->getOperand(2));
+    Assert(Value,
+           "SemanticInterposition metadata requires constant integer argument");
+  }
+
   if (ID->getString() == "CG Profile") {
     for (const MDOperand &MDO : cast<MDNode>(Op->getOperand(2))->operands())
       visitModuleFlagCGProfileEntry(MDO);

llvm/test/Transforms/Inline/inline-semantic-interposition.ll

@@ -0,0 +1,26 @@
+; Check that @callee1 gets inlined while @callee2 is not, because of
+; SemanticInterposition.
+
+; RUN: opt < %s -inline -S | FileCheck %s
+
+define internal i32 @callee1(i32 %A) {
+  ret i32 %A
+}
+
+define i32 @callee2(i32 %A) {
+  ret i32 %A
+}
+
+; CHECK-LABEL: @caller
+define i32 @caller(i32 %A) {
+; CHECK-NOT: call i32 @callee1(i32 %A)
+  %A1 = call i32 @callee1(i32 %A)
+; CHECK: %A2 = call i32 @callee2(i32 %A)
+  %A2 = call i32 @callee2(i32 %A)
+; CHECK: add i32 %A, %A2
+  %R = add i32 %A1, %A2
+  ret i32 %R
+}
+
+!llvm.module.flags = !{!0}
+!0 = !{i32 1, !"SemanticInterposition", i32 1}

llvm/test/Verifier/module-flags-semantic-interposition.ll

@@ -0,0 +1,12 @@
+; RUN: not llvm-as < %s -o /dev/null 2>&1 | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@foo = dso_local global i32 1, align 4
+
+!llvm.module.flags = !{!0}
+
+!0 = !{i32 1, !"SemanticInterposition", float 1.}
+
+; CHECK: SemanticInterposition metadata requires constant integer argument