[Bug tree-optimization/94274] fold phi whose incoming args are defined from binary operations

["z.zhanghaijian at huawei dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94274

--- Comment #3 from z.zhanghaijian at huawei dot com <z.zhanghaijian at huawei dot com> ---
(In reply to Marc Glisse from comment #1)
> Detecting common beginnings / endings in branches is something gcc does very
> seldom. Even at -Os, for if(cond)f(b);else f(c); we need to wait until
> rtl-optimizations to get a single call to f. (of course the reverse
> transformation of duplicating a statement that was after the branches into
> them, if it simplifies, is nice as well, and they can conflict)
> I don't know if handling one such very specific case (binary operations with
> a common argument) separately is a good idea when we don't even handle unary
> operations.

I tried to test this fold on specint2017 and found some performance gains on
500.perlbench_r. Then compared the assemble and found some improvements.

For example:

S_invlist_max, which is inlined by many functions, such as
S__append_range_to_invlist, S_ssc_anything, Perl__invlist_invert ...

invlist_inline.h:
#define FROM_INTERNAL_SIZE(x) ((x)/ sizeof(UV))

S_invlist_max(inlined by S__append_range_to_invlist, S_ssc_anything,
Perl__invlist_invert, ....)：
    return SvLEN(invlist) == 0  /* This happens under _new_invlist_C_array */
           ? FROM_INTERNAL_SIZE(SvCUR(invlist)) - 1
           : FROM_INTERNAL_SIZE(SvLEN(invlist)) - 1;

Dump tree phiopt：

<bb 3> [local count: 536870911]:
  _46 = pretmp_112 >> 3;
  iftmp.1123_47 = _46 + 18446744073709551615;
  goto <bb 5>; [100.00%]

  <bb 4> [local count: 536870911]:
  _48 = _44 >> 3;
  iftmp.1123_49 = _48 + 18446744073709551615;

  <bb 5> [local count: 1073741823]:
  # iftmp.1123_50 = PHI <iftmp.1123_47(3), iftmp.1123_49(4)>

Which can replaces with：

  <bb 3> [local count: 536870912]:

  <bb 4> [local count: 1073741823]:
  # _48 = PHI <_44(2), pretmp_112(3)>
  _49 = _48 >> 3;
  iftmp.1123_50 = _49 + 18446744073709551615;

Assemble：

lsr     x5, x6, #3
lsr     x3, x3, #3
sub     x20, x5, #0x1
sub     x3, x3, #0x1
csel    x20, x3, x20, ne

Replaces with：

csel    x3, x3, x4, ne
lsr     x3, x3, #3
sub     x20, x3, #0x1

This can eliminate two instruction.