[JITLink] Fix splitBlock if there are symbols span across the boundary

Fix `splitBlock` so that it can handle the case when the block being
split has symbols span across the split boundary. This is an error
case in general but for EHFrame splitting on macho platforms, there is an
anonymous symbol that marks the entire block. Current implementation
will leave a symbol that is out of bound of the underlying block. Fix
the problem by dropping such symbols when the block is split.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D113912

Author: cachemeifyoucan

llvm/lib/ExecutionEngine/JITLink/JITLink.cpp

@@ -213,7 +213,12 @@ Block &LinkGraph::splitBlock(Block &B, size_t SplitIndex,
     // Transfer all symbols with offset less than SplitIndex to NewBlock.
     while (!BlockSymbols.empty() &&
            BlockSymbols.back()->getOffset() < SplitIndex) {
-      BlockSymbols.back()->setBlock(NewBlock);
+      auto *Sym = BlockSymbols.back();
+      // If the symbol extends beyond the split, update the size to be within
+      // the new block.
+      if (Sym->getOffset() + Sym->getSize() > SplitIndex)
+        Sym->setSize(SplitIndex - Sym->getOffset());
+      Sym->setBlock(NewBlock);
       BlockSymbols.pop_back();
     }
 

llvm/unittests/ExecutionEngine/JITLink/LinkGraphTests.cpp

@@ -493,6 +493,9 @@ TEST(LinkGraphTest, SplitBlock) {
                                 false, false);
   auto &S4 = G.addDefinedSymbol(B1, 12, "S4", 4, Linkage::Strong,
                                 Scope::Default, false, false);
+  // Add a symbol that extends beyond the split.
+  auto &S5 = G.addDefinedSymbol(B1, 0, "S5", 16, Linkage::Strong,
+                                Scope::Default, false, false);
 
   // Add an extra block, EB, and target symbols, and use these to add edges
   // from B1 to EB.
@@ -538,6 +541,11 @@ TEST(LinkGraphTest, SplitBlock) {
   EXPECT_EQ(&S4.getBlock(), &B1);
   EXPECT_EQ(S4.getOffset(), 4U);
 
+  EXPECT_EQ(&S5.getBlock(), &B2);
+  EXPECT_EQ(S5.getOffset(), 0U);
+  // Size shrinks to fit.
+  EXPECT_EQ(S5.getSize(), 8U);
+
   // Check that edges in B1 have been transferred as expected:
   // Both blocks should now have two edges each at offsets 0 and 4.
   EXPECT_EQ(llvm::size(B1.edges()), 2);