[LLD] [MinGW] Implement the --[no-]gc-sections and --icf options

[mstorsjo]

Summary

GNU ld doesn't seem to support --icf at all, but this was suggested in D39885, and GNU gold seems to support it.

[rnk]

MinGW/Driver.cpp

173–175

I think this might be a more idiomatic way to write this:

if (Args.hasFlag(OPT_gc_sections, OPT_no_gc_sections, false))
  Add("-opt:ref");
else
  Add("-opt:noref");

180

I don't think LLD has support for safe ICF. Safe ICF leveraged certain x86_32 ELF relocations that COFF lacks, and failing that, relies on knowledge about the Itanium C++ name mangling rules. LLD doesn't implement anything like that. LLD's ICF totally breaks function pointer identity. I would make --icf=all imply -opt:icf, otherwise -opt:noicf.

Also, this is likely to need changes in the near future if we make -opt:icf fold identical comdats of readonly data again. We'll need a way to say "only fold identical code".

[mstorsjo]

Map --icf=safe to -opt:noicf, use Args.hasFlag.

[rnk]

lgtm

[ruiu]

MinGW/Driver.cpp

172–187

Please add a blank line before a new if ~ else if ~ else block.

198

Don't you want to report an error for an unknown argument?

[mstorsjo]

MinGW/Driver.cpp

172–187

This is context from another one of the diffs that later was revised - I think it looks better now.

198

I guess that could be good as well, will update the diff.

[mstorsjo]

Error out on unknown --icf parameters.

[ruiu]

LGTM

MinGW/Driver.cpp

else
     Add("-dynamicbase:no");
   if (Args.hasArg(OPT_nxcompat))
     Add("-nxcompat");
   else
     Add("-nxcompat:no");
   if (Args.hasArg(OPT_tsaware))
     Add("-tsaware");
   else
     Add("-tsaware:no");
+  if (auto *A = Args.getLastArg(OPT_gc_sections, OPT_no_gc_sections))
+    Add(A->getOption().getID() == OPT_gc_sections ? "-opt:ref" : "-opt:noref");
+  else
+    Add("-opt:noref");
+
+  if (auto *A = Args.getLastArg(OPT_icf)) {
+    StringRef S = A->getValue();
+    if (S == "none")
+      Add("-opt:noicf");
+    else
+      Add("-opt:icf");
+  } else {
+    Add("-opt:noicf");
+  }
 
   if (auto *A = Args.getLastArg(OPT_m)) {
     StringRef S = A->getValue();
     if (S == "i386pe")
       Add("-machine:x86");
     else if (S == "i386pep")
       Add("-machine:x64");
     else if (S == "thumb2pe")
       Add("-machine:arm");
     else if (S == "arm64pe")
       Add("-machine:arm64");
     else
       error("unknown parameter: -m" + S);
   }
 
   for (auto *A : Args.filtered(OPT_mllvm))
     Add("-mllvm:" + StringRef(A->getValue()));
 
   for (auto *A : Args.filtered(OPT_Xlink))
     Add(A->getValue());

MinGW/Options.td

 include "llvm/Option/OptParser.td"
 
 class F<string name>: Flag<["--", "-"], name>;
 class J<string name>: Joined<["--", "-"], name>;
 class S<string name>: Separate<["--", "-"], name>;
 
 def L: JoinedOrSeparate<["-"], "L">, MetaVarName<"<dir>">,
   HelpText<"Add a directory to the library search path">;
 def dynamicbase: Flag<["--"], "dynamicbase">, HelpText<"Enable ASLR">;
 def entry: S<"entry">, MetaVarName<"<entry>">,
   HelpText<"Name of entry point symbol">;
 def export_all_symbols: F<"export-all-symbols">,
     HelpText<"Export all symbols even if a def file or dllexport attributes are used">;
+def gc_sections: F<"gc-sections">, HelpText<"Remove unused sections">;
+def icf: J<"icf=">, HelpText<"Identical code folding">;
 def image_base: S<"image-base">, HelpText<"Base address of the program">;
 def l: JoinedOrSeparate<["-"], "l">, MetaVarName<"<libName>">,
   HelpText<"Root name of library to use">;
 def m: JoinedOrSeparate<["-"], "m">, HelpText<"Set target emulation">;
 def no_whole_archive: F<"no-whole-archive">,
     HelpText<"No longer include all object files for following archives">;
 def large_address_aware: Flag<["--"], "large-address-aware">,
     HelpText<"Enable large addresses">;
+def no_gc_sections: F<"no-gc-sections">, HelpText<"Don't remove unused sections">;
 def nxcompat: Flag<["--"], "nxcompat">,
     HelpText<"Enable data execution prevention">;
 def o: JoinedOrSeparate<["-"], "o">, MetaVarName<"<path>">,
   HelpText<"Path to file to write output">;
 def out_implib: Separate<["--"], "out-implib">, HelpText<"Import library name">;
 def out_implib_eq: Joined<["--"], "out-implib=">, Alias<out_implib>;
 def output_def: S<"output-def">, HelpText<"Output def file">;
 def shared: F<"shared">, HelpText<"Build a shared object">;

test/MinGW/driver.test

 RUN: ld.lld -### -m i386pep foo.o --nxcompat | FileCheck -check-prefix NXCOMPAT %s
 NXCOMPAT: -nxcompat -
 
 RUN: ld.lld -### -m i386pep foo.o --tsaware | FileCheck -check-prefix TSAWARE %s
 TSAWARE: -tsaware -
 
 RUN: ld.lld -### -m i386pep foo.o --image-base 0x1230000 | FileCheck -check-prefix IMAGE-BASE %s
 IMAGE-BASE: -base:0x1230000
+
+RUN: ld.lld -### -m i386pep foo.o | FileCheck -check-prefix NO-GC-SECTIONS %s
+RUN: ld.lld -### -m i386pep foo.o --gc-sections --no-gc-sections | FileCheck -check-prefix NO-GC-SECTIONS %s
+NO-GC-SECTIONS: -opt:noref
+
+RUN: ld.lld -### -m i386pep foo.o --gc-sections | FileCheck -check-prefix GC-SECTIONS %s
+GC-SECTIONS: -opt:ref
+
+RUN: ld.lld -### -m i386pep foo.o | FileCheck -check-prefix ICF-NONE %s
+RUN: ld.lld -### -m i386pep foo.o --icf=none | FileCheck -check-prefix ICF-NONE %s
+ICF-NONE: -opt:noicf
+
+RUN: ld.lld -### -m i386pep foo.o --icf=all | FileCheck -check-prefix ICF %s
+RUN: ld.lld -### -m i386pep foo.o --icf=safe | FileCheck -check-prefix ICF %s
+ICF: -opt:icf