Re: [patch][gold] Don't put rodata and text in the same segment.

[Rafael Espindola <espindola at google dot com>]

Attached is a new version of the rosegment patch that also generalizes
and documents the logic for deciding if we should move forward by a
page.

2010-09-08  Rafael Espindola  <espindola@google.com>

        * layout.cc (Layout::attach_allocated_section_to_segment): Add support
	for --rosegment.
	(Layout::find_first_load_seg): Search all segments to find the first
	one.
	(Layout::set_segment_offsets): Generalize the decision of when to
	move forward by a page.
	* options.h (rosegment): New.

Cheers,
-- 
Rafael Ãvila de EspÃndola

diff --git a/gold/layout.cc b/gold/layout.cc
index bfe6a5e..c2bab29 100644
--- a/gold/layout.cc
+++ b/gold/layout.cc
@@ -1171,10 +1171,11 @@ Layout::attach_allocated_section_to_segment(Output_section* os)
   bool is_address_set = parameters->options().section_start(os->name(), &addr);
 
   // In general the only thing we really care about for PT_LOAD
-  // segments is whether or not they are writable, so that is how we
-  // search for them.  Large data sections also go into their own
-  // PT_LOAD segment.  People who need segments sorted on some other
-  // basis will have to use a linker script.
+  // segments is whether or not they are writable or executable,
+  // so that is how we search for them.
+  // Large data sections also go into their own PT_LOAD segment.
+  // People who need segments sorted on some other basis will
+  // have to use a linker script.
 
   Segment_list::const_iterator p;
   for (p = this->segment_list_.begin();
@@ -1186,6 +1187,9 @@ Layout::attach_allocated_section_to_segment(Output_section* os)
       if (!parameters->options().omagic()
 	  && ((*p)->flags() & elfcpp::PF_W) != (seg_flags & elfcpp::PF_W))
 	continue;
+      if (parameters->options().rosegment()
+          && ((*p)->flags() & elfcpp::PF_X) != (seg_flags & elfcpp::PF_X))
+        continue;
       // If -Tbss was specified, we need to separate the data and BSS
       // segments.
       if (parameters->options().user_set_Tbss())
@@ -1454,6 +1458,7 @@ Layout::define_group_signatures(Symbol_table* symtab)
 Output_segment*
 Layout::find_first_load_seg()
 {
+  Output_segment* best = NULL;
   for (Segment_list::const_iterator p = this->segment_list_.begin();
        p != this->segment_list_.end();
        ++p)
@@ -1462,8 +1467,13 @@ Layout::find_first_load_seg()
 	  && ((*p)->flags() & elfcpp::PF_R) != 0
 	  && (parameters->options().omagic()
 	      || ((*p)->flags() & elfcpp::PF_W) == 0))
-	return *p;
+        {
+          if (best == NULL || this->segment_precedes(*p, best))
+            best = *p;
+        }
     }
+  if (best != NULL)
+    return best;
 
   gold_assert(!this->script_options_->saw_phdrs_clause());
 
@@ -2567,7 +2577,7 @@ Layout::set_segment_offsets(const Target* target, Output_segment* load_seg,
   const bool check_sections = parameters->options().check_sections();
   Output_segment* last_load_segment = NULL;
 
-  bool was_readonly = false;
+  elfcpp::Elf_Word old_flags = -1;
   for (Segment_list::iterator p = this->segment_list_.begin();
        p != this->segment_list_.end();
        ++p)
@@ -2614,21 +2624,26 @@ Layout::set_segment_offsets(const Target* target, Output_segment* load_seg,
 
 	  if (!are_addresses_set)
 	    {
-	      // If the last segment was readonly, and this one is
-	      // not, then skip the address forward one page,
-	      // maintaining the same position within the page.  This
-	      // lets us store both segments overlapping on a single
-	      // page in the file, but the loader will put them on
-	      // different pages in memory.
+	      // If the R, W or X flags changed, skip the address forward one
+	      // page, maintaining the same position within the page.
+	      // Advancing to the next page lets the loader set the flags
+	      // correctly for each segment.
+	      // Maintaining the same position within the page lets us store
+	      // both segments overlapping on a single page in the file, but
+	      // the loader will put them on different pages in memory.
+	      // We will revisit this decision once we know the size of the
+	      // segment.
 
 	      addr = align_address(addr, (*p)->maximum_alignment());
 	      aligned_addr = addr;
 
-	      if (was_readonly && ((*p)->flags() & elfcpp::PF_W) != 0)
-		{
-		  if ((addr & (abi_pagesize - 1)) != 0)
-		    addr = addr + abi_pagesize;
-		}
+              const elfcpp::Elf_Word cur_flags =
+                ((*p)->flags() & (elfcpp::PF_W | elfcpp::PF_R | elfcpp::PF_X));
+
+	      if (old_flags != static_cast<elfcpp::Elf_Word>(-1)
+		  && cur_flags != old_flags
+		  && (addr & (abi_pagesize - 1)) != 0)
+                addr = addr + abi_pagesize;
 
 	      off = orig_off + ((addr - orig_addr) & (abi_pagesize - 1));
 	    }
@@ -2686,8 +2701,8 @@ Layout::set_segment_offsets(const Target* target, Output_segment* load_seg,
 
 	  addr = new_addr;
 
-	  if (((*p)->flags() & elfcpp::PF_W) == 0)
-	    was_readonly = true;
+          old_flags =
+            ((*p)->flags() & (elfcpp::PF_W | elfcpp::PF_R | elfcpp::PF_X));
 
 	  // Implement --check-sections.  We know that the segments
 	  // are sorted by LMA.
diff --git a/gold/options.h b/gold/options.h
index c5e24cc..c990961 100644
--- a/gold/options.h
+++ b/gold/options.h
@@ -808,6 +808,10 @@ class General_options
               N_(" Only search directories specified on the command line."),
               NULL);
 
+  DEFINE_bool(rosegment, options::TWO_DASHES, '\0', false,
+              N_(" Put read-only non-executable sections in their own segment"),
+              NULL);
+
   DEFINE_string(m, options::EXACTLY_ONE_DASH, 'm', "",
                 N_("Ignored for compatibility"), N_("EMULATION"));