[ELF] - Implemented --print-gc-sections command line argument.

List all sections removed by garbage collection. This option is only effective if garbage collection has been enabled via the `--gc-sections' option.

Differential revision: http://reviews.llvm.org/D15327

llvm-svn: 255235

lld/ELF/Config.h

@@ -58,6 +58,7 @@ struct Configuration {
   bool Mips64EL = false;
   bool NoInhibitExec;
   bool NoUndefined;
+  bool PrintGcSections;
   bool Shared;
   bool Static = false;
   bool StripAll;

lld/ELF/Driver.cpp

@@ -163,6 +163,7 @@ void LinkerDriver::createFiles(opt::InputArgList &Args) {
   Config->GcSections = Args.hasArg(OPT_gc_sections);
   Config->NoInhibitExec = Args.hasArg(OPT_noinhibit_exec);
   Config->NoUndefined = Args.hasArg(OPT_no_undefined);
+  Config->PrintGcSections = Args.hasArg(OPT_print_gc_sections);
   Config->Shared = Args.hasArg(OPT_shared);
   Config->StripAll = Args.hasArg(OPT_strip_all);
   Config->Verbose = Args.hasArg(OPT_verbose);

lld/ELF/Options.td

@@ -79,6 +79,9 @@ def no_undefined : Flag<["--"], "no-undefined">,
 def o : Separate<["-"], "o">, MetaVarName<"<path>">,
   HelpText<"Path to file to write output">;
 
+def print_gc_sections: Flag<["--"], "print-gc-sections">,
+  HelpText<"List removed unused sections">;
+
 def rpath : Separate<["-"], "rpath">,
   HelpText<"Add a DT_RUNPATH to the output">;
 

lld/ELF/Writer.cpp

@@ -16,6 +16,7 @@
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Support/FileOutputBuffer.h"
+#include "llvm/Support/raw_ostream.h"
 #include "llvm/Support/StringSaver.h"
 
 using namespace llvm;
@@ -531,6 +532,15 @@ StringRef Writer<ELFT>::getOutputSectionName(StringRef S) const {
   return S;
 }
 
+template <class ELFT>
+void reportDiscarded(InputSectionBase<ELFT> *IS,
+                     const std::unique_ptr<ObjectFile<ELFT>> &File) {
+  if (!Config->PrintGcSections || !IS || IS->isLive())
+    return;
+  llvm::errs() << "removing unused section from '" << IS->getSectionName()
+               << "' in file '" << File->getName() << "'\n";
+}
+
 template <class ELFT>
 bool Writer<ELFT>::isDiscarded(InputSectionBase<ELFT> *IS) const {
   if (!IS || !IS->isLive() || IS == &InputSection<ELFT>::Discarded)
@@ -564,8 +574,10 @@ template <class ELFT> void Writer<ELFT>::createSections() {
 
   for (const std::unique_ptr<ObjectFile<ELFT>> &F : Symtab.getObjectFiles()) {
     for (InputSectionBase<ELFT> *C : F->getSections()) {
-      if (isDiscarded(C))
+      if (isDiscarded(C)) {
+        reportDiscarded(C, F);
         continue;
+      }
       const Elf_Shdr *H = C->getSectionHdr();
       uintX_t OutFlags = H->sh_flags & ~SHF_GROUP;
       // For SHF_MERGE we create different output sections for each sh_entsize.

lld/test/ELF/gc-sections-print.s

@@ -0,0 +1,23 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: ld.lld %t --gc-sections --print-gc-sections -o %t2 2>&1 | FileCheck -check-prefix=PRINT %s
+
+# PRINT:      removing unused section from '.text.x' in file
+# PRINT-NEXT: removing unused section from '.text.y' in file
+
+.globl _start
+.protected a, x, y
+_start:
+ call a
+
+.section .text.a,"ax",@progbits
+a:
+ nop
+
+.section .text.x,"ax",@progbits
+x:
+ nop
+
+.section .text.y,"ax",@progbits
+y:
+ nop