ld: Early detection of orphans we know will be discarded

When processing an orphan section we first call lang_place_orphans, this
function handles a few sections for which the behaviour is known COMMON
sections, or sections marked as SEC_EXCLUDE.

Any orphans that are not handled in lang_place_orphans are passed on to
ldlang_place_orphan, this is where we decide where to put the orphan,
and then call lang_add_section to perform the actual orphan placement.

We previously had a larger set of checks at the start of the function
lang_add_section to discard some sections that we _knew_ should not be
added into the output file, this was where .group sections (in a final
link) and .debug* sections (with --strip-debug) were dropped.

The problem with dropping these sections at the lang_add_section stage
is that a user might also be using --orphan-handling=error to prevent
orphans.  If they are then they should not be get errors about sections
that we know will be discarded, and which are not mentioned in the
linker script.

The solution proposed in this patch is to move the "will this section be
discarded" check into a separate function, and use this in
lang_place_orphans to have the early discard phase discard sections that
we know should not be included in the output file.

ld/ChangeLog:

	PR 21961
	* ldlang.c (lang_discard_section_p): New function.
	(lang_add_section): Checks moved out into new function, which is
	now called.
	(lang_place_orphans): Call lang_discard_section_p instead of
	duplicating some of the checks from lang_add_section.
	* testsuite/ld-elf/orphan-11.d: New file.
	* testsuite/ld-elf/orphan-11.ld: New file.
	* testsuite/ld-elf/orphan-11.s: New file.
	* testsuite/ld-elf/orphan-12.d: New file.
	* testsuite/ld-elf/orphan-12.s: New file.

ld/ChangeLog

@@ -1,3 +1,17 @@
+2017-08-17  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	PR 21961
+	* ldlang.c (lang_discard_section_p): New function.
+	(lang_add_section): Checks moved out into new function, which is
+	now called.
+	(lang_place_orphans): Call lang_discard_section_p instead of
+	duplicating some of the checks from lang_add_section.
+	* testsuite/ld-elf/orphan-11.d: New file.
+	* testsuite/ld-elf/orphan-11.ld: New file.
+	* testsuite/ld-elf/orphan-11.s: New file.
+	* testsuite/ld-elf/orphan-12.d: New file.
+	* testsuite/ld-elf/orphan-12.s: New file.
+
 2017-08-24  Alan Modra  <amodra@gmail.com>
 
 	* testsuite/ld-undefined/fundef.s: New test.

ld/ldlang.c

@@ -2273,6 +2273,34 @@ section_already_linked (bfd *abfd, asection *sec, void *data)
     bfd_section_already_linked (abfd, sec, &link_info);
 }
 
+
+/* Returns true if SECTION is one we know will be discarded based on its
+   section flags, otherwise returns false.  */
+
+static bfd_boolean
+lang_discard_section_p (asection *section)
+{
+  bfd_boolean discard;
+  flagword flags = section->flags;
+
+  /* Discard sections marked with SEC_EXCLUDE.  */
+  discard = (flags & SEC_EXCLUDE) != 0;
+
+  /* Discard the group descriptor sections when we're finally placing the
+     sections from within the group.  */
+  if ((flags & SEC_GROUP) != 0
+      && link_info.resolve_section_groups)
+    discard = TRUE;
+
+  /* Discard debugging sections if we are stripping debugging
+     information.  */
+  if ((link_info.strip == strip_debugger || link_info.strip == strip_all)
+      && (flags & SEC_DEBUGGING) != 0)
+    discard = TRUE;
+
+  return discard;
+}
+
 /* The wild routines.
 
    These expand statements like *(.text) and foo.o to a list of
@@ -2294,26 +2322,14 @@ lang_add_section (lang_statement_list_type *ptr,
   lang_input_section_type *new_section;
   bfd *abfd = link_info.output_bfd;
 
-  /* Discard sections marked with SEC_EXCLUDE.  */
-  discard = (flags & SEC_EXCLUDE) != 0;
+  /* Is this section one we know should be discarded?  */
+  discard = lang_discard_section_p (section);
 
   /* Discard input sections which are assigned to a section named
      DISCARD_SECTION_NAME.  */
   if (strcmp (output->name, DISCARD_SECTION_NAME) == 0)
     discard = TRUE;
 
-  /* Discard the group descriptor sections when we're finally placing the
-     sections from within the group.  */
-  if ((section->flags & SEC_GROUP) == SEC_GROUP
-      && link_info.resolve_section_groups)
-    discard = TRUE;
-
-  /* Discard debugging sections if we are stripping debugging
-     information.  */
-  if ((link_info.strip == strip_debugger || link_info.strip == strip_all)
-      && (flags & SEC_DEBUGGING) != 0)
-    discard = TRUE;
-
   if (discard)
     {
       if (section->output_section == NULL)
@@ -6446,7 +6462,7 @@ lang_place_orphans (void)
 
 	      if (file->flags.just_syms)
 		bfd_link_just_syms (file->the_bfd, s, &link_info);
-	      else if ((s->flags & SEC_EXCLUDE) != 0)
+	      else if (lang_discard_section_p (s))
 		s->output_section = bfd_abs_section_ptr;
 	      else if (strcmp (s->name, "COMMON") == 0)
 		{

ld/testsuite/ld-elf/orphan-11.d

@@ -0,0 +1,9 @@
+#source: orphan-11.s
+#ld: -T orphan-11.ld --orphan-handling=error
+#objdump: -wh
+#notarget: d30v-* dlx-* fr30-* frv-* ft32-* i860-* i960-* iq2000-* mn10200-* moxie-* ms1-* msp430-* mt-* pj-*
+
+#...
+  . \.text .*
+  . \.data .*
+#pass

ld/testsuite/ld-elf/orphan-11.ld

@@ -0,0 +1,16 @@
+SECTIONS
+{
+  . = SIZEOF_HEADERS;
+  .text : { *(.text .text.*) }
+  .data : { *(.data .data.*) }
+  .bss : { *(.bss .bss.*) *(COMMON) }
+  .sbss : { *(.sbss .sbss.*) }
+  .note : { *(.note .note.*) }
+  .rela : { *(.rela .rela.*) }
+  .rel : { *(.rel .rel.*) }
+
+  /DISCARD/ : {
+            *(.reginfo) *(.MIPS.abiflags) *(.trampolines) *(.iplt*)
+            *(.note*) *(.got*) *(.igot*) *(.*.attributes) *(.*.info)
+            *(.pdr) "linker stubs*"(*)  }
+}

ld/testsuite/ld-elf/orphan-11.s

@@ -0,0 +1,11 @@
+	.section .text.foo,"axG",%progbits,foo_group
+	.word 0
+
+	.section .data.foo,"waG",%progbits,foo_group
+	.word 1
+
+	.section .text, "ax"
+	.word 0
+
+	.section .data, "wa"
+	.word 1

ld/testsuite/ld-elf/orphan-12.d

@@ -0,0 +1,9 @@
+#source: orphan-12.s
+#ld: -T orphan-11.ld --strip-debug --orphan-handling=error
+#objdump: -wh
+#notarget: d30v-* dlx-* fr30-* frv-* ft32-* i860-* i960-* iq2000-* mn10200-* moxie-* ms1-* msp430-* mt-* pj-*
+
+#...
+  . \.text .*
+  . \.data .*
+#pass

ld/testsuite/ld-elf/orphan-12.s

@@ -0,0 +1,8 @@
+	.section .debug_info, "",%progbits
+	.word 0
+
+	.section .text, "ax"
+	.word 0
+
+	.section .data, "wa"
+	.word 1