[lld-macho]Define a flag for adjusting slop scale (#164295)

Co-authored-by: Ellis Hoag <[email protected]>

Author: oontvoo

lld/MachO/ConcatOutputSection.cpp

@@ -306,7 +306,7 @@ void TextOutputSection::finalize() {
     // contains several branch instructions in succession, then the distance
     // from the current position to the position where the thunks are inserted
     // grows. So leave room for a bunch of thunks.
-    unsigned slop = 256 * thunkSize;
+    unsigned slop = config->slopScale * thunkSize;
     while (finalIdx < endIdx) {
       uint64_t expectedNewSize =
           alignToPowerOf2(addr + size, inputs[finalIdx]->align) +
@@ -384,7 +384,9 @@ void TextOutputSection::finalize() {
         // above. If you hit this: For the current algorithm, just bumping up
         // slop above and trying again is probably simplest. (See also PR51578
         // comment 5).
-        fatal(Twine(__FUNCTION__) + ": FIXME: thunk range overrun");
+        fatal(Twine(__FUNCTION__) +
+              ": FIXME: thunk range overrun. Consider increasing the "
+              "slop-scale with `--slop-scale=<unsigned_int>`.");
       }
       thunkInfo.isec =
           makeSyntheticInputSection(isec->getSegName(), isec->getName());

lld/MachO/Config.h

@@ -224,6 +224,7 @@ struct Configuration {
   bool disableVerify;
   bool separateCstringLiteralSections;
   bool tailMergeStrings;
+  unsigned slopScale = 256;
 
   bool callGraphProfileSort = false;
   llvm::StringRef printSymbolOrder;

lld/MachO/Driver.cpp

@@ -2015,6 +2015,14 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
                    OPT_no_separate_cstring_literal_sections, false);
   config->tailMergeStrings =
       args.hasFlag(OPT_tail_merge_strings, OPT_no_tail_merge_strings, false);
+  if (auto *arg = args.getLastArg(OPT_slop_scale_eq)) {
+    StringRef v(arg->getValue());
+    unsigned slop = 0;
+    if (!llvm::to_integer(v, slop))
+      error(arg->getSpelling() +
+            ": expected a non-negative integer, but got '" + v + "'");
+    config->slopScale = slop;
+  }
 
   auto IncompatWithCGSort = [&](StringRef firstArgStr) {
     // Throw an error only if --call-graph-profile-sort is explicitly specified

lld/MachO/Options.td

@@ -1095,6 +1095,14 @@ defm tail_merge_strings
     : BB<"tail-merge-strings", "Enable string tail merging",
          "Disable string tail merging to improve link-time performance">,
       Group<grp_rare>;
+def slop_scale_eq
+    : Joined<["--"], "slop_scale=">,
+      MetaVarName<"<unsigned_int>">,
+      HelpText<"Specify the slop scale. Default value is 256. If your binary "
+               "has too many consecutive branch instructions resulting in "
+               "thunk-range overrun, then you need to increase this value to a "
+               "higher value, such as 512 or 1024, etc">,
+      Group<grp_rare>;
 
 def grp_deprecated : OptionGroup<"deprecated">, HelpText<"DEPRECATED">;
 

lld/test/MachO/set-slop-scale.s

@@ -0,0 +1,11 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-darwin %s -o %t.o
+# RUN: %lld -o /dev/null %t.o --slop_scale=1
+# RUN: not %lld -o /dev/null %t.o --slop_scale=-1 2>&1 | FileCheck %s
+# CHECK: error: --slop_scale=: expected a non-negative integer, but got '-1'
+
+.text
+.global _main
+_main:
+  mov $0, %rax
+  ret