Merge pull request ldc-developers#4748 from kinke/fix3589

ELF: Emit (most) instantiated symbols in COMDATs

CHANGELOG.md

@@ -12,6 +12,7 @@
 #### 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() {

tests/codegen/gh3589.d

@@ -0,0 +1,22 @@
+// UNSUPPORTED: Windows || Darwin
+
+// emit duplicate instantiated globals into two object files:
+// RUN: %ldc -c %S/inputs/gh3589_a.d -I%S/inputs -of=%t_a.o
+// RUN: %ldc -c %S/inputs/gh3589_b.d -I%S/inputs -of=%t_b.o
+
+// link & run:
+// RUN: %ldc -I%S/inputs %t_a.o %t_b.o -run %s
+
+extern extern(C) __gshared {
+    // magic linker symbols to refer to the start and end of test_section
+    byte __start_test_section;
+    byte __stop_test_section;
+}
+
+void main() {
+    import gh3589_a, gh3589_b;
+    assert(a_info == b_info);
+
+    const sectionSize = cast(size_t) (&__stop_test_section - &__start_test_section);
+    assert(sectionSize == 4);
+}

tests/codegen/inputs/gh3589_a.d

@@ -0,0 +1,2 @@
+import gh3589_templ;
+shared a_info = &getInfo!0; 

tests/codegen/inputs/gh3589_b.d

@@ -0,0 +1,2 @@
+import gh3589_templ;
+shared b_info = &getInfo!0; 

tests/codegen/inputs/gh3589_templ.di

@@ -0,0 +1,4 @@
+import ldc.attributes;
+template getInfo(int I) {
+    @(section("test_section")) @assumeUsed shared int getInfo = I;
+}