ELF: Emit (most) instantiated symbols in COMDATs

To fix issue #3589.

Author: kinke

CHANGELOG.md

@@ -11,6 +11,7 @@
 #### Bug fixes
 - Fix potentially corrupt IR layouts for bit fields. (#4646, #4708)
 - Fix potentially corrupt IR layouts for explicitly under-aligned aggregates, a regression introduced in LDC v1.31. (#4734, #4736)
+- ELF: Emit (most) instantiated symbols in COMDATs for proper link-time culling. (#3589, #4748)
 
 # LDC 1.39.0 (2024-07-04)
 

gen/tollvm.cpp

@@ -259,7 +259,13 @@ LLGlobalValue::LinkageTypes DtoLinkageOnly(Dsymbol *sym) {
 }
 
 LinkageWithCOMDAT DtoLinkage(Dsymbol *sym) {
-  return {DtoLinkageOnly(sym), needsCOMDAT()};
+  const auto linkage = DtoLinkageOnly(sym);
+  const bool inCOMDAT = needsCOMDAT() ||
+                        // ELF needs some help for ODR linkages:
+                        // https://github.com/ldc-developers/ldc/issues/3589
+                        (linkage == templateLinkage &&
+                         global.params.targetTriple->isOSBinFormatELF());
+  return {linkage, inCOMDAT};
 }
 
 bool needsCOMDAT() {