[lld-macho] Only enable `__DATA_CONST` for newer platforms

[int3]

Summary

Matches ld64.

[thakis]

lld/MachO/Driver.cpp

852

(Please feel free to ignore!)

PlatformKind is an enum with sequential numbers.

The code as is at first execution creates a red-black tree with 8 entries, then walks it.

A switch statement on the other hand should produce much smaller and simpler machine code, without being more verbose.

I doubt it matters in practice, but it's a good opportunity for more "silicon sympathy" :)

[int3]

lld/MachO/Driver.cpp

852

a switch statement would put every VersionTuple on a different line than the case, and would also require another line for the breaks, so it's considerably more verbose...

good point about the sequential numbers though, a lookup array would probably be better here.

lld/MachO/Driver.cpp

if (!isPowerOf2_32(align)) {
       continue;
     }
     sectAligns.push_back({segName, sectName, align});
   }
   return sectAligns;
 }
 
 static bool dataConstDefault(const InputArgList &args) {
+  static const std::map<PlatformKind, VersionTuple> minVersion = {
+      {PlatformKind::macOS, VersionTuple(10, 15)},
+      {PlatformKind::iOS, VersionTuple(13, 0)},
+      {PlatformKind::iOSSimulator, VersionTuple(13, 0)},
+      {PlatformKind::tvOS, VersionTuple(13, 0)},
+      {PlatformKind::tvOSSimulator, VersionTuple(13, 0)},
+      {PlatformKind::watchOS, VersionTuple(6, 0)},
+      {PlatformKind::watchOSSimulator, VersionTuple(6, 0)},
+      {PlatformKind::bridgeOS, VersionTuple(4, 0)}};
+  auto it = minVersion.find(config->platformInfo.target.Platform);
+  if (it != minVersion.end())
+    if (config->platformInfo.minimum < it->second)
+      return false;
+
   switch (config->outputType) {
   case MH_EXECUTE:
     return !args.hasArg(OPT_no_pie);
   case MH_BUNDLE:
     // FIXME: return false when -final_name ...
     // has prefix "/System/Library/UserEventPlugins/"
     // or matches "/usr/libexec/locationd" "/usr/libexec/terminusd"
     return true;

lld/test/MachO/builtin-rename.s

 # REQUIRES: x86
 # RUN: rm -rf %t; split-file %s %t
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin \
 # RUN:     %t/main.s -o %t/main.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin \
 # RUN:     %t/renames.s -o %t/renames.o
 
 ## Check that section and segment renames happen as expected
 # RUN: %lld                -o %t/ydata %t/main.o %t/renames.o -lSystem
 # RUN: %lld -no_data_const -o %t/ndata %t/main.o %t/renames.o -lSystem
 # RUN: %lld -no_pie        -o %t/nopie %t/main.o %t/renames.o -lSystem
+# RUN: %lld -platform_version macos 10.14 11.0 -o %t/old %t/main.o %t/renames.o -lSystem
 
 # RUN: llvm-objdump --syms %t/ydata | \
 # RUN:     FileCheck %s --check-prefixes=CHECK,YDATA
 # RUN: llvm-objdump --syms %t/ndata | \
 # RUN:     FileCheck %s --check-prefixes=CHECK,NDATA
 # RUN: llvm-objdump --syms %t/nopie | \
 # RUN:     FileCheck %s --check-prefixes=CHECK,NDATA
+# RUN: llvm-objdump --syms %t/old | \
+# RUN:     FileCheck %s --check-prefixes=CHECK,NDATA
 
 # CHECK-LABEL: {{^}}SYMBOL TABLE:
 
 # CHECK-DAG: __TEXT,__text __TEXT__StaticInit
 
 # NDATA-DAG: __DATA,__auth_got __DATA__auth_got
 # NDATA-DAG: __DATA,__auth_ptr __DATA__auth_ptr
 # NDATA-DAG: __DATA,__nl_symbol_ptr __DATA__nl_symbol_ptr