[lld-macho]Define a macro to allow specifying the slop size at build time.

We're seeing thunk size overrun in our large builds and it would be helpful to be able to increase the slop size.
However, this should probably not be changed between different runs of LLD, hence making it a build option so that it's fixed per lld binary.

Author: oontvoo

lld/MachO/CMakeLists.txt

@@ -1,3 +1,8 @@
+
+option(LLD_MACHO_SLOP_FACTOR
+    "Specify the slop factor. For very large build, the default of 256 might be too small, which can cause thunk-range overrun. Recommend increasing this value to 1024 or higher as needed.", 256)
+add_definitions("-DLLD_MACHO_SLOP_FACTOR=${LLD_MACHO_SLOP_FACTOR}")
+    
 set(LLVM_TARGET_DEFINITIONS Options.td)
 tablegen(LLVM Options.inc -gen-opt-parser-defs)
 add_public_tablegen_target(MachOOptionsTableGen)

lld/MachO/ConcatOutputSection.cpp

@@ -306,7 +306,12 @@ 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;
+#ifdef LLD_MACHO_SLOP_FACTOR
+    const unsigned kSlopFact = LLD_MACHO_SLOP_FACTOR;
+#else
+    const unsigned kSlopFact = 256;
+#endif
+    unsigned slop = kSlopFact * thunkSize;
     while (finalIdx < endIdx) {
       uint64_t expectedNewSize =
           alignToPowerOf2(addr + size, inputs[finalIdx]->align) +