[lld-macho] Increase slop to prevent thunk out of range again.

[glandium]

Summary

Building Firefox with -O0 on arm64 mac recently hit the
"FIXME: thunk range overrun" error on multiple occasions.

Doubling or tripling slop was not sufficient in some cases, so
quadruple it.

[int3]

Thanks. We should work on a less brittle fix soon...

[glandium]

Can you push this for me?

[glandium]

In D138174#3932672, @int3 wrote:

Thanks. We should work on a less brittle fix soon...

Yeah, we're already hitting the wall with the new value :(

[thakis]

Did you look into where all these thunk uses are coming from?

Back when I added this hack, just 3 was enough. It feels like something changed to require way higher values. It'd be interesting to find out _what_ changed.

(I added a bunch of printf() to this code to print the barrier progress and .o files that handle it to find out which .o file hit it last time.)

[glandium]

I can't dig into this at the moment, but I opened an issue with a reproducer (https://github.com/llvm/llvm-project/issues/59259)

lld/MachO/ConcatOutputSection.cpp

for (size_t callIdx = 0, endIdx = inputs.size(); callIdx < endIdx;
 
     // Assign addresses up-to the forward branch-range limit.
     // Every call instruction needs a small number of bytes (on Arm64: 4),
     // and each inserted thunk needs a slightly larger number of bytes
     // (on Arm64: 12). If a section starts with a branch instruction and
     // 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 = 1024 * thunkSize;
     while (finalIdx < endIdx && addr + size + inputs[finalIdx]->getSize() <
                                     isecVA + forwardBranchRange - slop)
       finalizeOne(inputs[finalIdx++]);
 
     if (!isec->hasCallSites)
       continue;
 
     if (finalIdx == endIdx && stubsInRangeVA == TargetInfo::outOfRangeVA) {