Set lambda linkage to LinkOnceODR (was Internal) (#5045)

This reverts #4415. The mangling of lambdas has been fixed (and is now stable and cannot clash with other modules anymore), so there is no longer a need for internal linkage. This allows merging of identical lambdas again (multiple instantiations).

This also fixes an instantiation bug, where the lambda's emission is sometimes culled because the frontend knows it is already instantiated in another module, but because of internal linkage it cannot resolve the symbol during linking (thus resulting in a missing symbol during linking).

Author: JohanEngelen

gen/tollvm.cpp

@@ -229,18 +229,15 @@ LLGlobalValue::LinkageTypes DtoLinkageOnly(Dsymbol *sym) {
       return LLGlobalValue::InternalLinkage;
 
   /* Function (incl. delegate) literals are emitted into each referencing
-   * compilation unit, so use internal linkage for all lambdas and all global
+   * compilation unit, so use linkonce_odr for all lambdas and all global
    * variables they define.
-   * This makes sure these symbols don't accidentally collide when linking
-   * object files compiled by different compiler invocations (lambda mangles
-   * aren't stable - see https://issues.dlang.org/show_bug.cgi?id=23722).
    */
   auto potentialLambda = sym;
   if (auto vd = sym->isVarDeclaration())
     if (vd->isDataseg())
       potentialLambda = vd->toParent2();
   if (potentialLambda->isFuncLiteralDeclaration())
-    return LLGlobalValue::InternalLinkage;
+    return LLGlobalValue::LinkOnceODRLinkage;
 
   if (sym->isInstantiated())
     return templateLinkage;

tests/codegen/lambdas_gh3648.d

@@ -1,4 +1,4 @@
-// Tests that lambdas and contained globals are emitted with internal linkage.
+// Tests that lambdas and contained globals are emitted as linkonce_odr.
 
 // RUN: %ldc -output-ll -of=%t.ll %s && FileCheck %s < %t.ll
 
@@ -25,22 +25,22 @@ void foo()
     }(123);
 }
 
-// the global variables should be defined as internal:
-// CHECK: _D14lambdas_gh364815__lambda_L5_C12FZ10global_bari{{.*}} = internal thread_local global
-// CHECK: _D14lambdas_gh364816__lambda_L10_C20FZ18global_bar_inlinedOi{{.*}} = internal global
-// CHECK: _D14lambdas_gh36483fooFZ__T15__lambda_L21_C5TiZQuFiZ12lambda_templi{{.*}} = internal global
+// the global variables should be defined as linkonce_odr:
+// CHECK: _D14lambdas_gh364815__lambda_L5_C12FZ10global_bari{{.*}} = linkonce_odr {{(hidden )?}}thread_local global
+// CHECK: _D14lambdas_gh364816__lambda_L10_C20FZ18global_bar_inlinedOi{{.*}} = linkonce_odr {{(hidden )?}}global
+// CHECK: _D14lambdas_gh36483fooFZ__T15__lambda_L21_C5TiZQuFiZ12lambda_templi{{.*}} = linkonce_odr {{(hidden )?}}global
 
 // foo() should only call two lambdas:
 // CHECK: define {{.*}}_D14lambdas_gh36483fooFZv
 // CHECK-NEXT: call {{.*}}__lambda_L5_C12
 // CHECK-NEXT: call {{.*}}__T15__lambda_L21_C5
 // CHECK-NEXT: ret void
 
-// bar() should be defined as internal:
-// CHECK: define internal {{.*}}__lambda_L5_C12
+// bar() should be defined as linkonce_odr:
+// CHECK: define linkonce_odr {{.*}}__lambda_L5_C12
 
 // bar_inlined() should NOT have made it to the .ll:
 // CHECK-NOT: define {{.*}}__lambda_L10_C20
 
-// the template lambda instance should be defined as internal:
-// CHECK: define internal {{.*}}__T15__lambda_L21_C5
+// the template lambda instance should be defined as linkonce_odr:
+// CHECK: define linkonce_odr {{.*}}__T15__lambda_L21_C5

tests/codegen/lambdas_gh3648b.d

@@ -1,4 +1,4 @@
-// Tests that *imported* lambdas and contained globals are emitted with internal linkage.
+// Tests that *imported* lambdas and contained globals are emitted as linkonce_odr.
 
 // RUN: %ldc -output-ll -of=%t.ll %s -I%S && FileCheck %s < %t.ll
 
@@ -10,17 +10,17 @@ void foo()
     bar_inlined();
 }
 
-// the global variables should be defined as internal:
-// CHECK: _D14lambdas_gh364815__lambda_L5_C12FZ10global_bari{{.*}} = internal thread_local global
-// CHECK: _D14lambdas_gh364816__lambda_L10_C20FZ18global_bar_inlinedOi{{.*}} = internal global
+// the global variables should be defined as linkonce_odr:
+// CHECK: _D14lambdas_gh364815__lambda_L5_C12FZ10global_bari{{.*}} = linkonce_odr {{(hidden )?}}thread_local global
+// CHECK: _D14lambdas_gh364816__lambda_L10_C20FZ18global_bar_inlinedOi{{.*}} = linkonce_odr {{(hidden )?}}global
 
 // foo() should only call one lambda:
 // CHECK: define {{.*}}_D15lambdas_gh3648b3fooFZv
 // CHECK-NEXT: call {{.*}}__lambda_L5_C12
 // CHECK-NEXT: ret void
 
-// bar() should be defined as internal:
-// CHECK: define internal {{.*}}__lambda_L5_C12
+// bar() should be defined as linkonce_odr:
+// CHECK: define linkonce_odr {{.*}}__lambda_L5_C12
 
 // bar_inlined() should NOT have made it to the .ll:
 // CHECK-NOT: define {{.*}}__lambda_L10_C20