[Bug tree-optimization/105835] New: Dead Code Elimination Regression at -O1 (trunk vs. 12.1.0)

[Bug tree-optimization/105835] New: Dead Code Elimination Regression at -O1 (trunk vs. 12.1.0)

[theodort at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105835

            Bug ID: 105835
           Summary: Dead Code Elimination Regression at -O1 (trunk vs.
                    12.1.0)
           Product: gcc
           Version: 13.0
            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 case.c
void foo();

static int b;

static short a(short c, unsigned short d) { return c - d; }

int main() {
    int e = -(0 < b);
    if (a(1, e))
        b = 0;
    else
        foo();
}

`gcc-1982fe2692b6c3b7f969ffc4edac59f9d4359e91 (trunk) -O1` can not eliminate
`foo` but `gcc-releases/gcc-12.1.0 -O1` can.

`gcc-1982fe2692b6c3b7f969ffc4edac59f9d4359e91 (trunk) -O1 -S -o /dev/stdout
case.c`
--------- OUTPUT ---------
main:
.LFB1:
        .cfi_startproc
        cmpl    $0, b(%rip)
        movl    $65535, %eax
        movl    $0, %edx
        cmovle  %edx, %eax
        cmpw    $1, %ax
        je      .L2
        movl    $0, b(%rip)
        movl    $0, %eax
        ret
.L2:
        subq    $8, %rsp
        .cfi_def_cfa_offset 16
        movl    $0, %eax
        call    foo
        movl    $0, %eax
        addq    $8, %rsp
        .cfi_def_cfa_offset 8
        ret
---------- END OUTPUT ---------


`gcc-releases/gcc-12.1.0 -O1 -S -o /dev/stdout case.c`
--------- OUTPUT ---------
main:
.LFB1:
        .cfi_startproc
        movl    $0, b(%rip)
        movl    $0, %eax
        ret
---------- END OUTPUT ---------


Bisects to:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=8fb94fc6097c0a934aac0d89c9c5e2038da67655

[Bug tree-optimization/105835] Dead Code Elimination Regression at -O1 (trunk vs. 12.1.0)

[roger at nextmovesoftware dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105835

Roger Sayle <roger at nextmovesoftware dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-06-03
     Ever confirmed|0                           |1

--- Comment #1 from Roger Sayle <roger at nextmovesoftware dot com> ---
Hmm.  There might be something missing in CCP (with -O1)...

  # RANGE [0, 1] NONZERO 1
  _3 = (intD.6) _2;
  # RANGE [0, 65535] NONZERO 65535
  _10 = _3 * 65535;
  d_11 = (short unsigned intD.18) _10;
  if (d_11 != 1)
    goto <bb 3>; [67.00%]
  else
    goto <bb 4>; [33.00%]

So it knows _10 is either 0 or 65535, so if it knew d_11 had the
range {0 or -1}, i.e. [-1,0] it would know d_11 != 1, and hence
goto <bb 3>.

[Bug tree-optimization/105835] Dead Code Elimination Regression at -O1 (trunk vs. 12.1.0)

[roger at nextmovesoftware dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105835

Roger Sayle <roger at nextmovesoftware dot com> changed:

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

--- Comment #2 from Roger Sayle <roger at nextmovesoftware dot com> ---
Patch proposed
https://gcc.gnu.org/pipermail/gcc-patches/2022-June/596200.html

[Bug tree-optimization/105835] [13 Regression] Dead Code Elimination Regression at -O1 (trunk vs. 12.1.0)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105835

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Dead Code Elimination       |[13 Regression] Dead Code
                   |Regression at -O1 (trunk    |Elimination Regression at
                   |vs. 12.1.0)                 |-O1 (trunk vs. 12.1.0)
   Target Milestone|---                         |13.0

[Bug tree-optimization/105835] [13 Regression] Dead Code Elimination Regression at -O1 (trunk vs. 12.1.0)

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105835

--- Comment #3 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:9991d84d2a84355fd3fc9afc89a963f45991bfa9

commit r13-1162-g9991d84d2a84355fd3fc9afc89a963f45991bfa9
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Sat Jun 18 09:06:20 2022 +0100

    PR tree-optimization/105835: Two narrowing patterns for match.pd.

    This patch resolves PR tree-optimization/105835, which is a code quality
    (dead code elimination) regression at -O1 triggered/exposed by a recent
    change to canonicalize X&-Y as X*Y.  The new (shorter) form exposes some
    missed optimization opportunities that can be handled by adding some
    extra simplifications to match.pd.

    One transformation is to simplify "(short)(x ? 65535 : 0)" into the
    equivalent "x ? -1 : 0", or more accurately x ? (short)-1 : (short)0",
    as INTEGER_CSTs record their type, and integer conversions can be
    pushed inside COND_EXPRs reducing the number of gimple statements.

    The other transformation is that (short)(X * 65535), where X is [0,1],
    into the equivalent (short)X * -1, (or again (short)-1 where tree's
    INTEGER_CSTs encode their type).  This is valid because multiplications
    where one operand is [0,1] are guaranteed not to overflow, and hence
    integer conversions can also be pushed inside these multiplications.

    These narrowing conversion optimizations can be identified by range
    analyses, such as EVRP, but these are only performed at -O2 and above,
    which is why this regression is only visible with -O1.

    2022-06-18  Roger Sayle  <roger@nextmovesoftware.com>

    gcc/ChangeLog
            PR tree-optimization/105835
            * match.pd (convert (mult zero_one_valued_p@1 INTEGER_CST@2)):
            Narrow integer multiplication by a zero_one_valued_p operand.
            (convert (cond @1 INTEGER_CST@2 INTEGER_CST@3)): Push integer
            conversions inside COND_EXPR where both data operands are
            integer constants.

    gcc/testsuite/ChangeLog
            PR tree-optimization/105835
            * gcc.dg/pr105835.c: New test case.

[Bug tree-optimization/105835] [13 Regression] Dead Code Elimination Regression at -O1 (trunk vs. 12.1.0)

[roger at nextmovesoftware dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105835

Roger Sayle <roger at nextmovesoftware dot com> changed:

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

--- Comment #4 from Roger Sayle <roger at nextmovesoftware dot com> ---
This should now be fixed on mainline.