[binutils-gdb] ld: clarify comments on /DISCARD/ output section behavior

[Matthieu Longo]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=fe4a952931334dee80378c9f0606745976d8b30f

commit fe4a952931334dee80378c9f0606745976d8b30f
Author: Matthieu Longo <matthieu.longo@arm.com>
Date:   Fri Sep 12 17:26:38 2025 +0100

    ld: clarify comments on /DISCARD/ output section behavior
    
    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. If the input section can't be assigned
      to the output section for a later reason, an error will be raised.
      Otherwise the input section will be assigned as intended to an output
      section specified by one of the matching clauses previous to /DISCARD/.

Diff:
---
 ld/ldlang.c                                        | 27 ++++++++++++++++++----
 .../non-contiguous-mem/non-contiguous-ok-5.warn    |  4 ++--
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/ld/ldlang.c b/ld/ldlang.c
index bc008c59df1..656edeb4981 100644
--- a/ld/ldlang.c
+++ b/ld/ldlang.c
@@ -2743,17 +2743,36 @@ 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 follows clauses that assign the input section to
+	 an output section. Since /DISCARD/ does not have the precedence,
+	 /DISCARD/ is ignored.
+	 1. If the input section can be assigned to an output section,
+	    the link will succeed. The warning below is emitted with
+	    --enable-non-contiguous-regions-warnings so that the user can
+	    notice that /DISCARD/ did not do what he might have expected,
+	    i.e. discarding the input section.
+	 2. If the input section cannot be assigned, the link will fail
+	    with an error. The warning below is emitted with
+	    --enable-non-contiguous-regions-warnings so that the user can
+	    notice that /DISCARD/ was ignored for this input section, then
+	    leading to a link failure caused by not enough space in the output
+	    section for the input section.  */
+      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 .* \(assigned to \.ramu, but additional match: \.ramz\)