Speedup cse_insn

When cse_insn prunes src{,_folded,_eqv_here,_related} with the
equivalence set in the *_same_value chain it also searches for
an equivalence to the destination of the instruction with

          /* This is the same as the destination of the insns, we want
             to prefer it.  Copy it to src_related.  The code below will
             then give it a negative cost.  */
          if (GET_CODE (dest) == code && rtx_equal_p (p->exp, dest))
            src_related = p->exp;

this picks up the last such equivalence and in particular any
later duplicate will be pruned by the preceeding

          else if (src_related && GET_CODE (src_related) == code
                   && rtx_equal_p (src_related, p->exp))
            src_related = 0;

first.  This wastes cycles doing extra rtx_equal_p checks.  The
following instead searches for the first destination equivalence
separately in this loop and delays using src_related for it until
we are about to process that, avoiding another redundant rtx_equal_p
check.

I've came here because of a testcase with very large equivalence
lists and compile-time of cse_insn.  The patch below doesn't speed
it up significantly since there's no equivalence on the destination.

In theory this opens the possibility to track dest_related
separately, avoiding the implicit pruning of any previous
value in src_related.  As is the change should be a no-op for
code generation.

	* cse.cc (cse_insn): Track an equivalence to the destination
	separately and delay using src_related for it.

0 files changed, 0 insertions, 0 deletions