[lld] [lld-macho] Fix branch extension thunk estimation logic (PR #120529)

[YongKang Zhu via llvm-commits]

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@@ -184,15 +184,45 @@ uint64_t TextOutputSection::estimateStubsInRangeVA(size_t callIdx) const {
     InputSection *isec = inputs[i];
     isecEnd = alignToPowerOf2(isecEnd, isec->align) + isec->getSize();
   }
+
+  // Tally up any thunks that have already been placed that have VA higher than
+  // inputs[callIdx]. First, find the index of the first thunk that is beyond
+  // the current inputs[callIdx].
+  auto itPostcallIdxThunks =
+      llvm::partition_point(thunks, [isecVA](const ConcatInputSection *t) {
+        return t->getVA() <= isecVA;
+      });
+  uint64_t existingForwardThunks = thunks.end() - itPostcallIdxThunks;
+
   // Estimate the address after which call sites can safely call stubs
   // directly rather than through intermediary thunks.
   uint64_t forwardBranchRange = target->forwardBranchRange;
   assert(isecEnd > forwardBranchRange &&
          "should not run thunk insertion if all code fits in jump range");
   assert(isecEnd - isecVA <= forwardBranchRange &&
          "should only finalize sections in jump range");
-  uint64_t stubsInRangeVA = isecEnd + maxPotentialThunks * target->thunkSize +
-                            in.stubs->getSize() - forwardBranchRange;
+
+  // Estimate the maximum size of the code, right before the stubs section.
+  uint64_t maxTextSize = 0;
+  // Add the size of all the inputs, including the unprocessed ones.
+  maxTextSize += isecEnd;
+
+  // Add the size of the thunks that have already been created that are ahead of
+  // inputs[callIdx]. These are already created thunks that will be interleaved
+  // with inputs[callIdx...end].
+  maxTextSize += existingForwardThunks * target->thunkSize;
+
+  // Add the size of the thunks that may be created in the future. Since
+  // 'maxPotentialThunks' overcounts, this is an estimate of the upper limit.
+  maxTextSize += maxPotentialThunks * target->thunkSize;
+
+  // Estimated maximum VA of last stub.
+  uint64_t maxVAOfLastStub = maxTextSize + in.stubs->getSize();
+
+  // Calculaate the first address that is gueranteed to not need a thunk to
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yozhu wrote:

Maybe use (move) the original comment on Line 197 here.

https://github.com/llvm/llvm-project/pull/120529