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Re: [PATCH] PR ld/17550: Section groups (comdat/linkonce) create undefined symbols unnecessarily

From: "H.J. Lu" <hjl dot tools at gmail dot com>
To: Alan Modra <amodra at gmail dot com>
Cc: Binutils <binutils at sourceware dot org>
Date: Fri, 6 May 2016 09:49:09 -0700
Subject: Re: [PATCH] PR ld/17550: Section groups (comdat/linkonce) create undefined symbols unnecessarily
Authentication-results: sourceware.org; auth=none
References: <20160505130054 dot GA19324 at intel dot com> <20160506012057 dot GB31342 at bubble dot grove dot modra dot org>

On Thu, May 5, 2016 at 6:20 PM, Alan Modra <amodra@gmail.com> wrote:
> On Thu, May 05, 2016 at 06:00:54AM -0700, H.J. Lu wrote:
>> When a global symbol is defined in COMDAT group, we shouldn't leave an
>> undefined symbol in symbol table when the symbol section is discarded
>> unless there is a reference to the symbol outside of COMDAT group.
>>
>> OK for master?
>
> OK if you've tested this with our big list of targets.
>

This is what I checked in.

-- 
H.J.

From ae9f0baf952618833005cf40cb3e85b7fd67e9d3 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Thu, 5 May 2016 05:35:01 -0700
Subject: [PATCH] Strip global symbol defined in discarded section

When a global symbol is defined in COMDAT group, we shouldn't leave an
undefined symbol in symbol table when the symbol section is discarded
unless there is a reference to the symbol outside of COMDAT group.

bfd/

	PR ld/17550
	* elf-bfd.h (elf_link_hash_entry): Update comments for indx,
	documenting that indx == -3 if symbol is defined in a discarded
	section.
	* elflink.c (elf_link_add_object_symbols): Set indx to -3 if
	symbol is defined in a discarded section.
	(elf_link_output_extsym): Strip a global symbol defined in a
	discarded section.

ld/

	PR ld/17550
	* testsuite/ld-elf/pr17550-1.s: New file.
	* testsuite/ld-elf/pr17550-2.s: Likewise.
	* testsuite/ld-elf/pr17550-3.s: Likewise.
	* testsuite/ld-elf/pr17550-4.s: Likewise.
	* testsuite/ld-elf/pr17550a.d: Likewise.
	* testsuite/ld-elf/pr17550b.d: Likewise.
	* testsuite/ld-elf/pr17550c.d: Likewise.
	* testsuite/ld-elf/pr17550d.d: Likewise.
---
 bfd/elf-bfd.h                   |  3 ++-
 bfd/elflink.c                   | 12 ++++++++++++
 ld/testsuite/ld-elf/pr17550-1.s |  9 +++++++++
 ld/testsuite/ld-elf/pr17550-2.s |  6 ++++++
 ld/testsuite/ld-elf/pr17550-3.s | 14 ++++++++++++++
 ld/testsuite/ld-elf/pr17550-4.s | 15 +++++++++++++++
 ld/testsuite/ld-elf/pr17550a.d  | 14 ++++++++++++++
 ld/testsuite/ld-elf/pr17550b.d  | 14 ++++++++++++++
 ld/testsuite/ld-elf/pr17550c.d  |  9 +++++++++
 ld/testsuite/ld-elf/pr17550d.d  | 13 +++++++++++++
 10 files changed, 108 insertions(+), 1 deletion(-)
 create mode 100644 ld/testsuite/ld-elf/pr17550-1.s
 create mode 100644 ld/testsuite/ld-elf/pr17550-2.s
 create mode 100644 ld/testsuite/ld-elf/pr17550-3.s
 create mode 100644 ld/testsuite/ld-elf/pr17550-4.s
 create mode 100644 ld/testsuite/ld-elf/pr17550a.d
 create mode 100644 ld/testsuite/ld-elf/pr17550b.d
 create mode 100644 ld/testsuite/ld-elf/pr17550c.d
 create mode 100644 ld/testsuite/ld-elf/pr17550d.d

diff --git a/bfd/elf-bfd.h b/bfd/elf-bfd.h
index 9067dd9..863fc39 100644
--- a/bfd/elf-bfd.h
+++ b/bfd/elf-bfd.h
@@ -124,7 +124,8 @@ struct elf_link_hash_entry
   struct bfd_link_hash_entry root;
 
   /* Symbol index in output file.  This is initialized to -1.  It is
-     set to -2 if the symbol is used by a reloc.  */
+     set to -2 if the symbol is used by a reloc.  It is set to -3 if
+     this symbol is defined in a discarded section.  */
   long indx;
 
   /* Symbol index as a dynamic symbol.  Initialized to -1, and remains
diff --git a/bfd/elflink.c b/bfd/elflink.c
index b6ff6b6..6ccd5fc 100644
--- a/bfd/elflink.c
+++ b/bfd/elflink.c
@@ -4082,6 +4082,7 @@ error_free_dyn:
       bfd_boolean old_weak;
       bfd_boolean override;
       bfd_boolean common;
+      bfd_boolean discarded;
       unsigned int old_alignment;
       bfd *old_bfd;
       bfd_boolean matched;
@@ -4092,6 +4093,7 @@ error_free_dyn:
       sec = NULL;
       value = isym->st_value;
       common = bed->common_definition (isym);
+      discarded = FALSE;
 
       bind = ELF_ST_BIND (isym->st_info);
       switch (bind)
@@ -4142,6 +4144,7 @@ error_free_dyn:
 	      /* Symbols from discarded section are undefined.  We keep
 		 its visibility.  */
 	      sec = bfd_und_section_ptr;
+	      discarded = TRUE;
 	      isym->st_shndx = SHN_UNDEF;
 	    }
 	  else if ((abfd->flags & (EXEC_P | DYNAMIC)) != 0)
@@ -4385,6 +4388,11 @@ error_free_dyn:
 	     || h->root.type == bfd_link_hash_warning)
 	h = (struct elf_link_hash_entry *) h->root.u.i.link;
 
+      /* Setting the index to -3 tells elf_link_output_extsym that
+	 this symbol is defined in a discarded section.  */
+      if (discarded)
+	h->indx = -3;
+
       *sym_hash = h;
 
       new_weak = (flags & BSF_WEAK) != 0;
@@ -9201,6 +9209,10 @@ elf_link_output_extsym (struct bfd_hash_entry *bh, void *data)
 	      return FALSE;
 	    }
 	}
+
+      /* Strip a global symbol defined in a discarded section.  */
+      if (h->indx == -3)
+	return TRUE;
     }
 
   /* We should also warn if a forced local symbol is referenced from
diff --git a/ld/testsuite/ld-elf/pr17550-1.s b/ld/testsuite/ld-elf/pr17550-1.s
new file mode 100644
index 0000000..3da73dc
--- /dev/null
+++ b/ld/testsuite/ld-elf/pr17550-1.s
@@ -0,0 +1,9 @@
+	.section .data,"awG",%progbits,foo_group,comdat
+	.dc.a	x_alias
+	.type	x, %object
+	.p2align 2
+	.size	x, 4
+x:
+	.zero	4
+	.globl	x_alias
+	.set	x_alias,x
diff --git a/ld/testsuite/ld-elf/pr17550-2.s b/ld/testsuite/ld-elf/pr17550-2.s
new file mode 100644
index 0000000..eb4120f
--- /dev/null
+++ b/ld/testsuite/ld-elf/pr17550-2.s
@@ -0,0 +1,6 @@
+	.section .data,"awG",%progbits,foo_group,comdat
+	.type	x, %object
+	.p2align 2
+	.size	x, 4
+x:
+	.zero	4
diff --git a/ld/testsuite/ld-elf/pr17550-3.s b/ld/testsuite/ld-elf/pr17550-3.s
new file mode 100644
index 0000000..3d7c252
--- /dev/null
+++ b/ld/testsuite/ld-elf/pr17550-3.s
@@ -0,0 +1,14 @@
+	.data
+	.dc.a	y
+	.section .data,"awG",%progbits,foo_group,comdat
+	.type	y, %object
+	.size	y, 4
+y:
+	.zero	4
+	.globl	x
+	.type	x, %object
+	.size	x, 4
+x:
+	.zero	4
+	.globl	x_alias
+	.set	x_alias,x
diff --git a/ld/testsuite/ld-elf/pr17550-4.s b/ld/testsuite/ld-elf/pr17550-4.s
new file mode 100644
index 0000000..d0442fd
--- /dev/null
+++ b/ld/testsuite/ld-elf/pr17550-4.s
@@ -0,0 +1,15 @@
+	.data
+	.dc.a	y
+	.section .data,"awG",%progbits,foo_group,comdat
+	.globl	y
+	.type	y, %object
+	.size	y, 4
+y:
+	.zero	4
+	.globl	x
+	.type	x, %object
+	.size	x, 4
+x:
+	.zero	4
+	.globl	x_alias
+	.set	x_alias,x
diff --git a/ld/testsuite/ld-elf/pr17550a.d b/ld/testsuite/ld-elf/pr17550a.d
new file mode 100644
index 0000000..c8f0424
--- /dev/null
+++ b/ld/testsuite/ld-elf/pr17550a.d
@@ -0,0 +1,14 @@
+#source: pr17550-1.s
+#source: pr17550-2.s
+#ld: -r
+#readelf: -s --wide
+#notarget: alpha-*-* cr16-*-* crx-*-* d30v-*-* dlx-*-* i960-*-* pj*-*-*
+# Disabled on alpha because alpha has a different .set directive.
+# cr16 and crx use non-standard scripts with memory regions, which don't
+# play well with comdat group sections under ld -r.  Generic linker
+# targets don't support comdat group sections.
+
+#failif
+#...
+ +[0-9]+: +[0-9a-f]+ +0 +OBJECT +GLOBAL +DEFAULT +UND x_alias
+#...
diff --git a/ld/testsuite/ld-elf/pr17550b.d b/ld/testsuite/ld-elf/pr17550b.d
new file mode 100644
index 0000000..d189747
--- /dev/null
+++ b/ld/testsuite/ld-elf/pr17550b.d
@@ -0,0 +1,14 @@
+#source: pr17550-2.s
+#source: pr17550-1.s
+#ld: -r
+#readelf: -s --wide
+#notarget: alpha-*-* cr16-*-* crx-*-* d30v-*-* dlx-*-* i960-*-* pj*-*-*
+# Disabled on alpha because alpha has a different .set directive.
+# cr16 and crx use non-standard scripts with memory regions, which don't
+# play well with comdat group sections under ld -r.  Generic linker
+# targets don't support comdat group sections.
+
+#failif
+#...
+ +[0-9]+: +[0-9a-f]+ +0 +OBJECT +GLOBAL +DEFAULT +UND x_alias
+#...
diff --git a/ld/testsuite/ld-elf/pr17550c.d b/ld/testsuite/ld-elf/pr17550c.d
new file mode 100644
index 0000000..23a83f1
--- /dev/null
+++ b/ld/testsuite/ld-elf/pr17550c.d
@@ -0,0 +1,9 @@
+#source: pr17550-2.s
+#source: pr17550-3.s
+#ld: -r
+#error: .*: defined in discarded section `\.data\[foo_group\]'
+#notarget: alpha-*-* cr16-*-* crx-*-* d30v-*-* dlx-*-* i960-*-* pj*-*-*
+# Disabled on alpha because alpha has a different .set directive.
+# cr16 and crx use non-standard scripts with memory regions, which don't
+# play well with comdat group sections under ld -r.  Generic linker
+# targets don't support comdat group sections.
diff --git a/ld/testsuite/ld-elf/pr17550d.d b/ld/testsuite/ld-elf/pr17550d.d
new file mode 100644
index 0000000..e8fad96
--- /dev/null
+++ b/ld/testsuite/ld-elf/pr17550d.d
@@ -0,0 +1,13 @@
+#source: pr17550-2.s
+#source: pr17550-4.s
+#ld: -r
+#readelf: -s --wide
+#notarget: alpha-*-* cr16-*-* crx-*-* d30v-*-* dlx-*-* i960-*-* pj*-*-*
+# Disabled on alpha because alpha has a different .set directive.
+# cr16 and crx use non-standard scripts with memory regions, which don't
+# play well with comdat group sections under ld -r.  Generic linker
+# targets don't support comdat group sections.
+
+#...
+ +[0-9]+: +[0-9a-f]+ +0 +OBJECT +GLOBAL +DEFAULT +UND y
+#pass
-- 
2.5.5

References:
- [PATCH] PR ld/17550: Section groups (comdat/linkonce) create undefined symbols unnecessarily
  - From: H.J. Lu
- Re: [PATCH] PR ld/17550: Section groups (comdat/linkonce) create undefined symbols unnecessarily
  - From: Alan Modra

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