[lld] [ELF] Make start/stop symbols retain associated discardable output sections (PR #96343)

[Fangrui Song via llvm-commits]

================
@@ -2028,42 +2028,33 @@ template <class ELFT> void Writer<ELFT>::checkExecuteOnly() {
 // The linker is expected to define SECNAME_start and SECNAME_end
 // symbols for a few sections. This function defines them.
 template <class ELFT> void Writer<ELFT>::addStartEndSymbols() {
-  // If a section does not exist, there's ambiguity as to how we
-  // define _start and _end symbols for an init/fini section. Since
-  // the loader assume that the symbols are always defined, we need to
-  // always define them. But what value? The loader iterates over all
-  // pointers between _start and _end to run global ctors/dtors, so if
-  // the section is empty, their symbol values don't actually matter
-  // as long as _start and _end point to the same location.
-  //
-  // That said, we don't want to set the symbols to 0 (which is
-  // probably the simplest value) because that could cause some
-  // program to fail to link due to relocation overflow, if their
-  // program text is above 2 GiB. We use the address of the .text
-  // section instead to prevent that failure.
-  //
-  // In rare situations, the .text section may not exist. If that's the
-  // case, use the image base address as a last resort.
-  OutputSection *Default = findSection(".text");
-  if (!Default)
-    Default = Out::elfHeader;
-
-  auto define = [=](StringRef start, StringRef end, OutputSection *os) {
-    if (os && !script->isDiscarded(os)) {
+  // If the associated output section does not exist, there is ambiguity as to
+  // how we define _start and _end symbols for an init/fini section. Users
+  // expect no "undefined symbol" linker errors and loaders expect equal
+  // st_value but do not particularly care whether the symbols are defined or
+  // not. We retain the output section so that the section indexes will be
+  // correct.
+  auto define = [=](StringRef start, StringRef end, OutputSection *os,
+                    bool exidx = false) {
----------------
MaskRay wrote:

Thanks for the suggestion. With more thoughts I will go with `retainSec`

https://github.com/llvm/llvm-project/pull/96343