Add block range data structure for blocks with non-contiguous address ranges

This patch does the following:

- Introduces a block range data structure which is accessed via
  a new field in struct block.
- Defines several macros for accessing block ranges.
- Defines a new function, make_blockrange, which is responsible for
  creating the new data structure.

It should be noted that some support for non-contiguous ranges already
existed in GDB in the form of blockvector addrmaps.  This support
allowed GDB to quickly find a block containing a particular address
even when the block consists of non-contiguous addresses.  See
find_block_in_blockvector() in block.c, dwarf2_record_block_ranges()
in dwarf2read.c, and record_block_range() in buildsym.c.

Addrmaps do not provide a convenient way to examine address ranges
associated with a particular block.  This data structure (and its
interface) is set up for quickly finding the value (which in this case
is a block) associated with a particular address.  The interface
does not include a method for doing a reverse mapping from blocks to
addresses.  A linear time mapping might be attempted via use of the
addrmap's foreach method, but this is not as straightforward as it
might first appear due to the fact that blocks corresponding to inline
function instances and lexical blocks w/ variables end up getting
interspersed in in the set of transitions.

Note:  If this approach is deemed to be too expensive in terms of
space, an alternate approach might be to attempt the linear time
mapping noted above.  find_pc_partial_function() needs to be able to
quickly know whether there are discontiguous ranges, so a flag for
this property would have to be added to struct block.  Also integral
to this set of changes is the concept of an "entry pc" which might be
different from the block's start address.  An entry_pc field would
also need to be added to struct block.  This does not result in any
space savings in struct block though since the space for the flag and
entry_pc use more space than the blockranges struct pointer that I've
added.  There would, however, be some space savings due to the fact
that the new data structures that I've added for this patch would not
need to be allocated.  (I happen to like the approach I've come up
with, but I wanted to mention another possibility just in case someone
does not.)

gdb/ChangeLog:

	* block.h (blockrange, blockranges): New struct declarations.
	(struct block): Add new field named `ranges'.
	(BLOCK_RANGES, BLOCK_NRANGES, BLOCK_RANGE, BLOCK_CONTIGUOUS_P)
	(BLOCK_RANGE_START, BLOCK_RANGE_END, BLOCK_ENTRY_PC): New
	macros for accessing ranges in struct block.
	(make_blockranges): New declaration.
	block.c (make_blockranges): New function.

Author: KevinBuettner

gdb/ChangeLog

@@ -1,3 +1,14 @@
+2018-08-24  Kevin Buettner  <[email protected]>
+
+	PR gdb/23021
+	* block.h (blockrange, blockranges): New struct declarations.
+	(struct block): Add new field named `ranges'.
+	(BLOCK_RANGES, BLOCK_NRANGES, BLOCK_RANGE, BLOCK_CONTIGUOUS_P)
+	(BLOCK_RANGE_START, BLOCK_RANGE_END, BLOCK_ENTRY_PC): New
+	macros for accessing ranges in struct block.
+	(make_blockranges): New declaration.
+	block.c (make_blockranges): New function.
+
 2018-08-24  Pedro Alves  <[email protected]>
 	    Simon Marchi  <[email protected]>
 

gdb/block.c

@@ -807,3 +807,24 @@ block_find_non_opaque_type_preferred (struct symbol *sym, void *data)
   *best = sym;
   return 0;
 }
+
+/* See block.h.  */
+
+struct blockranges *
+make_blockranges (struct objfile *objfile,
+                  const std::vector<blockrange> &rangevec)
+{
+  struct blockranges *blr;
+  size_t n = rangevec.size();
+
+  blr = (struct blockranges *)
+    obstack_alloc (&objfile->objfile_obstack,
+                   sizeof (struct blockranges)
+		   + (n - 1) * sizeof (struct blockrange));
+
+  blr->nranges = n;
+  for (int i = 0; i < n; i++)
+    blr->range[i] = rangevec[i];
+  return blr;
+}
+

gdb/block.h

@@ -31,6 +31,37 @@ struct using_direct;
 struct obstack;
 struct addrmap;
 
+/* Blocks can occupy non-contiguous address ranges.  When this occurs,
+   startaddr and endaddr within struct block (still) specify the lowest
+   and highest addresses of all ranges, but each individual range is
+   specified by the addresses in struct blockrange.  */
+
+struct blockrange
+{
+  blockrange (CORE_ADDR startaddr_, CORE_ADDR endaddr_)
+    : startaddr (startaddr_),
+      endaddr (endaddr_)
+  {
+  }
+
+  /* Lowest address in this range.  */
+
+  CORE_ADDR startaddr;
+
+  /* One past the highest address in the range.  */
+
+  CORE_ADDR endaddr;
+};
+
+/* Two or more non-contiguous ranges in the same order as that provided
+   via the debug info.  */
+
+struct blockranges
+{
+  int nranges;
+  struct blockrange range[1];
+};
+
 /* All of the name-scope contours of the program
    are represented by `struct block' objects.
    All of these objects are pointed to by the blockvector.
@@ -86,6 +117,12 @@ struct block
      using directives and the current namespace scope.  */
 
   struct block_namespace_info *namespace_info;
+
+  /* Address ranges for blocks with non-contiguous ranges.  If this
+     is NULL, then there is only one range which is specified by
+     startaddr and endaddr above.  */
+
+  struct blockranges *ranges;
 };
 
 /* The global block is singled out so that we can provide a back-link
@@ -109,6 +146,49 @@ struct global_block
 #define BLOCK_DICT(bl)		(bl)->dict
 #define BLOCK_NAMESPACE(bl)	(bl)->namespace_info
 
+/* Accessor for ranges field within block BL.  */
+
+#define BLOCK_RANGES(bl)	(bl)->ranges
+
+/* Number of ranges within a block.  */
+
+#define BLOCK_NRANGES(bl)	(bl)->ranges->nranges
+
+/* Access range array for block BL.  */
+
+#define BLOCK_RANGE(bl)		(bl)->ranges->range
+
+/* Are all addresses within a block contiguous?  */
+
+#define BLOCK_CONTIGUOUS_P(bl)	(BLOCK_RANGES (bl) == nullptr \
+				 || BLOCK_NRANGES (bl) <= 1)
+
+/* Obtain the start address of the Nth range for block BL.  */
+
+#define BLOCK_RANGE_START(bl,n) (BLOCK_RANGE (bl)[n].startaddr)
+
+/* Obtain the end address of the Nth range for block BL.  */
+
+#define BLOCK_RANGE_END(bl,n)	(BLOCK_RANGE (bl)[n].endaddr)
+
+/* Define the "entry pc" for a block BL to be the lowest (start) address
+   for the block when all addresses within the block are contiguous.  If
+   non-contiguous, then use the start address for the first range in the
+   block.
+
+   At the moment, this almost matches what DWARF specifies as the entry
+   pc.  (The missing bit is support for DW_AT_entry_pc which should be
+   preferred over range data and the low_pc.)
+
+   Once support for DW_AT_entry_pc is added, I expect that an entry_pc
+   field will be added to one of these data structures.  Once that's done,
+   the entry_pc field can be set from the dwarf reader (and other readers
+   too).  BLOCK_ENTRY_PC can then be redefined to be less DWARF-centric.  */
+
+#define BLOCK_ENTRY_PC(bl)	(BLOCK_CONTIGUOUS_P (bl) \
+				 ? BLOCK_START (bl) \
+				 : BLOCK_RANGE_START (bl,0))
+
 struct blockvector
 {
   /* Number of blocks in the list.  */
@@ -322,4 +402,9 @@ extern int block_find_non_opaque_type_preferred (struct symbol *sym,
        (sym) != NULL;							\
        (sym) = block_iter_match_next ((name), &(iter)))
 
+/* Given a vector of pairs, allocate and build an obstack allocated
+   blockranges struct for a block.  */
+struct blockranges *make_blockranges (struct objfile *objfile,
+                                      const std::vector<blockrange> &rangevec);
+
 #endif /* BLOCK_H */