fix(cli): relocate data into split modules in optimized builds

CHANGELOG.md

@@ -1,5 +1,12 @@
 ## wasm-split-cli vfuture
 
+- Improve data layout in optimized builds, with less data ending up in the main module and more
+  data in the split modules, in particular when `wasm-ld` deduplicates strings and performs tail
+  merging (for details see #60).
+- Fix relocations inside a data symbol that is contained in another symbol being attributed to
+  the containing symbol. A module that only used the inner symbol could miss the relocation's
+  target. This was hidden by whole segments being kept in the main module.
+
 ## wasm-split-cli v0.2.3
 
 - Fix breakage from wasm-bindgen 0.128, which changes how casts are generated, leading to

crates/wasm_split_cli/src/dep_graph.rs

@@ -305,6 +305,8 @@ fn iter_data_dependencies<'m>(
                 }) {
                     let _ = overlap_candidates.pop();
                 }
+                // The relocation belongs to the innermost symbol containing its start,
+                // and must lie fully inside it.
                 let &target = emit_iter_err!(overlap_candidates.last().ok_or_else(|| anyhow!(
                     "Invalid relocation entry {entry:?} not overlapping any data symbols"
                 )));
@@ -351,6 +353,10 @@ fn iter_data_dependencies<'m>(
 
         // if we reach here, then the next symbol is contained in all items on the stack
         if let Some(container) = container {
+            // Keep the inner symbol as a candidate: relocations inside it must be attributed to
+            // it, the smallest symbol containing them, and not to its container. Otherwise a
+            // module that only needs the inner symbol would miss the relocation's target.
+            overlap_candidates.push(next_symbol);
             return Some(Ok(DataDependency::Containment {
                 container,
                 inner: next_symbol,