Re: [Patch,tree-optimization]: Add new path Splitting pass on tree ssa representation

[Jeff Law]

On 11/13/2015 03:13 AM, Richard Biener wrote:

diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index 34d2356..6613e83 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -1474,6 +1474,7 @@ OBJS = \
         tree-ssa-loop.o \
         tree-ssa-math-opts.o \
         tree-ssa-operands.o \
+       tree-ssa-path-split.o \

gimple-ssa-path-split please.
Agreed.   I'll make that change for Ajit.



         tree-ssa-phionlycprop.o \
         tree-ssa-phiopt.o \
         tree-ssa-phiprop.o \
diff --git a/gcc/common.opt b/gcc/common.opt
index 757ce85..3e946ca 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -2403,6 +2403,10 @@ ftree-vrp
  Common Report Var(flag_tree_vrp) Init(0) Optimization
  Perform Value Range Propagation on trees.

+ftree-path-split

fsplit-paths
And this plus related variable name fixes and such.



+@item -ftree-path-split
+@opindex ftree-path-split
+Perform Path Splitting on trees.  When the two execution paths of a
+if-then-else merge at the loop latch node, try to duplicate the
+merge node into two paths. This is enabled by default at @option{-O2}
+and above.
+

I think if we go into the detail of the transform we should mention the
effective result (creating a loop nest with disambiguation figuring out
which is the "better" inner loop).
It no longer creates a loop nest.  The overall shape of the CFG is 
maintained.  ie, we still have a single simple latch for the loop.  The 
blocks we create are internal to the loop.

I always struggle with the right level at which to document these 
options.   I'll take a look at this for Ajit.

BTW Do we have an API for indicating that new blocks have been added to 
a loop?  If so, then we can likely drop the LOOPS_NEED_FIXUP.



  @item -fsplit-ivs-in-unroller
  @opindex fsplit-ivs-in-unroller
  Enables expression of values of induction variables in later iterations
diff --git a/gcc/opts.c b/gcc/opts.c
index 9a3fbb3..9a0b27c 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -509,6 +509,7 @@ static const struct default_options
default_options_table[] =
      { OPT_LEVELS_2_PLUS, OPT_fisolate_erroneous_paths_dereference, NULL, 1
},
      { OPT_LEVELS_2_PLUS, OPT_fipa_ra, NULL, 1 },
      { OPT_LEVELS_2_PLUS, OPT_flra_remat, NULL, 1 },
+    { OPT_LEVELS_2_PLUS, OPT_ftree_path_split, NULL, 1 },

Is this transform a good idea for -Os?
In general, no because of the block duplication.

jeff