[Bug c/97225] New: Failure to optimize out conditional addition of zero

[Bug c/97225] New: Failure to optimize out conditional addition of zero

[osandov at osandov dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97225

            Bug ID: 97225
           Summary: Failure to optimize out conditional addition of zero
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: osandov at osandov dot com
  Target Milestone: ---

For the following code:

#include <stddef.h>

struct vector {
    int *data;
    size_t size;
};

int *vector_end(struct vector *vec)
{
    return vec->data + vec->size;
}

GCC 10.2.0 on x86-64 generates the following code (same on -O2, -O3, and -Os):

vector_end:
        movq    8(%rdi), %rdx
        movq    (%rdi), %rax
        leaq    (%rax,%rdx,4), %rax
        ret

However, vector_end() needs to handle empty vectors represented as { NULL, 0 }.
Pointer arithmetic on a null pointer is undefined behavior (even NULL + 0, as
far as I can tell from the C standard), so the correct code is:

int *vector_end(struct vector *vec)
{
    if (vec->size == 0)
        return vec->data;
    return vec->data + vec->size;
}

I'd expect this to generate the same code, but GCC 10.2.0 generates a
conditional move with -O2 and -O3:

vector_end:
        movq    8(%rdi), %rdx
        movq    (%rdi), %rax
        testq   %rdx, %rdx
        leaq    (%rax,%rdx,4), %rcx
        cmovne  %rcx, %rax
        ret

And a branch with -Os:

vector_end:
        movq    8(%rdi), %rdx
        movq    (%rdi), %rax
        testq   %rdx, %rdx
        je      .L1
        leaq    (%rax,%rdx,4), %rax
.L1:
        ret

Clang 10.0.1, on the other hand, generates the same code with and without the
size check (oddly enough, it also falls back to a conditional move if the size
member is an int or unsigned int instead of size_t/unsigned long):

vector_end:                             # @vector_end
        movq    8(%rdi), %rax
        shlq    $2, %rax
        addq    (%rdi), %rax
        retq

Can GCC avoid the conditional move/branch here?

[Bug tree-optimization/97225] Failure to optimize out conditional addition of zero

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97225

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-09-28
          Component|c                           |tree-optimization
           Keywords|                            |missed-optimization
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think we've seen a duplicate report for this.

Confirmed.  late phiopt sees (we need hoisting to get rid of the loads
in the two if arms):


  <bb 2> [local count: 1073741824]:
  _1 = vec_6(D)->size;
  pretmp_9 = vec_6(D)->data;
  if (_1 == 0)
    goto <bb 4>; [34.00%]
  else
    goto <bb 3>; [66.00%]

  <bb 3> [local count: 708669601]:
  _3 = _1 * 4;
  _7 = pretmp_9 + _3;

  <bb 4> [local count: 1073741824]:
  # _4 = PHI <_7(3), pretmp_9(2)>
  return _4;

where it misses the value-conversion, possibly either based on lack
of handling the mult+add or because of consideration of making the
not infrequent path more expensive.  Later if-converting this anyway
on RTL shows a disconnect in cost then.

[Bug tree-optimization/97225] Failure to optimize out conditional addition of zero

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97225

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement

[Bug tree-optimization/97225] Failure to optimize out conditional addition of zero

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97225

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu.org

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I noticed this also while creating a testcase for
https://gcc.gnu.org/pipermail/gcc-patches/2023-August/626117.html .