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Extend LLVM IR WME to use thunks for cross-module witness method calls. (swiftlang#39528)
This enables optimizing / dead-stripping of witness methods across modules at LTO time. - Under -internalize-at-link, restrict visibility of wtables to linkage unit. - Emit thunks for cross-module wcalls when WME is enabled. - Use thunks for wcalls across modules when WME is enabled. - Adjust TBDGen to account for witness method thunks when WME is enabled. - Add an IR test to check that thunks are used when doing cross-module calls. - Add an end-to-end test case for cross-module WME.
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lines changed

8 files changed

+145
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‎include/swift/TBDGen/TBDGen.h

Lines changed: 5 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -44,6 +44,9 @@ struct TBDGenOptions {
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/// Whether LLVM IR Virtual Function Elimination is enabled.
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bool VirtualFunctionElimination = false;
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/// Whether LLVM IR Witness Method Elimination is enabled.
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bool WitnessMethodElimination = false;
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/// The install_name to use in the TBD file.
4851
std::string InstallName;
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@@ -74,6 +77,7 @@ struct TBDGenOptions {
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lhs.LinkerDirectivesOnly == rhs.LinkerDirectivesOnly &&
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lhs.PublicSymbolsOnly == rhs.PublicSymbolsOnly &&
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lhs.VirtualFunctionElimination == rhs.VirtualFunctionElimination &&
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lhs.WitnessMethodElimination == rhs.WitnessMethodElimination &&
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lhs.InstallName == rhs.InstallName &&
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lhs.ModuleLinkName == rhs.ModuleLinkName &&
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lhs.CurrentVersion == rhs.CurrentVersion &&
@@ -91,6 +95,7 @@ struct TBDGenOptions {
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return hash_combine(
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opts.HasMultipleIGMs, opts.IsInstallAPI, opts.LinkerDirectivesOnly,
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opts.PublicSymbolsOnly, opts.VirtualFunctionElimination,
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opts.WitnessMethodElimination,
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opts.InstallName, opts.ModuleLinkName,
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opts.CurrentVersion, opts.CompatibilityVersion,
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opts.ModuleInstallNameMapPath,

‎lib/Frontend/CompilerInvocation.cpp

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Original file line numberDiff line numberDiff line change
@@ -1542,6 +1542,7 @@ static bool ParseTBDGenArgs(TBDGenOptions &Opts, ArgList &Args,
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Opts.IsInstallAPI = Args.hasArg(OPT_tbd_is_installapi);
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Opts.VirtualFunctionElimination = Args.hasArg(OPT_enable_llvm_vfe);
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Opts.WitnessMethodElimination = Args.hasArg(OPT_enable_llvm_wme);
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if (const Arg *A = Args.getLastArg(OPT_tbd_compatibility_version)) {
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Opts.CompatibilityVersion = A->getValue();

‎lib/IRGen/GenMeta.cpp

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -824,7 +824,7 @@ namespace {
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SILDeclRef func(entry.getFunction());
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// Emit the dispatch thunk.
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if (Resilient)
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if (Resilient || IGM.getOptions().WitnessMethodElimination)
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IGM.emitDispatchThunk(func);
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// Classify the function.

‎lib/IRGen/GenProto.cpp

Lines changed: 7 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -2193,8 +2193,13 @@ static void addWTableTypeMetadata(IRGenModule &IGM,
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break;
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case SILLinkage::Public:
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default:
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global->setVCallVisibilityMetadata(
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llvm::GlobalObject::VCallVisibility::VCallVisibilityPublic);
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if (IGM.getOptions().InternalizeAtLink) {
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global->setVCallVisibilityMetadata(
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llvm::GlobalObject::VCallVisibility::VCallVisibilityLinkageUnit);
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} else {
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global->setVCallVisibilityMetadata(
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llvm::GlobalObject::VCallVisibility::VCallVisibilityPublic);
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}
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break;
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}
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}

‎lib/IRGen/IRGenSIL.cpp

Lines changed: 12 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -6568,8 +6568,18 @@ void IRGenSILFunction::visitWitnessMethodInst(swift::WitnessMethodInst *i) {
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65696569
assert(member.requiresNewWitnessTableEntry());
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6571-
if (IGM.isResilient(conformance.getRequirement(),
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ResilienceExpansion::Maximal)) {
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bool shouldUseDispatchThunk = false;
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if (IGM.isResilient(conformance.getRequirement(), ResilienceExpansion::Maximal)) {
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shouldUseDispatchThunk = true;
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} else if (IGM.getOptions().WitnessMethodElimination) {
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// For WME, use a thunk if the target protocol is defined in another module.
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// This way, we guarantee all wmethod call sites are visible to the LLVM VFE
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// optimization in GlobalDCE.
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auto protoDecl = cast<ProtocolDecl>(member.getDecl()->getDeclContext());
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shouldUseDispatchThunk = protoDecl->getModuleContext() != IGM.getSwiftModule();
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}
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6582+
if (shouldUseDispatchThunk) {
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llvm::Constant *fnPtr = IGM.getAddrOfDispatchThunk(member, NotForDefinition);
65746584
llvm::Constant *secondaryValue = nullptr;
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‎lib/TBDGen/TBDGen.cpp

Lines changed: 1 addition & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1077,7 +1077,7 @@ void TBDGenVisitor::visitProtocolDecl(ProtocolDecl *PD) {
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: TBD(TBD), PD(PD), Resilient(PD->getParentModule()->isResilient()) {}
10781078

10791079
void addMethod(SILDeclRef declRef) {
1080-
if (Resilient) {
1080+
if (Resilient || TBD.Opts.WitnessMethodElimination) {
10811081
TBD.addDispatchThunk(declRef);
10821082
TBD.addMethodDescriptor(declRef);
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}

‎test/IRGen/witness-method-elimination-ir-thunks.swift

Lines changed: 46 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,46 @@
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// Tests that under -enable-llvm-wme, protocol witness table calls to protocols
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// defined by other modules are using thunks (instead of direct wtable loads).
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// RUN: %empty-directory(%t)
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// RUN: %target-build-swift -Xfrontend -enable-llvm-wme -parse-as-library %s -DLIBRARY -module-name Library -emit-module -o %t/Library.swiftmodule
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// RUN: %target-build-swift -Xfrontend -enable-llvm-wme -parse-as-library %s -DCLIENT -module-name Main -I%t -emit-ir -o - | %FileCheck %s
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#if LIBRARY
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public protocol MyLibraryProtocol {
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func library_req()
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}
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#endif
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#if CLIENT
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import Library
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public protocol MyLocalProtocol {
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func local_req()
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}
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extension MyLocalProtocol {
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func func1() {
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// CHECK: define hidden swiftcc void @"$s4Main15MyLocalProtocolPAAE5func1yyF"
27+
self.local_req()
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// CHECK: [[SLOT:%.*]] = getelementptr inbounds i8*, i8** {{.*}}, i32 1
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// CHECK: [[SLOTASPTR:%.*]] = bitcast i8** [[SLOT]] to i8*
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// CHECK: call { i8*, i1 } @llvm.type.checked.load(i8* [[SLOTASPTR]], i32 0, metadata !"$s4Main15MyLocalProtocolP9local_reqyyFTq")
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// CHECK: ret void
32+
}
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}
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extension MyLibraryProtocol {
36+
func func2() {
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// CHECK: define hidden swiftcc void @"$s7Library02MyA8ProtocolP4MainE5func2yyF"
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self.library_req()
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// CHECK: call swiftcc void @"$s7Library02MyA8ProtocolP11library_reqyyFTj"
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// CHECK-NOT: @llvm.type.checked.load
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// CHECK: ret void
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}
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}
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#endif

‎test/IRGen/witness-method-elimination-two-modules.swift

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,72 @@
1+
// Tests that under -enable-llvm-wme + -internalize-at-link, cross-module
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// witness method calls are done via thunks and LLVM GlobalDCE is able to remove
3+
// unused witness methods from a library based on a list of used symbols by a
4+
// client.
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6+
// RUN: %empty-directory(%t)
7+
8+
// (1) Build library swiftmodule
9+
// RUN: %target-build-swift -parse-as-library -Xfrontend -enable-llvm-wme \
10+
// RUN: %s -DLIBRARY -module-name Library \
11+
// RUN: -emit-module -o %t/Library.swiftmodule \
12+
// RUN: -emit-tbd -emit-tbd-path %t/libLibrary.tbd -Xfrontend -tbd-install_name=%t/libLibrary.dylib
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14+
// (2) Build client
15+
// RUN: %target-build-swift -parse-as-library -Xfrontend -enable-llvm-wme \
16+
// RUN: %s -DCLIENT -module-name Main -I%t -L%t -lLibrary -o %t/main
17+
18+
// (3) Extract a list of used symbols by client from library
19+
// RUN: %llvm-nm --undefined-only -m %t/main | grep 'from libLibrary' | awk '{print $3}' > %t/used-symbols
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21+
// (4) Now produce the .dylib with just the symbols needed by the client
22+
// RUN: %target-build-swift -parse-as-library -Xfrontend -enable-llvm-wme -Xfrontend -internalize-at-link \
23+
// RUN: %s -DLIBRARY -lto=llvm-full %lto_flags -module-name Library \
24+
// RUN: -emit-library -o %t/libLibrary.dylib \
25+
// RUN: -Xlinker -exported_symbols_list -Xlinker %t/used-symbols -Xlinker -dead_strip
26+
27+
// (5) Check list of symbols in library
28+
// RUN: %llvm-nm --defined-only %t/libLibrary.dylib | %FileCheck %s --check-prefix=NM
29+
30+
// (6) Execution test
31+
// RUN: %target-run %t/main | %FileCheck %s
32+
33+
// REQUIRES: executable_test
34+
35+
// Test disabled until LLVM GlobalDCE supports Swift wtables.
36+
// REQUIRES: rdar81868930
37+
38+
#if LIBRARY
39+
40+
public protocol MyProtocol {
41+
func func1_used()
42+
func func2_unused()
43+
}
44+
45+
public struct MyStruct : MyProtocol {
46+
public init() {}
47+
public func func1_used() { print("MyStruct.func1_used") }
48+
public func func2_unused() { print("MyStruct.func2_unused") }
49+
}
50+
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// NM: $s7Library8MyStructV10func1_usedyyF
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// NM-NOT: $s7Library8MyStructV12func2_unusedyyF
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// NM: $s7Library8MyStructVAA0B8ProtocolA2aDP10func1_usedyyFTW
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// NM-NOT: $s7Library8MyStructVAA0B8ProtocolA2aDP12func2_unusedyyFTW
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#endif
57+
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#if CLIENT
59+
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import Library
61+
62+
@_cdecl("main")
63+
func main() -> Int32 {
64+
let o: MyProtocol = MyStruct()
65+
o.func1_used()
66+
print("Done")
67+
// CHECK: MyStruct.func1_used
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// CHECK-NEXT: Done
69+
return 0
70+
}
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72+
#endif

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