[CodeGen] Add a flag to disable emitting block signature strings (llvm#96944)

Users who don't need the signature string to be emitted can use the flag
to reduce code size.

rdar://121933818

Author: ahatanak

clang/docs/ReleaseNotes.rst

@@ -445,6 +445,11 @@ New Compiler Flags
 - ``-Wc++2c-compat`` group was added to help migrating existing codebases
   to upcoming C++26.
 
+- ``-fdisable-block-signature-string`` instructs clang not to emit the signature
+  string for blocks. Disabling the string can potentially break existing code
+  that relies on it. Users should carefully consider this possibiilty when using
+  the flag.
+
 Deprecated Compiler Flags
 -------------------------
 

clang/docs/UsersManual.rst

@@ -913,6 +913,12 @@ Clang options that don't fit neatly into other categories.
   binary compatibility issues on older x86_64 targets, however, so use it with
   caution.
 
+.. option:: -fdisable-block-signature-string
+
+  Instruct clang not to emit the signature string for blocks. Disabling the
+  string can potentially break existing code that relies on it. Users should
+  carefully consider this possibiilty when using the flag.
+
 .. _configuration-files:
 
 Configuration files

clang/include/clang/Basic/CodeGenOptions.def

@@ -188,6 +188,7 @@ CODEGENOPT(NoImplicitFloat   , 1, 0) ///< Set when -mno-implicit-float is enable
 CODEGENOPT(NullPointerIsValid , 1, 0) ///< Assume Null pointer deference is defined.
 CODEGENOPT(OpenCLCorrectlyRoundedDivSqrt, 1, 0) ///< -cl-fp32-correctly-rounded-divide-sqrt
 CODEGENOPT(HIPCorrectlyRoundedDivSqrt, 1, 1) ///< -fno-hip-fp32-correctly-rounded-divide-sqrt
+CODEGENOPT(DisableBlockSignatureString, 1, 0) ///< Set when -fdisable-block-signature-string is enabled.
 CODEGENOPT(HIPSaveKernelArgName, 1, 0) ///< Set when -fhip-kernel-arg-name is enabled.
 CODEGENOPT(UniqueInternalLinkageNames, 1, 0) ///< Internal Linkage symbols get unique names.
 CODEGENOPT(SplitMachineFunctions, 1, 0) ///< Split machine functions using profile information.

clang/include/clang/Driver/Options.td

@@ -3477,6 +3477,11 @@ defm objc_avoid_heapify_local_blocks : BoolFOption<"objc-avoid-heapify-local-blo
   PosFlag<SetTrue, [], [ClangOption], "Try">,
   NegFlag<SetFalse, [], [ClangOption], "Don't try">,
   BothFlags<[], [CC1Option], " to avoid heapifying local blocks">>;
+defm disable_block_signature_string : BoolFOption<"disable-block-signature-string",
+  CodeGenOpts<"DisableBlockSignatureString">, DefaultFalse,
+  PosFlag<SetTrue, [], [ClangOption], "Disable">,
+  NegFlag<SetFalse, [], [ClangOption], "Don't disable">,
+  BothFlags<[], [CC1Option], " block signature string)">>;
 
 def fomit_frame_pointer : Flag<["-"], "fomit-frame-pointer">, Group<f_Group>,
   Visibility<[ClangOption, FlangOption]>,

clang/lib/CodeGen/CGBlocks.cpp

@@ -121,11 +121,15 @@ static std::string getBlockDescriptorName(const CGBlockInfo &BlockInfo,
     Name += "_";
   }
 
-  std::string TypeAtEncoding =
-      CGM.getContext().getObjCEncodingForBlock(BlockInfo.getBlockExpr());
-  /// Replace occurrences of '@' with '\1'. '@' is reserved on ELF platforms as
-  /// a separator between symbol name and symbol version.
-  std::replace(TypeAtEncoding.begin(), TypeAtEncoding.end(), '@', '\1');
+  std::string TypeAtEncoding;
+
+  if (!CGM.getCodeGenOpts().DisableBlockSignatureString) {
+    TypeAtEncoding =
+        CGM.getContext().getObjCEncodingForBlock(BlockInfo.getBlockExpr());
+    /// Replace occurrences of '@' with '\1'. '@' is reserved on ELF platforms
+    /// as a separator between symbol name and symbol version.
+    std::replace(TypeAtEncoding.begin(), TypeAtEncoding.end(), '@', '\1');
+  }
   Name += "e" + llvm::to_string(TypeAtEncoding.size()) + "_" + TypeAtEncoding;
   Name += "l" + CGM.getObjCRuntime().getRCBlockLayoutStr(CGM, BlockInfo);
   return Name;
@@ -201,9 +205,13 @@ static llvm::Constant *buildBlockDescriptor(CodeGenModule &CGM,
   }
 
   // Signature.  Mandatory ObjC-style method descriptor @encode sequence.
-  std::string typeAtEncoding =
-    CGM.getContext().getObjCEncodingForBlock(blockInfo.getBlockExpr());
-  elements.add(CGM.GetAddrOfConstantCString(typeAtEncoding).getPointer());
+  if (CGM.getCodeGenOpts().DisableBlockSignatureString) {
+    elements.addNullPointer(i8p);
+  } else {
+    std::string typeAtEncoding =
+        CGM.getContext().getObjCEncodingForBlock(blockInfo.getBlockExpr());
+    elements.add(CGM.GetAddrOfConstantCString(typeAtEncoding).getPointer());
+  }
 
   // GC layout.
   if (C.getLangOpts().ObjC) {
@@ -814,7 +822,8 @@ llvm::Value *CodeGenFunction::EmitBlockLiteral(const CGBlockInfo &blockInfo) {
     descriptor = buildBlockDescriptor(CGM, blockInfo);
 
     // Compute the initial on-stack block flags.
-    flags = BLOCK_HAS_SIGNATURE;
+    if (!CGM.getCodeGenOpts().DisableBlockSignatureString)
+      flags = BLOCK_HAS_SIGNATURE;
     if (blockInfo.HasCapturedVariableLayout)
       flags |= BLOCK_HAS_EXTENDED_LAYOUT;
     if (blockInfo.NeedsCopyDispose)
@@ -1300,7 +1309,9 @@ static llvm::Constant *buildGlobalBlock(CodeGenModule &CGM,
       fields.add(CGM.getNSConcreteGlobalBlock());
 
     // __flags
-    BlockFlags flags = BLOCK_IS_GLOBAL | BLOCK_HAS_SIGNATURE;
+    BlockFlags flags = BLOCK_IS_GLOBAL;
+    if (!CGM.getCodeGenOpts().DisableBlockSignatureString)
+      flags |= BLOCK_HAS_SIGNATURE;
     if (blockInfo.UsesStret)
       flags |= BLOCK_USE_STRET;
 

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5751,6 +5751,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   Args.AddLastArg(CmdArgs, options::OPT_fexperimental_omit_vtable_rtti,
                   options::OPT_fno_experimental_omit_vtable_rtti);
 
+  Args.AddLastArg(CmdArgs, options::OPT_fdisable_block_signature_string,
+                  options::OPT_fno_disable_block_signature_string);
+
   // Handle segmented stacks.
   Args.addOptInFlag(CmdArgs, options::OPT_fsplit_stack,
                     options::OPT_fno_split_stack);

clang/test/CodeGen/blocks.c

@@ -1,7 +1,14 @@
-// RUN: %clang_cc1 -triple i386-unknown-unknown %s -emit-llvm -Wno-strict-prototypes -o - -fblocks | FileCheck %s
+// RUN: %clang_cc1 -triple i386-unknown-unknown %s -emit-llvm -Wno-strict-prototypes -o - -fblocks | FileCheck --check-prefix=CHECK --check-prefix=SIG_STR %s
+// RUN: %clang_cc1 -triple i386-unknown-unknown %s -emit-llvm -Wno-strict-prototypes -o - -fblocks -fdisable-block-signature-string | FileCheck --check-prefix=CHECK --check-prefix=NO_SIG_STR %s
 
-// CHECK: @{{.*}} = internal constant { i32, i32, ptr, ptr, ptr, ptr } { i32 0, i32 24, ptr @__copy_helper_block_4_20r, ptr @__destroy_helper_block_4_20r, ptr @{{.*}}, ptr null }, align 4
-// CHECK: @[[BLOCK_DESCRIPTOR_TMP21:.*]] = internal constant { i32, i32, ptr, ptr, ptr, ptr } { i32 0, i32 24, ptr @__copy_helper_block_4_20r, ptr @__destroy_helper_block_4_20r, ptr @{{.*}}, ptr null }, align 4
+// SIG_STR: @[[STR:.*]] = private unnamed_addr constant [6 x i8] c"v4@?0\00", align 1
+// SIG_STR: @{{.*}} = internal constant { ptr, i32, i32, ptr, ptr } { ptr @_NSConcreteGlobalBlock, i32 1342177280, i32 0, ptr @f_block_invoke, ptr @{{.*}} }, align 4
+// NO_SIG_STR: @{{.*}} = internal constant { ptr, i32, i32, ptr, ptr } { ptr @_NSConcreteGlobalBlock, i32 268435456, i32 0, ptr @f_block_invoke, ptr @{{.*}} }, align 4
+
+// SIG_STR: @{{.*}} = internal constant { i32, i32, ptr, ptr, ptr, ptr } { i32 0, i32 24, ptr @__copy_helper_block_4_20r, ptr @__destroy_helper_block_4_20r, ptr @[[STR]], ptr null }, align 4
+// SIG_STR: @[[BLOCK_DESCRIPTOR_TMP21:.*]] = internal constant { i32, i32, ptr, ptr, ptr, ptr } { i32 0, i32 24, ptr @__copy_helper_block_4_20r, ptr @__destroy_helper_block_4_20r, ptr @[[STR]], ptr null }, align 4
+// NO_SIG_STR: @{{.*}} = internal constant { i32, i32, ptr, ptr, ptr, ptr } { i32 0, i32 24, ptr @__copy_helper_block_4_20r, ptr @__destroy_helper_block_4_20r, ptr null, ptr null }, align 4
+// NO_SIG_STR: @[[BLOCK_DESCRIPTOR_TMP21:.*]] = internal constant { i32, i32, ptr, ptr, ptr, ptr } { i32 0, i32 24, ptr @__copy_helper_block_4_20r, ptr @__destroy_helper_block_4_20r, ptr null, ptr null }, align 4
 
 void (^f)(void) = ^{};
 

clang/test/CodeGenObjC/blocks.m

@@ -1,9 +1,13 @@
-// RUN: %clang_cc1 -triple i386-apple-darwin9 -fobjc-runtime=macosx-fragile-10.5 -emit-llvm -fblocks -Wno-strict-prototypes -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple i386-apple-darwin9 -fobjc-runtime=macosx-fragile-10.5 -emit-llvm -fblocks -Wno-strict-prototypes -o - %s | FileCheck --check-prefix=CHECK --check-prefix=SIG_STR %s
+// RUN: %clang_cc1 -triple i386-apple-darwin9 -fobjc-runtime=macosx-fragile-10.5 -emit-llvm -fblocks -Wno-strict-prototypes -fdisable-block-signature-string -o - %s | FileCheck --check-prefix=CHECK --check-prefix=NO_SIG_STR %s
 
 // Check that there is only one capture (20o) in the copy/dispose function
 // names.
 
-// CHECK: @[[BLOCK_DESCRIPTOR0:.*]] = linkonce_odr hidden unnamed_addr constant { i32, i32, ptr, ptr, ptr, i32 } { i32 0, i32 28, ptr @__copy_helper_block_4_20o, ptr @__destroy_helper_block_4_20o, ptr @{{.*}}, i32 512 },
+// SIG_STR: @[[STR3:.*]] = private unnamed_addr constant [6 x i8] c"v4@?0\00", align 1
+// SIG_STR: @[[BLOCK_DESCRIPTOR0:"__block_descriptor_28_4_20o_e5_v4\\01\?0l"]] = linkonce_odr hidden unnamed_addr constant { i32, i32, ptr, ptr, ptr, i32 } { i32 0, i32 28, ptr @__copy_helper_block_4_20o, ptr @__destroy_helper_block_4_20o, ptr @[[STR3]], i32 512 },
+// NO_SIG_STR: @[[BLOCK_DESCRIPTOR0:__block_descriptor_28_4_20o_e0_l]] = linkonce_odr hidden unnamed_addr constant { i32, i32, ptr, ptr, ptr, i32 } { i32 0, i32 28, ptr @__copy_helper_block_4_20o, ptr @__destroy_helper_block_4_20o, ptr null, i32 512 },
+
 
 void (^gb0)(void);
 
@@ -132,6 +136,9 @@ void test5(A *a) {
 }
 
 // CHECK-LABEL: define void @test5(
+// CHECK: %[[FLAGS:.*]] = getelementptr inbounds <{ ptr, i32, i32, ptr, ptr, ptr, ptr }>, ptr %{{.*}}, i32 0, i32 1
+// SIG_STR: store i32 -1040187392, ptr %[[FLAGS]], align 4
+// NO_SIG_STR: store i32 -2113929216, ptr %[[FLAGS]], align 4
 // CHECK: %[[V0:.*]] = getelementptr inbounds <{ ptr, i32, i32, ptr, ptr, ptr, ptr }>, ptr %{{.*}}, i32 0, i32 4
 // CHECK: store ptr @[[BLOCK_DESCRIPTOR0]], ptr %[[V0]],