[clang] Limit bitcode option ignorelist to Darwin

[MichielDerhaeg]

Summary

This ignore list prevents embedding of bitcode for some platforms
without jumping through some hoops as several of the flags are passed by default.
E.g. On Android, since NDK 21, -ffunction-sections, -fdata-sections,
-mstackrealign, etc.. are added by default.

The ignore list is limited to Apple-based platforms.
Flags that are not expressed in some way in bitcode are now also passed to ensure they take effect.

[steven_wu]

LGTM. Can you add a test for your usecase?

[wanders]

clang/lib/Driver/ToolChains/Clang.cpp

4759–4760

This comment and variable name are not as accurate any longer. Maybe moving it inside if (RawTriple.isOSDarwin()) makes it clearer.

[MichielDerhaeg]

• Render options for other platforms and test
• comment and propagate list

[MichielDerhaeg]

clang/lib/Driver/ToolChains/Clang.cpp

4759–4760

Changed the comment to what I could derive from D61627: [clang driver] Allow -fembed-bitcode combined with -mno-red-zone.

[wanders]

This looks sensible to me. But I'm not an expert in the area (and still not 100% clear on the purpose of the ignorelist) so can't really say if the added comments captures the intent accurately.

clang/test/Driver/embed-bitcode-elf.c

1

Should this test have a // REQUIRES: aarch64-registered-target?

[MichielDerhaeg]

• add target requirement to lit test
• move condition

[steven_wu]

Thanks! LGTM.

clang/lib/Driver/ToolChains/Clang.cpp

if (C.getDriver().embedBitcodeInObject() && !IsUsingLTO &&
       (isa<BackendJobAction>(JA) || isa<AssembleJobAction>(JA))) {
     // Add flags implied by -fembed-bitcode.
     Args.AddLastArg(CmdArgs, options::OPT_fembed_bitcode_EQ);
     // Disable all llvm IR level optimizations.
     CmdArgs.push_back("-disable-llvm-passes");
 
     // Render target options.
     TC.addClangTargetOptions(Args, CmdArgs, JA.getOffloadingDeviceKind());
 
-    // reject options that shouldn't be supported in bitcode
-    // also reject kernel/kext
+    for (const auto &A : Args) {
+      // For Apple platforms, the following options are disallowed to limit the
+      // ability to change the backend behaviour and ABI. This is to ensure the
+      // bitcode can be retargeted to certain architectures.
       static const constexpr unsigned kBitcodeOptionIgnorelist[] = {
           options::OPT_mkernel,
           options::OPT_fapple_kext,
           options::OPT_ffunction_sections,
           options::OPT_fno_function_sections,
           options::OPT_fdata_sections,
           options::OPT_fno_data_sections,
           options::OPT_fbasic_block_sections_EQ,
           options::OPT_funique_internal_linkage_names,
           options::OPT_fno_unique_internal_linkage_names,
           options::OPT_funique_section_names,
           options::OPT_fno_unique_section_names,
           options::OPT_funique_basic_block_section_names,
           options::OPT_fno_unique_basic_block_section_names,
           options::OPT_mrestrict_it,
           options::OPT_mno_restrict_it,
           options::OPT_mstackrealign,
           options::OPT_mno_stackrealign,
           options::OPT_mstack_alignment,
           options::OPT_mcmodel_EQ,
           options::OPT_mlong_calls,
           options::OPT_mno_long_calls,
           options::OPT_ggnu_pubnames,
           options::OPT_gdwarf_aranges,
           options::OPT_fdebug_types_section,
           options::OPT_fno_debug_types_section,
           options::OPT_fdwarf_directory_asm,
           options::OPT_fno_dwarf_directory_asm,
           options::OPT_mrelax_all,
           options::OPT_mno_relax_all,
           options::OPT_ftrap_function_EQ,
           options::OPT_ffixed_r9,
           options::OPT_mfix_cortex_a53_835769,
           options::OPT_mno_fix_cortex_a53_835769,
           options::OPT_ffixed_x18,
           options::OPT_mglobal_merge,
           options::OPT_mno_global_merge,
           options::OPT_mred_zone,
           options::OPT_mno_red_zone,
           options::OPT_Wa_COMMA,
           options::OPT_Xassembler,
           options::OPT_mllvm,
       };
-    for (const auto &A : Args)
-      if (llvm::is_contained(kBitcodeOptionIgnorelist, A->getOption().getID()))
+      if (RawTriple.isOSDarwin() &&
+          llvm::is_contained(kBitcodeOptionIgnorelist, A->getOption().getID()))
         D.Diag(diag::err_drv_unsupported_embed_bitcode) << A->getSpelling();
+    }
+
+    for (const auto &A : Args) {
+      // These options are not represented by any attribute in bitcode. We
+      // simply propagate these so that they take effect and are preserved in
+      // `.llvmcmd`.
+      static const constexpr unsigned bitcodeOptionPropagateList[] = {
+          options::OPT_ffunction_sections,
+          options::OPT_fno_function_sections,
+          options::OPT_fdata_sections,
+          options::OPT_fno_data_sections,
+          options::OPT_fbasic_block_sections_EQ,
+          options::OPT_funique_section_names,
+          options::OPT_fno_unique_section_names,
+          options::OPT_funique_basic_block_section_names,
+          options::OPT_fno_unique_basic_block_section_names,
+          options::OPT_mrestrict_it, // Wonder what this does.
+          options::OPT_mno_restrict_it,
+          options::OPT_gdwarf_aranges,
+          options::OPT_fdebug_types_section,
+          options::OPT_fno_debug_types_section,
+          options::OPT_fdwarf_directory_asm,
+          options::OPT_fno_dwarf_directory_asm,
+          options::OPT_mrelax_all,
+          options::OPT_mno_relax_all,
+          options::OPT_mglobal_merge,
+          options::OPT_mno_global_merge,
+          options::OPT_Wa_COMMA,
+          options::OPT_Xassembler,
+          options::OPT_mllvm,
+      };
+      if (llvm::is_contained(bitcodeOptionPropagateList,
+                             A->getOption().getID()))
+        A->render(Args, CmdArgs);
+    }
 
     // Render the CodeGen options that need to be passed.
     Args.addOptOutFlag(CmdArgs, options::OPT_foptimize_sibling_calls,
                        options::OPT_fno_optimize_sibling_calls);
 
     RenderFloatingPointOptions(TC, D, isOptimizationLevelFast(Args), Args,
                                CmdArgs, JA);
 

clang/test/Driver/embed-bitcode-elf.c

+// RUN: %clang -c -target aarch64-linux-android21 %s -fembed-bitcode -o %t.o \
+// RUN: -ffunction-sections -fdata-sections -fstack-protector-strong
+// RUN: llvm-readobj -S %t.o | FileCheck --check-prefix=CHECK %s
+// CHECK:   Name: .text.foo
+// CHECK:   Name: .llvmbc
+// CHECK:   Name: .llvmcmd
+//
+// RUN: llvm-objcopy %t.o --dump-section=.llvmcmd=%t.llvmcmd
+// RUN: FileCheck --check-prefix=CMD %s < %t.llvmcmd
+// CMD: ffunction-sections
+// CMD: fdata-sections
+
+int foo() { return 42; }