[rtlopt] var-tracking - more detailed overall comment

[Josef Zlomek <zlomj9am at artax dot karlin dot mff dot cuni dot cz>]

Hi,

this patch makes the overall comment more detailed.

Josef

2003-02-04  Josef Zlomek  <zlomekj@suse.cz>

	* var-tracking.c: More detailed overall comment.

Index: var-tracking.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/Attic/var-tracking.c,v
retrieving revision 1.1.4.15
diff -c -3 -p -r1.1.4.15 var-tracking.c
*** var-tracking.c	3 Feb 2003 18:12:42 -0000	1.1.4.15
--- var-tracking.c	4 Feb 2003 11:59:27 -0000
***************
*** 38,45 ****
--- 38,69 ----
     The IN and OUT sets for basic blocks consist of a link list for each
     physical register and a hash table of link lists for memory references.
     The link lists should be pretty short so it is a good data structure here.
+    The IN set for basic block BB is computed as a union of OUT sets of BB's
+    predecessors, the OUT set for BB is computed from the IN set for BB and
+    the location references in BB (from array of locations created in the first
+    step).
     REG_EXPR and MEM_EXPR are used to find out which variable is associated
     with a given register/memory.
+    For a register, the link list contains the list of triplets
+    (loc, decl, offset) called "attributes" (loc is the register RTX,
+    decl and offset are from REG_ATTRS (loc)) describing variable parts which are
+    currently in a given register.  There may be more than one such a variable
+    part in a register, for example in the following example, register allocator
+    may assign same register to variables A and B, and both of them are stored in
+    the same register in CODE:
+ 
+      if (cond)
+        set A;
+      else
+        set B;
+      CODE;
+      if (cond)
+        use A;
+      else
+        use B;
+ 
+    For a memory, the link list are similar, one link list contains attributes
+    for the same decl (they may have different offsets).
  
     Finally, the NOTE_INSN_VAR_LOCATION notes describing the variable locations
     are emitted to appropriate positions in RTL code.  Each such a note describes