[PATCH v2 5/5] ld: clarify comments on /DISCARD/ output section behavior

[Matthieu Longo]

The previous comments made it difficult to understand how the /DISCARD/
output section interacts with non-contiguous regions.

In summary, the general rule is that the first (top-most) clause takes
precedence over subsequent ones:
- If /DISCARD/ appears first, the section is dropped. There is no need
  to warn about potential behavior changes with non-contiguous regions
  when the section is already discarded.
- If /DISCARD/ follows clauses that assign the input section to an output
  section, /DISCARD/ is ignored, and the section is kept. In this case,
  the linker must warn that the section may not be discarded.
---
 ld/ldlang.c                                      | 16 ++++++++++++----
 .../non-contiguous-mem/non-contiguous-ok-5.warn  |  4 ++--
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/ld/ldlang.c b/ld/ldlang.c
index c7c4ba8a019..818fa742d63 100644
--- a/ld/ldlang.c
+++ b/ld/ldlang.c
@@ -2709,17 +2709,25 @@ wont_add_section_p (asection *section,
 
   if (discard)
     {
+      /* /DISCARD/ is seen first and the top-most clause has precedence on the
+	 next ones, thus the section will be dropped.  No need to warn about
+	 potential change in behavior with non-contiguous regions when the
+	 section is already dropped.  */
       if (section->output_section == NULL)
 	{
 	  /* This prevents future calls from assigning this section or
 	     warning about it again.  */
 	  section->output_section = bfd_abs_section_ptr;
 	}
-      else if (bfd_is_abs_section (section->output_section))
-	;
-      else if (link_info.non_contiguous_regions_warnings)
+      /* The /DISCARD/ clause appears after previous ones which assigned the
+	 input section to an output section.  /DISCARD/ does not have the
+	 precedence, so the section will be kept.  */
+      else if (! bfd_is_abs_section (section->output_section)
+	       && link_info.non_contiguous_regions_warnings)
 	einfo (_("%P:%pS: warning: --enable-non-contiguous-regions makes "
-		 "section `%pA' from `%pB' match /DISCARD/ clause.\n"),
+		 "section `%pA' from `%pB' match /DISCARD/ clause.  If the "
+		 "section can be assigned to an output section, it won't be "
+		 "discarded.\n"),
 	       NULL, section, section->owner);
 
       return true;
diff --git a/ld/testsuite/ld-aarch64/non-contiguous-mem/non-contiguous-ok-5.warn b/ld/testsuite/ld-aarch64/non-contiguous-mem/non-contiguous-ok-5.warn
index 730599ef289..7cf8af73a56 100644
--- a/ld/testsuite/ld-aarch64/non-contiguous-mem/non-contiguous-ok-5.warn
+++ b/ld/testsuite/ld-aarch64/non-contiguous-mem/non-contiguous-ok-5.warn
@@ -2,8 +2,8 @@
 .*: warning: .* may change behaviour for section .\.code\.2. from .* \(assigned to \.raml, but additional match: \.ramu\)
 .*: warning: .* may change behaviour for section .\.code\.3. from .* \(assigned to \.raml, but additional match: \.ramu\)
 .*: warning: .* may change behaviour for section .\.code\.4. from .* \(assigned to \.raml, but additional match: \.ramu\)
-.*: warning: .* makes section .\.code\.2. from .* match /DISCARD/ clause\.
-.*: warning: .* makes section .\.code\.4. from .* match /DISCARD/ clause\.
+.*: warning: .* makes section .\.code\.2. from .* match /DISCARD/ clause\.  If the section can be assigned to an output section, it won't be discarded\.
+.*: warning: .* makes section .\.code\.4. from .* match /DISCARD/ clause\.  If the section can be assigned to an output section, it won't be discarded\.
 .*: warning: .* may change behaviour for section .\.code\.1. from .* \(assigned to \.ramu, but additional match: \.ramz\)
 .*: warning: .* may change behaviour for section .\.code\.2. from .* \(assigned to \.ramu, but additional match: \.ramz\)
 .*: warning: .* may change behaviour for section .\.code\.3. from .* \(assigned to \.ramu, but additional match: \.ramz\)
-- 
2.51.0