[PATCH 24/33]New parameter bound on number of selected candidates

[Bin Cheng]

Hi,
IVOPTs still have difficulty for outer loop (especially for large loop nest), 
and tend to select too many candidates.
It's generally bad because of unavoidable register spilling.  In this case, we 
probably want to compute iv_uses with
small number of bivs.  Though this results in more computation inside of loop, 
it could improve spilling.
This patch adds new parameter bound on number of selected candidates, it simply 
gives up if too many candidates are
selected.  So far it works loop by loop, I am not sure if we want to by pass 
whole loop nest once this bound is hit.
Is it OK?

Thanks,
bin
2017-04-11  Bin Cheng  <bin.ch...@arm.com>

        * doc/invoke.texi (iv-max-selected-candidates): New.
        * params.def (PARAM_IV_MAX_SELECTED_CANDIDATES): New.
        * tree-ssa-loop-ivopts.c (MAX_SELECTED_CANDIDATES): New.
        (tree_ssa_iv_optimize_loop): Skip if too many cands are selected.
From 40517ca836f868b8bd79bde56aa7c053ffef4fc2 Mon Sep 17 00:00:00 2001
From: Bin Cheng <binch...@e108451-lin.cambridge.arm.com>
Date: Tue, 7 Mar 2017 13:53:04 +0000
Subject: [PATCH 24/33] add-bound-on-selected-cands-20170221.txt

---
 gcc/doc/invoke.texi        | 4 ++++
 gcc/params.def             | 8 ++++++++
 gcc/tree-ssa-loop-ivopts.c | 8 +++++++-
 3 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 19a85b6..f9cbdbb 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -9922,6 +9922,10 @@ If the number of candidates in the set is smaller than 
this value,
 always try to remove unnecessary ivs from the set
 when adding a new one.
 
+@item iv-max-selected-candidates
+The induction variable optimizations give up on loops that more induction
+variable candidates are selected.
+
 @item avg-loop-niter
 Average number of iterations of a loop.
 
diff --git a/gcc/params.def b/gcc/params.def
index 1b058e4..7daab14 100644
--- a/gcc/params.def
+++ b/gcc/params.def
@@ -527,6 +527,14 @@ DEFPARAM(PARAM_IV_ALWAYS_PRUNE_CAND_SET_BOUND,
         "If number of candidates in the set is smaller, we always try to 
remove unused ivs during its optimization.",
         10, 0, 0)
 
+/* The induction variable optimizations give up on loops that more induction
+   variable candidates are selected.  */
+
+DEFPARAM(PARAM_IV_MAX_SELECTED_CANDIDATES,
+        "iv-max-selected-candidates",
+        "Bound on number of selected iv candidates for loops in iv 
optimizations.",
+        48, 0, 0)
+
 DEFPARAM(PARAM_AVG_LOOP_NITER,
         "avg-loop-niter",
         "Average number of iterations of a loop.",
diff --git a/gcc/tree-ssa-loop-ivopts.c b/gcc/tree-ssa-loop-ivopts.c
index dcc4618..8469782 100644
--- a/gcc/tree-ssa-loop-ivopts.c
+++ b/gcc/tree-ssa-loop-ivopts.c
@@ -667,6 +667,12 @@ struct iv_ca_delta
 #define ALWAYS_PRUNE_CAND_SET_BOUND \
   ((unsigned) PARAM_VALUE (PARAM_IV_ALWAYS_PRUNE_CAND_SET_BOUND))
 
+/* If there are more candidates slected, we just give up because it usually
+   causes high register pressure issue.  */
+
+#define MAX_SELECTED_CANDIDATES \
+  ((unsigned) PARAM_VALUE (PARAM_IV_MAX_SELECTED_CANDIDATES))
+
 /* The list of trees for that the decl_rtl field must be reset is stored
    here.  */
 
@@ -7382,7 +7388,7 @@ tree_ssa_iv_optimize_loop (struct ivopts_data *data, 
struct loop *loop)
 
   /* Find the optimal set of induction variables (item 3, part 2).  */
   iv_ca = find_optimal_iv_set (data);
-  if (!iv_ca)
+  if (!iv_ca || iv_ca->n_cands > MAX_SELECTED_CANDIDATES)
     goto finish;
   changed = true;
 
-- 
1.9.1