[Bug tree-optimization/96929] New: Failure to optimize right shift of -1 to -1

[Bug tree-optimization/96929] New: Failure to optimize right shift of -1 to -1

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96929

            Bug ID: 96929
           Summary: Failure to optimize right shift of -1 to -1
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

int f(int b)
{
    return -1 >> b;
}

This can be optimized to `return -1;`. This transformation is done by LLVM, but
not by GCC.

[Bug tree-optimization/96929] Failure to optimize right shift of -1 to -1

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96929

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-09-04
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
We already have a rule for this:
/* Optimize -1 >> x for arithmetic right shifts.  */
(simplify
 (rshift integer_all_onesp@0 @1)
 (if (!TYPE_UNSIGNED (type)
      && tree_expr_nonnegative_p (@1))
  @0))
but the rule requires that the shift count is non-negative.
That was added in PR38359 fix.
Even current wide_int_binop has:
    case RSHIFT_EXPR:
    case LSHIFT_EXPR:
      if (wi::neg_p (arg2))
        {
          tmp = -arg2;
          if (code == RSHIFT_EXPR)
            code = LSHIFT_EXPR;
          else
            code = RSHIFT_EXPR;
        }
      else
        tmp = arg2;
and so it treats rshift shifts by negative values as left shifts.
So, if we wanted to fix this PR, we'd need to remove the
tree_expr_nonnegative_p and change wide_int_binop to perhaps best punt on
negative arg2 instead of trying to handle it.  Wonder what it will break.

[Bug tree-optimization/96929] Failure to optimize right shift of -1 to -1

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96929

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 49603
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49603&action=edit
gcc11-pr96929.patch

Untested fix.

[Bug tree-optimization/96929] Failure to optimize right shift of -1 to -1

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96929

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:4866b2f5db117f9e89f82c44ffed57178c09cc49

commit r11-5271-g4866b2f5db117f9e89f82c44ffed57178c09cc49
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Nov 24 09:03:17 2020 +0100

    middle-end, c++: Treat shifts by negative as undefined [PR96929]

    The PR38359 change made the -1 >> x to -1 optimization less useful by
    requiring that the x must be non-negative.
    Shifts by negative amount are UB, but we for historic reasons had in some
    (but not all) places some hack to treat shifts by negative value as the
    other direction shifts by the negated amount.

    The following patch just removes that special handling, instead we punt on
    optimizing those (and ideally path isolation should catch that up and turn
    those into __builtin_unreachable, perhaps with __builtin_warning next to
    it).  Folding the shifts in some places as if they were rotates and in
other
    as if they were saturating just leads to inconsistencies.

    For C++ constexpr diagnostics and -fpermissive, I've added code to pretend
    fold-const.c has not changed, without -fpermissive it will be an error
    anyway and I think it is better not to change all the diagnostics.

    During x86_64-linux and i686-linux bootstrap/regtest, my statistics
    gathering patch noted 185 unique -m32/-m64 x TU x function_name x
shift_kind
    x fold-const/tree-ssa-ccp cases.  I have investigated the
    64 ../../gcc/config/i386/i386.c x86_output_aligned_bss LSHIFT_EXPR
wide_int_bitop
    64 ../../gcc/config/i386/i386-expand.c emit_memmov LSHIFT_EXPR
wide_int_bitop
    64 ../../gcc/config/i386/i386-expand.c ix86_expand_carry_flag_compare
LSHIFT_EXPR wide_int_bitop
    64 ../../gcc/expmed.c expand_divmod LSHIFT_EXPR wide_int_bitop
    64 ../../gcc/lra-lives.c process_bb_lives LSHIFT_EXPR wide_int_bitop
    64 ../../gcc/rtlanal.c nonzero_bits1 LSHIFT_EXPR wide_int_bitop
    64 ../../gcc/varasm.c optimize_constant_pool.isra LSHIFT_EXPR
wide_int_bitop
    cases and all of them are either during jump threading (dom) or during PRE.
    For jump threading, the most common case is 1 << floor_log2 (whatever)
where
    floor_log2 is return HOST_BITS_PER_WIDE_INT - 1 - clz_hwi (x);
    and clz_hwi is if (x == 0) return HOST_BITS_PER_WIDE_INT; return
__builtin_clz* (x);
    and so has range [-1, 63] and a comparison against == 0 which makes the
    threader think it might be nice to jump thread the case leading to 1 << -1.
    I think it is better to keep the 1 << -1 s in the IL for this and let path
    isolation turn that into __builtin_unreachable () if the user wishes so.

    2020-11-24  Jakub Jelinek  <jakub@redhat.com>

            PR tree-optimization/96929
            * fold-const.c (wide_int_binop) <case LSHIFT_EXPR, case
RSHIFT_EXPR>:
            Return false on negative second argument rather than trying to
handle
            it as shift in the other direction.
            * tree-ssa-ccp.c (bit_value_binop) <case LSHIFT_EXPR,
            case RSHIFT_EXPR>: Punt on negative shift count rather than trying
            to handle it as shift in the other direction.
            * match.pd (-1 >> x to -1): Remove tree_expr_nonnegative_p check.

            * constexpr.c (cxx_eval_binary_expression): For shifts by constant
            with MSB set, emulate older wide_int_binop behavior to preserve
            diagnostics and -fpermissive behavior.

            * gcc.dg/tree-ssa/pr96929.c: New test.

[Bug tree-optimization/96929] Failure to optimize right shift of -1 to -1

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96929

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Fixed on the trunk.