Make assignments to dot keep an empty output section

[Alan Modra <amodra at gmail dot com>]

On Tue, Jan 21, 2014 at 01:54:04PM +0000, Douglas Graham wrote:
> So I guess a simple workaround  is to add a ". = . + 4" inside .sdata2 before aligning, to force that section to always contain at least 4  bytes.

Yes, that will work.  I'm also going to commit the following so this
problem doesn't bite other people.

An assignment to dot in an output section that allocates space of
course keeps the output section.  Here, I'm changing the behaviour for
assignments that don't allocate space.  The idea is not so much to
allow people to force output of an empty section with ". = .", but
to fix cases where an otherwise empty section has padding added by an
alignment expression that changes with relaxation or .eh_frame
editing.  Such a section might have zero size before relaxation and so
be stripped incorrectly.

An alternate fix is to delay stripping of sections, but that would
require patches all over the place.

ld/
	* ld.texinfo (Output Section Discarding): Mention assigning to dot
	as a way of keeping otherwise empty sections.
	* ldexp.c (is_dot, is_value, is_sym_value, is_dot_ne_0,
	is_dot_plus_0, is_align_conditional): New predicates.
	(exp_fold_tree_1): Set SEC_KEEP when assigning to dot inside an
	output section, except for some special cases.
	* scripttempl/elfmicroblaze.sc: Use canonical form to align at
	end of .heap and .stack.
ld/testsuite/
	* ld-shared/elf-offset.ld: Align end of .bss with canonical form
	of ALIGN that allows an empty .bss to be removed.
	* ld-arm/arm-dyn.ld: Likewise.
	* ld-arm/arm-lib.ld: Likewise.
	* ld-elfvsb/elf-offset.ld: Likewise.
	* ld-mips-elf/mips-dyn.ld: Likewise.
	* ld-mips-elf/mips-lib.ld: Likewise.
	* ld-arm/arm-no-rel-plt.ld: Remove duplicate ALIGN.
	* ld-powerpc/vle-multiseg-1.ld: Remove ALIGN at start of section.
	ALIGN address of section instead.
	* ld-powerpc/vle-multiseg-2.ld: Likewise.
	* ld-powerpc/vle-multiseg-3.ld: Likewise.
	* ld-powerpc/vle-multiseg-4.ld: Likewise.
	* ld-powerpc/vle-multiseg-6.ld: Likewise.
	* ld-scripts/empty-aligned.d: Check section headers not program
	headers.  Remove xfail and notarget.
	* ld-scripts/empty-aligned.t: Use canonical ALIGN for end of .text2.

diff --git a/ld/ld.texinfo b/ld/ld.texinfo
index ae3d568..a8e5ea6 100644
--- a/ld/ld.texinfo
+++ b/ld/ld.texinfo
@@ -4406,9 +4406,9 @@ scripts.
 @cindex discarding sections
 @cindex sections, discarding
 @cindex removing sections
-The linker will not create output sections with no contents.  This is
-for convenience when referring to input sections that may or may not
-be present in any of the input files.  For example:
+The linker will not normally create output sections with no contents.
+This is for convenience when referring to input sections that may or
+may not be present in any of the input files.  For example:
 @smallexample
 .foo : @{ *(.foo) @}
 @end smallexample
@@ -4416,7 +4416,12 @@ be present in any of the input files.  For example:
 will only create a @samp{.foo} section in the output file if there is a
 @samp{.foo} section in at least one input file, and if the input
 sections are not all empty.  Other link script directives that allocate
-space in an output section will also create the output section.
+space in an output section will also create the output section.  So
+too will assignments to dot even if the assignment does not create
+space, except for @samp{. = 0}, @samp{. = . + 0}, @samp{. = sym},
+@samp{. = . + sym} and @samp{. = ALIGN (. != 0, expr, 1)} when
+@samp{sym} is an absolute symbol of value 0 defined in the script.
+This allows you to force output of an empty section with @samp{. = .}.
 
 The linker will ignore address assignments (@pxref{Output Section Address})
 on discarded output sections, except when the linker script defines
diff --git a/ld/ldexp.c b/ld/ldexp.c
index 9a529db..cc3be6d 100644
--- a/ld/ldexp.c
+++ b/ld/ldexp.c
@@ -790,6 +790,89 @@ fold_name (etree_type *tree)
     }
 }
 
+/* Return true if TREE is '.'.  */
+ 
+static bfd_boolean
+is_dot (const etree_type *tree)
+{
+  return (tree->type.node_class == etree_name
+	  && tree->type.node_code == NAME
+	  && tree->name.name[0] == '.'
+	  && tree->name.name[1] == 0);
+}
+
+/* Return true if TREE is a constant equal to VAL.  */
+
+static bfd_boolean
+is_value (const etree_type *tree, bfd_vma val)
+{
+  return (tree->type.node_class == etree_value
+	  && tree->value.value == val);
+}
+
+/* Return true if TREE is an absolute symbol equal to VAL defined in
+   a linker script.  */
+
+static bfd_boolean
+is_sym_value (const etree_type *tree, bfd_vma val)
+{
+  struct bfd_link_hash_entry *h;
+  struct lang_definedness_hash_entry *def;
+
+  return (tree->type.node_class == etree_name
+	  && tree->type.node_code == NAME
+	  && (def = lang_symbol_defined (tree->name.name)) != NULL
+	  && def->by_script
+	  && def->iteration == (lang_statement_iteration & 1)
+	  && (h = bfd_wrapped_link_hash_lookup (link_info.output_bfd,
+						&link_info,
+						tree->name.name,
+						FALSE, FALSE, TRUE)) != NULL
+	  && h->type == bfd_link_hash_defined
+	  && h->u.def.section == bfd_abs_section_ptr
+	  && h->u.def.value == val);
+}
+
+/* Return true if TREE is ". != 0".  */
+
+static bfd_boolean
+is_dot_ne_0 (const etree_type *tree)
+{
+  return (tree->type.node_class == etree_binary
+	  && tree->type.node_code == NE
+	  && is_dot (tree->binary.lhs)
+	  && is_value (tree->binary.rhs, 0));
+}
+
+/* Return true if TREE is ". = . + 0" or ". = . + sym" where sym is an
+   absolute constant with value 0 defined in a linker script.  */
+
+static bfd_boolean
+is_dot_plus_0 (const etree_type *tree)
+{
+  return (tree->type.node_class == etree_binary
+	  && tree->type.node_code == '+'
+	  && is_dot (tree->binary.lhs)
+	  && (is_value (tree->binary.rhs, 0)
+	      || is_sym_value (tree->binary.rhs, 0)));
+}
+
+/* Return true if TREE is "ALIGN (. != 0 ? some_expression : 1)".  */
+
+static bfd_boolean
+is_align_conditional (const etree_type *tree)
+{
+  if (tree->type.node_class == etree_unary
+      && tree->type.node_code == ALIGN_K)
+    {
+      tree = tree->unary.child;
+      return (tree->type.node_class == etree_trinary
+	      && is_dot_ne_0 (tree->trinary.cond)
+	      && is_value (tree->trinary.rhs, 1));
+    }
+  return 0;
+}
+
 static void
 exp_fold_tree_1 (etree_type *tree)
 {
@@ -854,6 +937,20 @@ exp_fold_tree_1 (etree_type *tree)
 	      exp_fold_tree_1 (tree->assign.src);
 	      expld.assigning_to_dot = FALSE;
 
+	      /* If we are assigning to dot inside an output section
+		 arrange to keep the section, except for certain
+		 expressions that evaluate to zero.  We ignore . = 0,
+		 . = . + 0, and . = ALIGN (. != 0 ? expr : 1).  */
+	      if (expld.phase == lang_mark_phase_enum
+		  && expld.section != bfd_abs_section_ptr
+		  && !(expld.result.valid_p
+		       && expld.result.value == 0
+		       && (is_value (tree->assign.src, 0)
+			   || is_sym_value (tree->assign.src, 0)
+			   || is_dot_plus_0 (tree->assign.src)
+			   || is_align_conditional (tree->assign.src))))
+		expld.section->flags |= SEC_KEEP;
+
 	      if (!expld.result.valid_p)
 		{
 		  if (expld.phase != lang_mark_phase_enum)
diff --git a/ld/scripttempl/elfmicroblaze.sc b/ld/scripttempl/elfmicroblaze.sc
index a54b891..8d2e6f2 100644
--- a/ld/scripttempl/elfmicroblaze.sc
+++ b/ld/scripttempl/elfmicroblaze.sc
@@ -183,7 +183,7 @@ SECTIONS
     ${RELOCATING+*(.bss.*)}
     ${RELOCATING+*(.gnu.linkonce.b.*)}
     ${RELOCATING+*(COMMON)}
-    ${RELOCATING+. = ALIGN(4);}  
+    ${RELOCATING+. = ALIGN(. != 0 ? 4 : 1);}
     
     ${RELOCATING+PROVIDE (__bss_end = .);}
 
@@ -203,7 +203,7 @@ SECTIONS
   .stack : {
     ${RELOCATING+ _stack_end = .;}
     ${RELOCATING+ . += _STACK_SIZE;}
-    ${RELOCATING+ . = ALIGN(8);}
+    ${RELOCATING+ . = ALIGN(. != 0 ? 8 : 1);}
     ${RELOCATING+ _stack = .;}
     ${RELOCATING+ _end = .;}
   }
diff --git a/ld/testsuite/ld-arm/arm-dyn.ld b/ld/testsuite/ld-arm/arm-dyn.ld
index bef9a18..71cb4c9 100644
--- a/ld/testsuite/ld-arm/arm-dyn.ld
+++ b/ld/testsuite/ld-arm/arm-dyn.ld
@@ -149,7 +149,7 @@ SECTIONS
    /* Align here to ensure that the .bss section occupies space up to
       _end.  Align after .bss to ensure correct alignment even if the
       .bss section disappears because there are no input sections.  */
-   . = ALIGN(32 / 8);
+   . = ALIGN(. != 0 ? 32 / 8 : 1);
   }
   . = ALIGN(32 / 8);
   _end = .;
diff --git a/ld/testsuite/ld-arm/arm-lib.ld b/ld/testsuite/ld-arm/arm-lib.ld
index c9482c3..f158c23 100644
--- a/ld/testsuite/ld-arm/arm-lib.ld
+++ b/ld/testsuite/ld-arm/arm-lib.ld
@@ -141,7 +141,7 @@ SECTIONS
    /* Align here to ensure that the .bss section occupies space up to
       _end.  Align after .bss to ensure correct alignment even if the
       .bss section disappears because there are no input sections.  */
-   . = ALIGN(32 / 8);
+   . = ALIGN(. != 0 ? 32 / 8 : 1);
   }
   . = ALIGN(32 / 8);
   _end = .;
diff --git a/ld/testsuite/ld-arm/arm-no-rel-plt.ld b/ld/testsuite/ld-arm/arm-no-rel-plt.ld
index 439909c..d8e5c9d 100644
--- a/ld/testsuite/ld-arm/arm-no-rel-plt.ld
+++ b/ld/testsuite/ld-arm/arm-no-rel-plt.ld
@@ -181,7 +181,6 @@ SECTIONS
   }
   _bss_end__ = . ; __bss_end__ = . ;
   . = ALIGN(32 / 8);
-  . = ALIGN(32 / 8);
   __end__ = . ;
   _end = .; PROVIDE (end = .);
   /* Stabs debugging sections.  */
diff --git a/ld/testsuite/ld-elfvsb/elf-offset.ld b/ld/testsuite/ld-elfvsb/elf-offset.ld
index 7c64824..cd90bcc 100644
--- a/ld/testsuite/ld-elfvsb/elf-offset.ld
+++ b/ld/testsuite/ld-elfvsb/elf-offset.ld
@@ -131,7 +131,7 @@ SECTIONS
    /* Align here to ensure that the .bss section occupies space up to
       _end.  Align after .bss to ensure correct alignment even if the
       .bss section disappears because there are no input sections.  */
-   . = ALIGN(32 / 8);
+   . = ALIGN(. != 0 ? 32 / 8 : 1);
   }
   . = ALIGN(32 / 8);
   _end = . ;
diff --git a/ld/testsuite/ld-mips-elf/mips-dyn.ld b/ld/testsuite/ld-mips-elf/mips-dyn.ld
index e4f90d2..b931e1b 100644
--- a/ld/testsuite/ld-mips-elf/mips-dyn.ld
+++ b/ld/testsuite/ld-mips-elf/mips-dyn.ld
@@ -179,7 +179,7 @@ SECTIONS
    /* Align here to ensure that the .bss section occupies space up to
       _end.  Align after .bss to ensure correct alignment even if the
       .bss section disappears because there are no input sections.  */
-   . = ALIGN(32 / 8);
+   . = ALIGN(. != 0 ? 32 / 8 : 1);
   }
   . = ALIGN(32 / 8);
   _end = .;
diff --git a/ld/testsuite/ld-mips-elf/mips-lib.ld b/ld/testsuite/ld-mips-elf/mips-lib.ld
index 5073d9f..1d66c62 100644
--- a/ld/testsuite/ld-mips-elf/mips-lib.ld
+++ b/ld/testsuite/ld-mips-elf/mips-lib.ld
@@ -173,7 +173,7 @@ SECTIONS
    /* Align here to ensure that the .bss section occupies space up to
       _end.  Align after .bss to ensure correct alignment even if the
       .bss section disappears because there are no input sections.  */
-   . = ALIGN(32 / 8);
+   . = ALIGN(. != 0 ? 32 / 8 : 1);
   }
   . = ALIGN(32 / 8);
   _end = .;
diff --git a/ld/testsuite/ld-powerpc/vle-multiseg-1.ld b/ld/testsuite/ld-powerpc/vle-multiseg-1.ld
index f2ff319..3fe37b9 100644
--- a/ld/testsuite/ld-powerpc/vle-multiseg-1.ld
+++ b/ld/testsuite/ld-powerpc/vle-multiseg-1.ld
@@ -4,7 +4,6 @@ SECTIONS
   { *(.data) *(.ctors) *(.dtors) *(.eh_frame) *(.jcr) }
   .text_vle	0x00001000 :
   {
-	. = ALIGN(16);
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.text_vle)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.text)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.init)
@@ -12,6 +11,6 @@ SECTIONS
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.fini)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.fini_vle) 		
   }
-  .text_iv	. : { . = ALIGN(16); *(.text_iv) }
+  .text_iv	ALIGN(16)  : { *(.text_iv) }
   .iv_handlers	0x0001F000 : { *(.iv_handlers) }
 }
diff --git a/ld/testsuite/ld-powerpc/vle-multiseg-2.ld b/ld/testsuite/ld-powerpc/vle-multiseg-2.ld
index 2320b61..da9a79d 100644
--- a/ld/testsuite/ld-powerpc/vle-multiseg-2.ld
+++ b/ld/testsuite/ld-powerpc/vle-multiseg-2.ld
@@ -2,7 +2,6 @@ SECTIONS
 {
   .text_vle	0x00001000 :
   {
-	. = ALIGN(16);
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.text_vle)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.text)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.init)
@@ -12,6 +11,6 @@ SECTIONS
   }
   .data		0x00001400 :
   { *(.data) *(.ctors) *(.dtors) *(.eh_frame) *(.jcr) }
-  .text_iv	. : { . = ALIGN(16); *(.text_iv) }
+  .text_iv	ALIGN(16)  : { *(.text_iv) }
   .iv_handlers	0x0001F000 : { *(.iv_handlers) }
 }
diff --git a/ld/testsuite/ld-powerpc/vle-multiseg-3.ld b/ld/testsuite/ld-powerpc/vle-multiseg-3.ld
index 0ed2f44..cfb1d88 100644
--- a/ld/testsuite/ld-powerpc/vle-multiseg-3.ld
+++ b/ld/testsuite/ld-powerpc/vle-multiseg-3.ld
@@ -2,7 +2,6 @@ SECTIONS
 {
   .text_vle	0x00001000 :
   {
-	. = ALIGN(16);
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.text_vle)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.text)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.init)
@@ -10,7 +9,7 @@ SECTIONS
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.fini)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.fini_vle) 		
   }
-  .text_iv	. : { . = ALIGN(16); *(.text_iv) }
+  .text_iv	ALIGN(16)  : { *(.text_iv) }
   .data		0x00001400 :
   { *(.data) *(.ctors) *(.dtors) *(.eh_frame) *(.jcr) }
   .iv_handlers	0x0001F000 : { *(.iv_handlers) }
diff --git a/ld/testsuite/ld-powerpc/vle-multiseg-4.ld b/ld/testsuite/ld-powerpc/vle-multiseg-4.ld
index 503fe06..2130427 100644
--- a/ld/testsuite/ld-powerpc/vle-multiseg-4.ld
+++ b/ld/testsuite/ld-powerpc/vle-multiseg-4.ld
@@ -2,7 +2,6 @@ SECTIONS
 {
   .text_vle	0x00001000 :
   {
-	. = ALIGN(16);
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.text_vle)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.text)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.init)
@@ -10,7 +9,7 @@ SECTIONS
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.fini)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.fini_vle) 		
   }
-  .text_iv	. : { . = ALIGN(16); *(.text_iv) }
+  .text_iv	ALIGN(16)  : { *(.text_iv) }
   .iv_handlers	0x0001F000 : { *(.iv_handlers) }
   .data		0x00020400 :
   { *(.data) *(.ctors) *(.dtors) *(.eh_frame) *(.jcr) }
diff --git a/ld/testsuite/ld-powerpc/vle-multiseg-6.ld b/ld/testsuite/ld-powerpc/vle-multiseg-6.ld
index c8d88dd..2db76cc 100644
--- a/ld/testsuite/ld-powerpc/vle-multiseg-6.ld
+++ b/ld/testsuite/ld-powerpc/vle-multiseg-6.ld
@@ -16,7 +16,6 @@ SECTIONS
   }
   .text_vle	0x00001000 :
   {
-	. = ALIGN(16);
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.text*)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.init*)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.fini*)
@@ -24,14 +23,12 @@ SECTIONS
 
   .text_iv	0x100000 :
   {
-	. = ALIGN(16);
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.text_iv)
 	INPUT_SECTION_FLAGS (SHF_PPC_VLE) *(.iv_handlers)
   } >vle_seg2
 
   .text 	0x101000 :
   {
-	. = ALIGN(16);
 	INPUT_SECTION_FLAGS (!SHF_PPC_VLE) *(.text*)
   }
 }
diff --git a/ld/testsuite/ld-scripts/empty-aligned.d b/ld/testsuite/ld-scripts/empty-aligned.d
index d9916dd..ff131eb 100644
--- a/ld/testsuite/ld-scripts/empty-aligned.d
+++ b/ld/testsuite/ld-scripts/empty-aligned.d
@@ -1,14 +1,8 @@
 #source: empty-aligned.s
 #ld: -T empty-aligned.t
-#readelf: -l --wide
-#xfail: "hppa64-*-*"
-#notarget: frv-*-*linux*
+#readelf: -S --wide
 
 #...
-Program Headers:
- +Type +Offset +VirtAddr +PhysAddr +FileSiz +MemSiz +Flg +Align
- +LOAD +0x[0-9a-f]+ 0x[0-9a-f]+ 0x[0-9a-f]+ 0x[0-9a-f]+ 0x[0-9a-f]+ [RWE ]+ +0x[0-9a-f]+
-! +LOAD .*
-#...
- +Segment Sections\.\.\.
- +00 +.text 
+.* .text .*
+!.* .text[234] .*
+#pass
diff --git a/ld/testsuite/ld-scripts/empty-aligned.t b/ld/testsuite/ld-scripts/empty-aligned.t
index e59bc20..5f6a38d 100644
--- a/ld/testsuite/ld-scripts/empty-aligned.t
+++ b/ld/testsuite/ld-scripts/empty-aligned.t
@@ -6,14 +6,14 @@ SECTIONS
   {
     *(.text1)
   }
-  /* Same for alignment at beginning and end.  */
+  /* Same for alignment at beginning and end, although we need to be
+     careful in the expression used to align.  */
   .text2 ALIGN (4096) :
   {
     *(.text2)
-    . = ALIGN (4096);
+    . = ALIGN (. != 0 ? 4096 : 1);
   }
-  /* Same for alignment just at end, although we need to be careful in
-     the expression used to align.  */
+  /* Same for alignment just at end.  */
   .text3 :
   {
     *(.text3)
diff --git a/ld/testsuite/ld-shared/elf-offset.ld b/ld/testsuite/ld-shared/elf-offset.ld
index 125d879..ee587f9 100644
--- a/ld/testsuite/ld-shared/elf-offset.ld
+++ b/ld/testsuite/ld-shared/elf-offset.ld
@@ -128,7 +128,7 @@ SECTIONS
    /* Align here to ensure that the .bss section occupies space up to
       _end.  Align after .bss to ensure correct alignment even if the
       .bss section disappears because there are no input sections.  */
-   . = ALIGN(32 / 8);
+   . = ALIGN(. != 0 ? 32 / 8 : 1);
   }
   . = ALIGN(32 / 8);
   _end = . ;


-- 
Alan Modra
Australia Development Lab, IBM