[llvm-objcopy] Add -o option to llvm-objcopy

[jakehehrlich]

Summary

llvm-objcopy needs to have an llvm-strip alias. The llvm-strip alias needs to be command line compatible with GNU strip. This change adds a -o option to llvm-objcopy to allow the user to specify the output file name using -o rather than a positional argument.

[jhenderson]

There's an issue here, based on my reading of the man page for strip. Strip takes a list of objects that are to be stripped, and modifies them in place. As things stand, this patch will mean llvm-strip a.o b.o will strip a.o and emit it as b.o, which is a potentially very confusing issue if trying to use it as a drop-in replacement. I don't mind if you don't at this point support multiple inputs, but I think that the implicit output needs to be illegal for llvm-strip.

Aside from that, this change looks fine to me.

[jakehehrlich]

In D41125#953571, @jhenderson wrote:

There's an issue here, based on my reading of the man page for strip. Strip takes a list of objects that are to be stripped, and modifies them in place. As things stand, this patch will mean llvm-strip a.o b.o will strip a.o and emit it as b.o, which is a potentially very confusing issue if trying to use it as a drop-in replacement. I don't mind if you don't at this point support multiple inputs, but I think that the implicit output needs to be illegal for llvm-strip.

Aside from that, this change looks fine to me.

Yikes! I didn't realize this worked this way. I think a more drastic change is required now. I'm looking at not using the symlink option because it seems like it would just be better if I had different options for each executable.

test/tools/llvm-objcopy/basic-strip.test

 # RUN: yaml2obj %s > %t
-# RUN: llvm-strip %t %t2
+# RUN: llvm-strip %t -o %t2
 # RUN: llvm-readobj -file-headers -sections %t2 | FileCheck %s
 
 !ELF
 FileHeader:
   Class:           ELFCLASS64
   Data:            ELFDATA2LSB
   Type:            ET_REL
   Machine:         EM_X86_64

test/tools/llvm-objcopy/explicit-out.test

+# RUN: yaml2obj %s > %t
+# RUN: llvm-objcopy %t -o %t2
+# RUN: llvm-objcopy %t %t3
+# RUN: diff %t2 %t3 
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_X86_64
+Sections:
+  - Name:            .bss
+    Type:            SHT_NOBITS
+    Flags:           [ SHF_ALLOC ]
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+  - Name:            .blarg
+    Type:            SHT_PROGBITS
+    Flags:           [ ]
+  - Name:            .gnu.warning.foo
+    Type:            SHT_PROGBITS

tools/llvm-objcopy/llvm-objcopy.cpp

void CopyConfig<ELFT>::copyBinary() {
   WriteObjectFile(*Obj, OutputFilename);
 }
 
 // This namespace hides our flags so that only our driver functions see them.
 namespace flags {
 
 static cl::opt<std::string> InputFilename(cl::Positional, cl::desc("<input>"));
 static cl::opt<std::string> OutputFilename(cl::Positional, cl::desc("<output>"));
+static cl::alias ExplicitOutput("o", cl::desc("file to output to"),
+                                cl::aliasopt(OutputFilename));
 static cl::opt<std::string>
     OutputFormat("O", cl::desc("Set output format to one of the following:"
                                "\n\tbinary\n\telf"), cl::init("elf"));
 static cl::list<std::string> ToRemove("remove-section",
                                       cl::desc("Remove <section>"),
                                       cl::value_desc("section"));
 static cl::alias ToRemoveA("R", cl::desc("Alias for remove-section"),
                            cl::aliasopt(ToRemove));