[LTO][Legacy] Add new API to check presence of ctor/dtor functions

[w2yehia]

Summary

On AIX, the linker needs to check whether a given lto_module_t contains any constructor/destructor functions, in order to implement the behavior of the -bcdtors:all flag. See https://www.ibm.com/docs/en/aix/7.2?topic=l-ld-command for the flag's documentation.
In llvm IR, constructor (destructor) functions are added to a special global array @llvm.global_ctors (@llvm.global_dtors).
However, because these two symbols are artificial, they are not visited during the symbol traversal (using the lto_module_get_[num_symbols|symbol_name|symbol_attribute] API).

This patch adds a new function to the libLTO interface that checks the presence of one or both of these two symbols.

[steven_wu]

Comment inline.

It would be good if you can find a way to write testcase.

llvm/lib/LTO/LTOModule.cpp

695

Can you simply this code a bit by using consume_front() on the name?

Also you need to check for nullptr for PointerUnion:

if (auto *GV = Sym.dyn_cast<>)

[w2yehia]

address code review comment.

[w2yehia]

Added testcase for LTOModule::hasCtorDtor()

@steven_wu FYI

[steven_wu]

LGTM in general. Just two small comments inline. Thanks!

llvm/tools/llvm-lto/llvm-lto.cpp

404 ↗

(On Diff #362148)

Can you update the comments here to reflect the current function?

420 ↗

(On Diff #362148)

Can you make the output to stdout more verbose? Filename : {0|1} is not clear even for a testing tool.

[w2yehia]

commited version

llvm/include/llvm-c/lto.h

  * @{
  */
 
-#define LTO_API_VERSION 28
+#define LTO_API_VERSION 29
 
 /**
  * \since prior to LTO_API_VERSION=3

                                                unsigned int *out_cputype,
                                                unsigned int *out_cpusubtype);
 
+/**
+ * This function can be used by the linker to check if a given module has
+ * any constructor or destructor functions.
+ *
+ * Returns true if the module has either the @llvm.global_ctors or the
+ * @llvm.global_dtors symbol. Otherwise returns false.
+ *
+ * \since LTO_API_VERSION=29
+ */
+extern lto_bool_t lto_module_has_ctor_dtor(lto_module_t mod);
 /**
  * Diagnostic severity.
  *

llvm/include/llvm/LTO/legacy/LTOModule.h

 
   Expected<uint32_t> getMachOCPUSubType() const;
 
+  bool hasCtorDtor() const;
+
 private:
   /// Parse metadata from the module
   // FIXME: it only parses "llvm.linker.options" metadata at the moment

llvm/lib/LTO/LTOModule.cpp

 Expected<uint32_t> LTOModule::getMachOCPUSubType() const {
   return MachO::getCPUSubType(Triple(Mod->getTargetTriple()));
 }
+
+bool LTOModule::hasCtorDtor() const {
+  for (auto Sym : SymTab.symbols()) {
+    if (auto *GV = Sym.dyn_cast<GlobalValue *>()) {
+      StringRef Name = GV->getName();
+      if (Name.consume_front("llvm.global_")) {
+        if (Name.equals("ctors") || Name.equals("dtors"))
+          return true;
+      }
+    }
+  }
+  return false;
+}

llvm/tools/lto/lto.cpp

   unwrap(cg)->setShouldEmbedUselists(ShouldEmbedUselists);
 }
 
+lto_bool_t lto_module_has_ctor_dtor(lto_module_t mod) {
+  return unwrap(mod)->hasCtorDtor();
+}
+
 // ThinLTO API below
 
 thinlto_code_gen_t thinlto_create_codegen(void) {

llvm/tools/lto/lto.exports

 lto_module_create_from_memory_with_path
 lto_module_create_in_local_context
 lto_module_create_in_codegen_context
+lto_module_has_ctor_dtor
 lto_module_get_linkeropts
 lto_module_get_macho_cputype
 lto_module_get_num_symbols