[Bug tree-optimization/102950] New: [11 Regression] Dead Code Elimination Regression at -O3 (trunk&11.2.0 vs 10.3.0)

[Bug tree-optimization/102950] New: [11 Regression] Dead Code Elimination Regression at -O3 (trunk&11.2.0 vs 10.3.0)

[theodort at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102950

            Bug ID: 102950
           Summary: [11 Regression] Dead Code Elimination Regression at
                    -O3 (trunk&11.2.0 vs 10.3.0)
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: theodort at inf dot ethz.ch
  Target Milestone: ---

cat test.c
void foo(void);

static char a;
static short d(unsigned e) {
  char b;
  short c;
  a = b = e;
  if (b)
    return 0;
  if (1 >= e) {
    c = e == 0;
    if (c)
      foo();
  }
  return 0;
}
int main() { d(a ^ 233); }

10.3.0 at -O3 can eliminate the call to foo but neither trunk nor 11.2.0 at -O3
can:

gcc-10 -O3 -S test.c -o /dev/stdout
main:
.LFB1:
        .cfi_startproc
        xorb    $-23, a(%rip)
        xorl    %eax, %eax
        ret
        .cfi_endproc

gcc-11 -O3 -S test.c -o /dev/stdout
...
main:
.LFB1:
        .cfi_startproc
        movsbl  a(%rip), %eax
        xorb    $-23, %al
        movb    %al, a(%rip)
        cmpl    $1, %eax
        ja      .L10
        testb   %al, %al
        je      .L14
.L10:
        xorl    %eax, %eax
        ret
.L14:
        pushq   %rax
        .cfi_def_cfa_offset 16
        call    foo
        xorl    %eax, %eax
        popq    %rdx
        .cfi_def_cfa_offset 8
        ret
        .cfi_endproc


gcc-trunk -O3 -S test.c -o /dev/stdout
main:
.LFB1:
        .cfi_startproc
        movsbl  a(%rip), %eax
        xorb    $-23, %al
        movb    %al, a(%rip)
        cmpl    $1, %eax
        ja      .L10
        testb   %al, %al
        je      .L14
.L10:
        xorl    %eax, %eax
        ret
.L14:
        pushq   %rax
        .cfi_def_cfa_offset 16
        call    foo
        xorl    %eax, %eax
        popq    %rdx
        .cfi_def_cfa_offset 8
        ret
        .cfi_endproc


gcc-trunk -v
Using built-in specs.
Target: x86_64-pc-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 12.0.0 20211022 (experimental) (GCC)

Introduced with
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=fcae5121154d1c3382b056bcc2c563cedac28e74

[Bug tree-optimization/102950] [11/12 Regression] Dead Code Elimination Regression at -O3 (trunk&11.2.0 vs 10.3.0)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102950

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |11.3
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-10-27
            Summary|[11 Regression] Dead Code   |[11/12 Regression] Dead
                   |Elimination Regression at   |Code Elimination Regression
                   |-O3 (trunk&11.2.0 vs        |at -O3 (trunk&11.2.0 vs
                   |10.3.0)                     |10.3.0)
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |missed-optimization

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

In GCC 10, forwprop2 gets:
  a.0_1 = a;
  _2 = (int) a.0_1;
  _3 = _2 ^ 233;
  _4 = (unsigned int) _3;
  b_7 = (char) _4;
  a = b_7;
  if (b_7 != 0)
    goto <bb 6>; [34.00%]
  else
    goto <bb 3>; [66.00%]

  <bb 3> [local count: 708669601]:
  if (_4 <= 1)
    goto <bb 4>; [41.00%]
  else
    goto <bb 6>; [59.00%]

  <bb 4> [local count: 290554533]:
  if (_4 == 0)
    goto <bb 5>; [33.00%]
  else
    goto <bb 6>; [67.00%]

  <bb 5> [local count: 95882995]:
  foo ();

While in GCC 11 we get:
  a.0_1 = a;
  _2 = (int) a.0_1;
  _3 = _2 ^ 233;
  _4 = (unsigned int) _3;
  b_7 = (char) _4;
  a = b_7;
  if (b_7 != 0)
    goto <bb 5>; [34.00%]
  else
    goto <bb 3>; [66.00%]

  <bb 3> [local count: 708669601]:
  if (_4 <= 1)
    goto <bb 4>; [25.50%]
  else
    goto <bb 5>; [74.50%]


I still can't figure out why forwprop2 can do it in GCC 10 but not in GCC 11.

[Bug tree-optimization/102950] [11/12 Regression] Dead Code Elimination Regression at -O3 (trunk&11.2.0 vs 10.3.0)

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102950

--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> ---
r11-3685 is bad and r11-3683 is good.

[Bug tree-optimization/102950] [11/12 Regression] Dead Code Elimination Regression at -O3 (trunk&11.2.0 vs 10.3.0)

[amacleod at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102950

--- Comment #3 from Andrew Macleod <amacleod at redhat dot com> ---
Im not sure what the pre-ranger trick was, but the shortcoming we have it the
following:
    a.0_1 = a;
    _2 = (int) a.0_1;
    _3 = _2 ^ 233;
    _4 = (unsigned int) _3;
    b_8 = (char) _3;
    a = b_8;
    if (b_8 != 0)

we know 
 _2  : int [-128, 127]

but when we calculate _3, 
 [-128, 127] ^ 233 uses the original bitwise XOR code, and it returns VARYING
for that range. therefore We only know _3 is VARYING and therefore

2->3  (F) _3 :  int [-INF, -256][0, 0][256, +INF]
2->3  (F) _4 :  unsigned int [0, 0][256, 4294967040]

When when we later get to 
    if (_4 <= 1)
      goto <bb 4>; [25.50%]

we're kinda of stuck.

whereas in reality, properly calculated, we'd know that 
_3  = [-128, 127],  _4 = [-128, 127]

And as you can see on the outgoing edges, we see thru the casts to trim out the
other bits in _3 and _4 on the 2->3 edge,  so with those proper inputs, we
would end up with _4 and _3 == [0,0].

so, if no one else gets to it, I'll eventually teach
range-op.cc::operator_bitwise_xor::wi_fold to do something about this.  special
case constants, or maybe look at the ranges and if the RHS fits within the LHS
effective precision, produce a better result.

[Bug tree-optimization/102950] [11/12 Regression] Dead Code Elimination Regression at -O3 (trunk&11.2.0 vs 10.3.0)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102950

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug tree-optimization/102950] [11/12 Regression] Dead Code Elimination Regression at -O3 (trunk&11.2.0 vs 10.3.0)

[roger at nextmovesoftware dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102950

Roger Sayle <roger at nextmovesoftware dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |roger at nextmovesoftware dot com
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |roger at nextmovesoftware dot com

--- Comment #4 from Roger Sayle <roger at nextmovesoftware dot com> ---
Patch proposed
https://gcc.gnu.org/pipermail/gcc-patches/2022-February/589569.html

[Bug tree-optimization/102950] [11/12 Regression] Dead Code Elimination Regression at -O3 (trunk&11.2.0 vs 10.3.0)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102950

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.3                        |11.4

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 11.3 is being released, retargeting bugs to GCC 11.4.

[Bug tree-optimization/102950] [11/12/13 Regression] Dead Code Elimination Regression at -O3 (trunk&11.2.0 vs 10.3.0)

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102950

--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Roger Sayle <sayle@gcc.gnu.org>:

https://gcc.gnu.org/g:b3e98eb3396e16ae8b20c94916bc2bd7862d2c97

commit r13-89-gb3e98eb3396e16ae8b20c94916bc2bd7862d2c97
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Tue May 3 14:38:50 2022 -0400

    PR tree-optimization/102950: Improved EVRP for signed BIT_XOR_EXPR.

    This patch fixes PR tree-optimization/102950, which is a P2 regression,
    by providing better range bounds for BIT_XOR_EXPR, BIT_AND_EXPR and
    BIT_IOR_EXPR on signed integer types.  In general terms, any binary
    bitwise operation on sign-extended or zero-extended integer types will
    produce results that are themselves sign-extended or zero-extended.
    More precisely, we can derive signed bounds from the number of leading
    redundant sign bit copies, from the equation:
            clrsb(X op Y) >= min (clrsb (X), clrsb(Y))
    and from the property that for any (signed or unsigned) range [lb, ub]
    that clrsb([lb, ub]) >= min (clrsb(lb), clrsb(ub)).

    These can be used to show that [-1, 0] op [-1, 0] is [-1, 0] or that
    [-128, 127] op [-128, 127] is [-128, 127], even when tracking nonzero
    bits would result in VARYING (as every bit can be 0 or 1).  This is
    equivalent to determining the minimum type precision in which the
    operation can be performed then sign extending the result.

    One additional refinement is to observe that X ^ Y can never be
    zero if the ranges of X and Y don't overlap, i.e. X can't be equal
    to Y.

    Previously, the expression "(int)(char)a ^ 233" in the PR was considered
    VARYING, but with the above changes now has the range [-256, -1][1, 255],
    which is sufficient to optimize away the call to foo.

    2022-05-03  Roger Sayle  <roger@nextmovesoftware.com>

    gcc/ChangeLog
            PR tree-optimization/102950
            * range-op.cc (wi_optimize_signed_bitwise_op): New function to
            determine bounds of bitwise operations on signed types.
            (operator_bitwise_and::wi_fold): Call the above function.
            (operator_bitwise_or::wi_fold): Likewise.
            (operator_bitwise_xor::wi_fold): Likewise.  Additionally, the
            result can't be zero if the operands can't be equal.

    gcc/testsuite/ChangeLog
            PR tree-optimization/102950
            * gcc.dg/pr102950.c: New test case.
            * gcc.dg/tree-ssa/evrp10.c: New test case.

[Bug tree-optimization/102950] [11/12 Regression] Dead Code Elimination Regression at -O3 (11.2.0 vs 10.3.0)

[roger at nextmovesoftware dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102950

Roger Sayle <roger at nextmovesoftware dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
            Summary|[11/12/13 Regression] Dead  |[11/12 Regression] Dead
                   |Code Elimination Regression |Code Elimination Regression
                   |at -O3 (trunk&11.2.0 vs     |at -O3 (11.2.0 vs 10.3.0)
                   |10.3.0)                     |
   Target Milestone|11.4                        |13.0
      Known to work|                            |13.0
           Assignee|roger at nextmovesoftware dot com  |unassigned at gcc dot gnu.org

--- Comment #7 from Roger Sayle <roger at nextmovesoftware dot com> ---
This has now been fixed on mainline.

[Bug tree-optimization/102950] [11/12 Regression] Dead Code Elimination Regression at -O3 (11.2.0 vs 10.3.0)

[amacleod at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102950

Andrew Macleod <amacleod at redhat dot com> changed:

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

--- Comment #8 from Andrew Macleod <amacleod at redhat dot com> ---
Fixed. Thanks Roger.