[lld-macho] Allow deduplicate-literals to be overridden

[thevinster]

Summary

It's still uncertain but whether we want to have deduplicate-literals be the
default flag for LLD out of the box or not. If deduplicate-literals is the default
behavior, then we will need a way override it and not deduplicate. Luckily, we
have no_deduplicate to fill this gap. For now, I've set the default to be false
which aligns with the existing behavior. That can only always be changed after
discussions on D117250.

[int3]

lgtm

[smeenai]

It's worth noting that -no_deduplicate in ld64 actually turns off ICF, not string merging. I don't know if ld64 even has a way to turn off string merging, since IIRC it's implemented in the symbol table itself.

lld/MachO/Driver.cpp

1285

I'm confused ... was the second argument meant to be OPT_no_deduplicate? The test seems to be passing though...

[int3]

lld/MachO/Driver.cpp

1285

-no_deduplicate aliases to setting OPT_icf_eq to false :)

[thevinster]

It's worth noting that -no_deduplicate in ld64 actually turns off ICF, not string merging. I don't know if ld64 even has a way to turn off string merging, since IIRC it's implemented in the symbol table itself.

Yup. LLD's -no_deduplicate also turns off ICF but it _also_ turns off string merging. It's probably worth documenting this as well.

[thevinster]

Update differences in rst

lld/MachO/Driver.cpp

bool macho::link(ArrayRef<const char *> argsArr, bool canExitEarly,
   config->demangle = args.hasArg(OPT_demangle);
   config->implicitDylibs = !args.hasArg(OPT_no_implicit_dylibs);
   config->emitFunctionStarts =
       args.hasFlag(OPT_function_starts, OPT_no_function_starts, true);
   config->emitBitcodeBundle = args.hasArg(OPT_bitcode_bundle);
   config->emitDataInCodeInfo =
       args.hasFlag(OPT_data_in_code_info, OPT_no_data_in_code_info, true);
   config->icfLevel = getICFLevel(args);
-  config->dedupLiterals = args.hasArg(OPT_deduplicate_literals) ||
-                          config->icfLevel != ICFLevel::none;
+  config->dedupLiterals =
+      args.hasFlag(OPT_deduplicate_literals, OPT_icf_eq, false) ||
   config->warnDylibInstallName = args.hasFlag(
       OPT_warn_dylib_install_name, OPT_no_warn_dylib_install_name, false);
   config->callGraphProfileSort = args.hasFlag(
       OPT_call_graph_profile_sort, OPT_no_call_graph_profile_sort, true);
   config->printSymbolOrder = args.getLastArgValue(OPT_print_symbol_order);
 
   // FIXME: Add a commandline flag for this too.
   config->zeroModTime = getenv("ZERO_AR_DATE");

lld/MachO/ld64-vs-lld.rst

 ==================
 LD64 vs LLD-MACHO
 ==================
 
 This doc lists all significant deliberate differences in behavior between LD64 and LLD-MachO.
 
+``-no_deduplicate`` Flag
+**********************
+- LD64:
+   * This turns off ICF (deduplication pass) in the linker.
+- LLD
+   * This turns off ICF and string merging in the linker.
+
 ObjC symbols treatment
 **********************
 There are differences in how LLD and LD64 handle ObjC symbols loaded from archives.
 
 - LD64:
    * Duplicate ObjC symbols from the same archives will not raise an error. LD64 will pick the first one.
    * Duplicate ObjC symbols from different archives will raise a "duplicate symbol" error.
 - LLD:
    * Duplicate symbols, regardless of which archives they are from, will raise errors.

lld/test/MachO/literal-dedup.s

 # REQUIRES: x86
 # RUN: rm -rf %t; split-file %s %t
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/test.s -o %t/test.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/qux.s -o %t/qux.o
 # RUN: %lld -dylib --deduplicate-literals %t/test.o %t/qux.o -o %t/test
 # RUN: llvm-objdump --macho --section="__TEXT,__literals" --section="__DATA,ptrs" --syms %t/test | FileCheck %s
 # RUN: llvm-readobj --section-headers %t/test | FileCheck %s --check-prefix=HEADER
 
+## Make sure literal deduplication can be overridden or that the later flag wins.
+# RUN: %lld -dylib --deduplicate-literals -no_deduplicate %t/test.o %t/qux.o -o %t/no-dedup-test
+# RUN: llvm-objdump --macho --section="__TEXT,__literals" --section="__DATA,ptrs" %t/no-dedup-test | FileCheck %s --check-prefix=NO-DEDUP
+
+# RUN: %lld -dylib -no_deduplicate --deduplicate-literals %t/test.o %t/qux.o -o %t/test
+# RUN: llvm-objdump --macho --section="__TEXT,__literals" --section="__DATA,ptrs" --syms %t/test | FileCheck %s
+# RUN: llvm-readobj --section-headers %t/test | FileCheck %s --check-prefix=HEADER
+
+# NO-DEDUP-NOT:  Contents of (__TEXT,__literals) section
+# NO-DEDUP:      Contents of (__DATA,ptrs) section
+# NO-DEDUP-NEXT: __TEXT:__literal16:0xdeadbeef 0xdeadbeef 0xdeadbeef 0xdeadbeef
+# NO-DEDUP-NEXT: __TEXT:__literal16:0xdeadbeef 0xdeadbeef 0xdeadbeef 0xdeadbeef
+# NO-DEDUP-NEXT: __TEXT:__literal16:0xfeedface 0xfeedface 0xfeedface 0xfeedface
+# NO-DEDUP-NEXT: __TEXT:__literal16:0xdeadbeef 0xdeadbeef 0xdeadbeef 0xdeadbeef
+# NO-DEDUP-NEXT: __TEXT:__literal8:0xdeadbeef 0xdeadbeef
+# NO-DEDUP-NEXT: __TEXT:__literal8:0xdeadbeef 0xdeadbeef
+# NO-DEDUP-NEXT: __TEXT:__literal8:0xfeedface 0xfeedface
+# NO-DEDUP-NEXT: __TEXT:__literal8:0xdeadbeef 0xdeadbeef
+# NO-DEDUP-NEXT: __TEXT:__literal4:0xdeadbeef
+# NO-DEDUP-NEXT: __TEXT:__literal4:0xdeadbeef
+# NO-DEDUP-NEXT: __TEXT:__literal4:0xfeedface
+# NO-DEDUP-NEXT: __TEXT:__literal4:0xdeadbeef
+
 # CHECK:      Contents of (__TEXT,__literals) section
 # CHECK-NEXT: [[#%.16x,DEADBEEF16:]] ef be ad de ef be ad de ef be ad de ef be ad de
 # CHECK-NEXT: [[#%.16x,FEEDFACE16:]] ce fa ed fe ce fa ed fe ce fa ed fe ce fa ed fe
 # CHECK-NEXT: [[#%.16x,DEADBEEF8:]]  ef be ad de ef be ad de ce fa ed fe ce fa ed fe
 # CHECK-NEXT: [[#%.16x,DEADBEEF4:]]  ef be ad de ce fa ed fe
 # CHECK-NEXT: Contents of (__DATA,ptrs) section
 # CHECK-NEXT: 0000000000001000  0x[[#%x,DEADBEEF16]]
 # CHECK-NEXT: 0000000000001008  0x[[#%x,DEADBEEF16]]

lld/test/MachO/silent-ignore.s

 # REQUIRES: x86
 
 ## Check that we correctly parse these flags, even though they are
 ## unimplemented. ## We may still emit warnings or errors for some of the
 ## unimplemented ones (but those ## errors are squelched because of the
 ## `--version` flag.)
 # RUN: %lld --version \
 # RUN:   -dynamic \
-# RUN:   -no_deduplicate \
 # RUN:   -lto_library /lib/foo \
 # RUN:   -macosx_version_min 0 \
 # RUN:   -no_dtrace_dof \
 # RUN:   -dependency_info /path/to/dependency_info.dat \
 # RUN:   -lto_library ../lib/libLTO.dylib \
 # RUN:   -mllvm -time-passes \
 # RUN:   -objc_abi_version 2 \
 # RUN:   -ios_simulator_version_min 9.0.0 \