[lld-macho] Add support for -add_empty_section

[keith]

Summary

This is a ld64 option equivalent to -sectcreate seg sect /dev/null
that's useful for creating sections like the RESTRICT section.

[int3]

From the ld64 manpage:

-add_empty_section segname sectname
           An empty section named sectname in the segment segname. If any of the inputs contains a section (segname,sectname), that section will be included in the output, and this option will be ignored.

Can we add a test for the case where the inputs also contain a section of the same name?

lld/test/MachO/sectcreate.s

25

nit: I believe the double-underscore prefix is intended to be reserved for "special" section names, and user-created sections should just have regular un-prefixed names

[keith]

Improve test

[keith]

Add overlapping section

[keith]

Ah nice call, I added another case where it overlaps with sectcreate, and added one where it overlaps with DATA,data

[int3]

Thanks!

lld/MachO/Driver.cpp

for (const Arg *arg : args.filtered(OPT_sectcreate)) {
       StringRef segName = arg->getValue(0);
       StringRef sectName = arg->getValue(1);
       StringRef fileName = arg->getValue(2);
       Optional<MemoryBufferRef> buffer = readFile(fileName);
       if (buffer)
         inputFiles.insert(make<OpaqueFile>(*buffer, segName, sectName));
     }
 
+    for (const Arg *arg : args.filtered(OPT_add_empty_section)) {
+      StringRef segName = arg->getValue(0);
+      StringRef sectName = arg->getValue(1);
+      inputFiles.insert(make<OpaqueFile>(MemoryBufferRef(), segName, sectName));
+    }
+
     gatherInputSections();
     if (config->callGraphProfileSort)
       extractCallGraphProfile();
 
     if (config->deadStrip)
       markLive();
 
     // ICF assumes that all literals have been folded already, so we must run

lld/MachO/Options.td

def sectcreate : MultiArg<["-"], "sectcreate", 3>,
     MetaVarName<"<segment> <section> <file>">,
     HelpText<"Create <section> in <segment> from the contents of <file>">,
     Group<grp_content>;
 def segcreate : MultiArg<["-"], "segcreate", 3>,
     MetaVarName<"<segment> <section> <file>">,
     Alias<sectcreate>,
     HelpText<"Alias for -sectcreate">,
     Group<grp_content>;
+def add_empty_section : MultiArg<["-"], "add_empty_section", 2>,
+    MetaVarName<"<segment> <section>">,
+    HelpText<"Create an empty <section> in <segment>">,
+    Group<grp_content>;
 def filelist : Separate<["-"], "filelist">,
     MetaVarName<"<file>">,
     HelpText<"Read names of files to link from <file>">,
     Group<grp_content>;
 def dtrace : Separate<["-"], "dtrace">,
     MetaVarName<"<script>">,
     HelpText<"Enable DTrace static probes according to declarations in <script>">,
     Flags<[HelpHidden]>,

lld/test/MachO/sectcreate.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t.o
 # RUN: echo "-sectcreate 1.1" >%t1
 # RUN: echo "-sectcreate 1.2" >%t2
 # RUN: echo "-sectcreate 2" >%t3
 # RUN: %lld \
 # RUN:     -sectcreate SEG SEC1 %t1 \
 # RUN:     -segcreate SEG SEC2 %t3 \
 # RUN:     -sectcreate SEG SEC1 %t2 \
+# RUN:     -add_empty_section __DATA __data \
 # RUN:     -o %t %t.o
 # RUN: llvm-objdump -s %t | FileCheck %s
 
 ## -dead_strip does not strip -sectcreate sections,
 ## but also doesn't set S_ATTR_NO_DEAD_STRIP on them.
 # RUN: %lld -dead_strip \
 # RUN:     -sectcreate SEG SEC1 %t1 \
 # RUN:     -segcreate SEG SEC2 %t3 \
 # RUN:     -sectcreate SEG SEC1 %t2 \
+# RUN:     -add_empty_section SEG SEC1 \
 # RUN:     -o %t %t.o
 # RUN: llvm-objdump -s %t | FileCheck --check-prefix=STRIPPED %s
 # RUN: llvm-readobj --sections %t | FileCheck --check-prefix=STRIPPEDSEC %s
 
+# RUN: %lld -add_empty_section foo bar -o %t %t.o
+# RUN: llvm-readobj --sections %t | FileCheck --check-prefix=EMPTYSECTION %s
+
+# RUN: %lld -sectcreate SEG SEC1 %t1 -add_empty_section SEG SEC1 -o %t %t.o
+# RUN: llvm-readobj --sections %t | FileCheck --check-prefix=CREATEDANDEMPTY %s
+
 # CHECK: Contents of section __TEXT,__text:
 # CHECK: Contents of section __DATA,__data:
 # CHECK: my string!.
 # CHECK: Contents of section SEG,SEC1:
 # CHECK: -sectcreate 1.1.
 # CHECK: -sectcreate 1.2.
 # CHECK: Contents of section SEG,SEC2:
 # CHECK: -sectcreate 2.
 
 # STRIPPED: Contents of section __TEXT,__text:
 # STRIPPED-NOT: Contents of section __DATA,__data:
 # STRIPPED-NOT: my string!.
 # STRIPPED: Contents of section SEG,SEC1:
 # STRIPPED: -sectcreate 1.1.
 # STRIPPED: -sectcreate 1.2.
 # STRIPPED: Contents of section SEG,SEC2:
 # STRIPPED: -sectcreate 2.
 
 # STRIPPEDSEC-NOT: NoDeadStrip
 
+# EMPTYSECTION: Name: bar
+# EMPTYSECTION: Segment: foo
+# EMPTYSECTION: Size: 0x0
+# EMPTYSECTION-NOT: Name:
+
+# CREATEDANDEMPTY: Name: SEC1
+# CREATEDANDEMPTY: Segment: SEG
+# CREATEDANDEMPTY: Size: 0x10
+# CREATEDANDEMPTY-NOT: Name:
+
 .text
 .global _main
 _main:
   mov $0, %eax
   ret
 
 .data
 .global my_string
 my_string:
   .string "my string!"
 
 .subsections_via_symbols