Hello.

The patch adds a new pass that identifies a series of if-elseif
statements and transform then into a GIMPLE switch (if possible).
The pass runs right after tree-ssa pass and I decided to implement
matching of various forms that are introduced by folder (fold_range_test):

1) if condition with equal operation:

   <bb 2> :
   if (argc_8(D) == 1)
     goto <bb 3>; [INV]
   else
     goto <bb 4>; [INV]

2) if condition with a range check:

   <bb 3> :
   _4 = c_13(D) + 198;
   if (_4 <= 1)
     goto <bb 7>; [INV]
   else
     goto <bb 4>; [INV]

3) mixture of 1) and 2) with a or condition:

   <bb 2> :
   _1 = aChar_8(D) == 1;
   _2 = aChar_8(D) == 10;
   _3 = _1 | _2;
   if (_3 != 0)
     goto <bb 5>; [INV]
   else
     goto <bb 3>; [INV]

   or:

   <bb 2> :
   aChar.1_1 = (unsigned int) aChar_10(D);
   _2 = aChar.1_1 + 4294967287;
   _3 = _2 <= 1;
   _4 = aChar_10(D) == 12;
   _5 = _3 | _4;
   if (_5 != 0)
     goto <bb 5>; [INV]
   else
     goto <bb 3>; [INV]

The motivation example in PR88702 is transformed now into:

IsHTMLWhitespace (int aChar)
{
   int iftmp.0_1;

   <bb 2> [local count: 1073741824]:
   switch (aChar_2(D)) <default: <L6> [50.00%], case 9 ... 10: <L7> [50.00%], case 12 ... 13: <L7> [50.00%], case 32: <L7> [50.00%]>

   <bb 3> [local count: 536870913]:
<L6>:

   <bb 4> [local count: 1073741824]:
   # iftmp.0_1 = PHI <1(2), 0(3)>
<L7>:
   return iftmp.0_1;

}

I'm also attaching if-elseif chains that are transformed in make all-host of the GCC compiler.
There are ~800 such transformations. The most beautiful transformation is this one:

$ cat -n gcc/c-family/c-common.c
...
   2895        /* This used to be a switch, but Genix compiler can't handle that.  */
   2896        if (code == NE_EXPR)
   2897          {
   2898            if (max_lt || min_gt)
   2899              val = truthvalue_true_node;
   2900          }
   2901        else if (code == EQ_EXPR)
   2902          {
   2903            if (max_lt || min_gt)
   2904              val = truthvalue_false_node;
   2905          }
...

Patch can bootstrap on x86_64-linux-gnu and survives regression tests.

Thoughts?
Thanks,
Martin


gcc/ChangeLog:

2019-11-04  Martin Liska  <mliska@suse.cz>

	PR tree-optimization/14799
	PR ipa/88702
	* Makefile.in: Include new tree-if-to-switch.o.
	* common.opt: Document -ftree-if-to-switch.
	* doc/invoke.texi: Likewise.
	* opts.c: Enable the pass with -O2+.
	* passes.def: Add ne pass.
	* timevar.def (TV_TREE_IF_TO_SWITCH): Add new
	timevar.
	* tree-if-to-switch.c: New file.
	* tree-pass.h (make_pass_if_to_switch): New.

gcc/testsuite/ChangeLog:

2019-11-04  Martin Liska  <mliska@suse.cz>

	PR tree-optimization/14799
	PR ipa/88702
	* gcc.dg/tree-ssa/if-to-switch-1.c: New test.
	* gcc.dg/tree-ssa/if-to-switch-2.c: New test.
	* gcc.dg/tree-ssa/if-to-switch-3.c: New test.
	* gcc.dg/tree-ssa/if-to-switch-4.c: New test.
	* gcc.dg/tree-ssa/if-to-switch-5.c: New test.
	* gcc.dg/tree-ssa/reassoc-32.c: Disable tree-if-to-switch
	in order to transform the range test.
	* gcc.dg/tree-ssa/reassoc-33.c: Likewise.
---
  gcc/Makefile.in                               |   1 +
  gcc/common.opt                                |   4 +
  gcc/doc/invoke.texi                           |  10 +-
  gcc/opts.c                                    |   1 +
  gcc/passes.def                                |   1 +
  .../gcc.dg/tree-ssa/if-to-switch-1.c          |  35 +
  .../gcc.dg/tree-ssa/if-to-switch-2.c          |  11 +
  .../gcc.dg/tree-ssa/if-to-switch-3.c          |  11 +
  .../gcc.dg/tree-ssa/if-to-switch-4.c          |  35 +
  .../gcc.dg/tree-ssa/if-to-switch-5.c          |  12 +
  gcc/testsuite/gcc.dg/tree-ssa/reassoc-32.c    |   2 +-
  gcc/testsuite/gcc.dg/tree-ssa/reassoc-33.c    |   2 +-
  gcc/timevar.def                               |   1 +
  gcc/tree-if-to-switch.c                       | 611 ++++++++++++++++++
  gcc/tree-pass.h                               |   1 +
  15 files changed, 735 insertions(+), 3 deletions(-)
  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/if-to-switch-1.c
  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/if-to-switch-2.c
  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/if-to-switch-3.c
  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/if-to-switch-4.c
  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/if-to-switch-5.c
  create mode 100644 gcc/tree-if-to-switch.c