GCC Middle and Back End API Reference
occurrence Struct Reference
Collaboration diagram for occurrence:

Data Fields

basic_block bb
tree recip_def
gimple recip_def_stmt
struct occurrencechildren
struct occurrencenext
int num_divisions
bool bb_has_division

Detailed Description


Global, SSA-based optimizations using mathematical identities. Copyright (C) 2005-2013 Free Software Foundation, Inc.

This file is part of GCC.

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   Currently, the only mini-pass in this file tries to CSE reciprocal
   operations.  These are common in sequences such as this one:

        modulus = sqrt(x*x + y*y + z*z);
        x = x / modulus;
        y = y / modulus;
        z = z / modulus;

   that can be optimized to

        modulus = sqrt(x*x + y*y + z*z);
        rmodulus = 1.0 / modulus;
        x = x * rmodulus;
        y = y * rmodulus;
        z = z * rmodulus;

   We do this for loop invariant divisors, and with this pass whenever
   we notice that a division has the same divisor multiple times.

   Of course, like in PRE, we don't insert a division if a dominator
   already has one.  However, this cannot be done as an extension of
   PRE for several reasons.

   First of all, with some experiments it was found out that the
   transformation is not always useful if there are only two divisions
   hy the same divisor.  This is probably because modern processors
   can pipeline the divisions; on older, in-order processors it should
   still be effective to optimize two divisions by the same number.
   We make this a param, and it shall be called N in the remainder of
   this comment.

   Second, if trapping math is active, we have less freedom on where
   to insert divisions: we can only do so in basic blocks that already
   contain one.  (If divisions don't trap, instead, we can insert
   divisions elsewhere, which will be in blocks that are common dominators
   of those that have the division).

   We really don't want to compute the reciprocal unless a division will
   be found.  To do this, we won't insert the division in a basic block
   that has less than N divisions *post-dominating* it.

   The algorithm constructs a subset of the dominator tree, holding the
   blocks containing the divisions and the common dominators to them,
   and walk it twice.  The first walk is in post-order, and it annotates
   each block with the number of divisions that post-dominate it: this
   gives information on where divisions can be inserted profitably.
   The second walk is in pre-order, and it inserts divisions as explained
   above, and replaces divisions by multiplications.

   In the best case, the cost of the pass is O(n_statements).  In the
   worst-case, the cost is due to creating the dominator tree subset,
   with a cost of O(n_basic_blocks ^ 2); however this can only happen
   for n_statements / n_basic_blocks statements.  So, the amortized cost
   of creating the dominator tree subset is O(n_basic_blocks) and the
   worst-case cost of the pass is O(n_statements * n_basic_blocks).

   More practically, the cost will be small because there are few
   divisions, and they tend to be in the same basic block, so insert_bb
   is called very few times.

   If we did this using domwalk.c, an efficient implementation would have
   to work on all the variables in a single pass, because we could not
   work on just a subset of the dominator tree, as we do now, and the
   cost would also be something like O(n_statements * n_basic_blocks).
   The data structures would be more complex in order to work on all the
   variables in a single pass.  
   FIXME: RTL headers have to be included here for optabs.  
   This structure represents one basic block that either computes a
   division, or is a common dominator for basic block that compute a

Field Documentation

basic_block occurrence::bb
     The basic block represented by this structure.  

Referenced by insert_reciprocals(), and occ_new().

bool occurrence::bb_has_division
     True if the basic block has a division, false if it is a common
     dominator for basic blocks that do.  If it is false and trapping
     math is active, BB is not a candidate for inserting a reciprocal.  
struct occurrence* occurrence::children
     Pointer to a list of "struct occurrence"s for blocks dominated
     by BB.  

Referenced by occ_new().

struct occurrence* occurrence::next
     Pointer to the next "struct occurrence"s in the list of blocks
     sharing a common dominator.  

Referenced by occ_new().

int occurrence::num_divisions
     The number of divisions that are in BB before compute_merit.  The
     number of divisions that are in BB or post-dominate it after
tree occurrence::recip_def
     If non-NULL, the SSA_NAME holding the definition for a reciprocal
     inserted in BB.  
gimple occurrence::recip_def_stmt
     If non-NULL, the GIMPLE_ASSIGN for a reciprocal computation that
     was inserted in BB.  

The documentation for this struct was generated from the following file: