[llvm-objcopy][MachO] Remove more sections with llvm-bitcode-strip

[keith]

Summary

This adds the other potential bitcode sections that can exist and should
be stripped with -r from llvm-bitcode-strip.

[rmaz]

I see the producers of the sections here:

• https://github.com/llvm/llvm-project/blob/main/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp#L4910-L4938
• https://github.com/apple/swift/blob/91b2abe99686600cd18daa2e898d8376440fba9e/lib/IRGen/IRGen.cpp#L1131-L1140
• https://github.com/llvm/llvm-project/blob/main/clang/tools/driver/cc1as_main.cpp#L517-L524

The only one not totally clear is the __asm section, is this the only time that section is used?

[jhenderson]

Functionally, this looks to do what it says it does - I have no comments there. I don't know enough about either bitcode or Mach-O to identify the sections that should be removed though. Soft LGTM apart from that point, with someone with more knowledge needing to confirm it or otherwise.

[keith]

In D132267#3738697, @rmaz wrote:

I see the producers of the sections here:

• https://github.com/llvm/llvm-project/blob/main/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp#L4910-L4938
• https://github.com/apple/swift/blob/91b2abe99686600cd18daa2e898d8376440fba9e/lib/IRGen/IRGen.cpp#L1131-L1140
• https://github.com/llvm/llvm-project/blob/main/clang/tools/driver/cc1as_main.cpp#L517-L524

The only one not totally clear is the __asm section, is this the only time that section is used?

According to https://jonasdevlieghere.com/embedded-bitcode/ and https://lists.llvm.org/pipermail/llvm-dev/2016-June/101019.html __LLVM,__asm is just a way to know that the source was built directly from assembly. I included it given this logic from ld64 https://github.com/keith/ld64/blob/eddf3fd275669887e70392a1f7d8355222fa295e/src/ld/parsers/macho_relocatable_file.cpp#L2631-L2643 and it looks like ld64 uses this knowledge for some logic https://github.com/keith/ld64/blob/eddf3fd275669887e70392a1f7d8355222fa295e/src/ld/passes/bitcode_bundle.cpp#L662-L664

Either way I think it's safe to remove as Apple's bitcode_strip does as well

[alexander-shaposhnikov]

LG, thanks!

llvm/test/tools/llvm-objcopy/MachO/bitcode-strip-object.s

+# REQUIRES: x86-registered-target
+
+## Verify llvm-bitcode-strip removes sections from object files
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t
+# RUN: llvm-bitcode-strip -r %t -o %t2
+# RUN: llvm-readobj --macho-segment --sections %t2 | FileCheck --implicit-check-not=__LLVM %s
+
+# CHECK:      Name: __text
+# CHECK-NEXT: Segment: __TEXT
+
+.section __LLVM,__bundle
+  .asciz "test"
+
+.section __LLVM,__bitcode
+  .asciz "test"
+
+.section __LLVM,__cmdline
+  .asciz "test"
+
+.section __LLVM,__swift_cmdline
+  .asciz "test"
+
+.section __LLVM,__asm
+  .asciz "test"
+
+.text
+.globl _main
+_main:
+  ret

llvm/tools/llvm-objcopy/ObjcopyOptions.cpp

objcopy::parseBitcodeStripOptions(ArrayRef<const char *> ArgsArr,
   Config.OutputFilename = InputArgs.getLastArgValue(BITCODE_STRIP_output);
 
   if (!InputArgs.hasArg(BITCODE_STRIP_remove))
     return createStringError(errc::invalid_argument, "no action specified");
 
   // We only support -r for now, which removes all bitcode sections and
   // the __LLVM segment if it's now empty.
   cantFail(Config.ToRemove.addMatcher(NameOrPattern::create(
+      "__LLVM,__asm", MatchStyle::Literal, ErrorCallback)));
+  cantFail(Config.ToRemove.addMatcher(NameOrPattern::create(
+      "__LLVM,__bitcode", MatchStyle::Literal, ErrorCallback)));
+  cantFail(Config.ToRemove.addMatcher(NameOrPattern::create(
       "__LLVM,__bundle", MatchStyle::Literal, ErrorCallback)));
+  cantFail(Config.ToRemove.addMatcher(NameOrPattern::create(
+      "__LLVM,__cmdline", MatchStyle::Literal, ErrorCallback)));
+  cantFail(Config.ToRemove.addMatcher(NameOrPattern::create(
+      "__LLVM,__swift_cmdline", MatchStyle::Literal, ErrorCallback)));
   MachOConfig.EmptySegmentsToRemove.insert("__LLVM");
 
   DC.CopyConfigs.push_back(std::move(ConfigMgr));
   return std::move(DC);
 }
 
 // parseStripOptions returns the config and sets the input arguments. If a
 // help flag is set then parseStripOptions will print the help messege and