[PATCH] Reduce file size for PT_GNU_RELRO segment

["H.J. Lu" <hongjiu dot lu at intel dot com>]

When a padding in file is used to align PT_GNU_RELRO segment, the maximum
padding size is maximum page size minus 1.  This patch trades memory size
for file size by increasing memory size to avoid padding if file size
will be reduced by more than maximum page minus a page and memory size
will be increased by less than a page.

OK for master?

Thanks.

H.J.
---
	* ldlang.c (output_prev_load_sec_find): New.
	(lang_size_sections_1): Reduce file size for PT_GNU_RELRO
	segment by increasing memory size.
---
 ld/ldlang.c | 63 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/ld/ldlang.c b/ld/ldlang.c
index 8880821..3450c9d 100644
--- a/ld/ldlang.c
+++ b/ld/ldlang.c
@@ -1718,6 +1718,31 @@ output_prev_sec_find (lang_output_section_statement_type *os)
   return NULL;
 }
 
+/* Find the last output load section with contents before given output
+   statement.  */
+
+static asection *
+output_prev_load_sec_find (lang_output_section_statement_type *os)
+{
+  lang_output_section_statement_type *lookup;
+
+  for (lookup = os->prev; lookup != NULL; lookup = lookup->prev)
+    {
+      if (lookup->constraint < 0)
+	continue;
+
+      if (lookup->bfd_section != NULL
+	  && lookup->bfd_section->owner != NULL
+	  && ((lookup->bfd_section->flags & (SEC_ALLOC
+					     | SEC_LOAD
+					     | SEC_HAS_CONTENTS))
+	      == (SEC_ALLOC | SEC_LOAD | SEC_HAS_CONTENTS)))
+	return lookup->bfd_section;
+    }
+
+  return NULL;
+}
+
 /* Look for a suitable place for a new output section statement.  The
    idea is to skip over anything that might be inside a SECTIONS {}
    statement in a script, before we find another output section
@@ -4776,6 +4801,12 @@ lang_size_sections_1
    bfd_boolean check_regions)
 {
   lang_statement_union_type *s;
+  bfd_boolean reduce_padding
+    = (link_info.relro
+       && expld.dataseg.phase == exp_dataseg_relro_adjust
+       && expld.dataseg.maxpagesize > expld.dataseg.pagesize);
+  bfd_vma pagsizediff
+    = expld.dataseg.maxpagesize - expld.dataseg.pagesize;
 
   /* Size up the sections from their constituent parts.  */
   for (s = *prev; s != NULL; s = s->header.next)
@@ -4918,6 +4949,38 @@ lang_size_sections_1
 		  {
 		    bfd_vma savedot = newdot;
 		    newdot = align_power (newdot, section_alignment);
+		    if (reduce_padding && expld.dataseg.base == newdot)
+		      {
+			/* If a padding in file will be used for
+			   PT_GNU_RELRO segment, check if we can reduce
+			   the padding by increase the address of the
+			   first relro section.  It is a tradeoff between
+			   memory size and file size.  We only do it if
+			   file size will be reduced by more than maximum
+			   page minus a page and memory size will be
+			   increased by less than a page.  */
+			asection *prev_sec
+			  = output_prev_load_sec_find (os);
+			if (prev_sec)
+			  {
+			    bfd_vma pad
+			      = ((newdot - (bfd_section_vma (prev_sec->owner,
+							     prev_sec)
+					    + bfd_get_section_size (prev_sec)))
+				 % expld.dataseg.maxpagesize);
+			    if (pad)
+			      {
+				bfd_vma nextdot
+				  = expld.dataseg.maxpagesize - pad;
+				nextdot = align_power (newdot + nextdot,
+						       section_alignment);
+				if (((nextdot - newdot)
+				     < expld.dataseg.pagesize)
+				    && pad > pagsizediff)
+				  newdot = nextdot;
+			      }
+			  }
+		      }
 
 		    dotdelta = newdot - savedot;
 		    if (dotdelta != 0
-- 
2.1.0