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From: "ibuclaw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug d/110359] New: d: Suboptimal codegen for __builtin_expect(cond,
 false) since PR96435
Date: Wed, 21 Jun 2023 18:44:59 +0000
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X-Bugzilla-Component: d
X-Bugzilla-Version: 11.3.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110359

            Bug ID: 110359
           Summary: d: Suboptimal codegen for __builtin_expect(cond,
                    false) since PR96435
           Product: gcc
           Version: 11.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: d
          Assignee: ibuclaw at gdcproject dot org
          Reporter: ibuclaw at gcc dot gnu.org
  Target Milestone: ---

Since pr96435, both boolean objects *and* expressions have been evaluated in
the following way.

    (*(ubyte*)&obj_or_expr) & 1

It has been noted that sometimes this can cause the back-end to optimize in
non-obvious ways - in particular with `__builtin_expect`.

---
double pow(in double x, in ulong p)
{
    import gcc.builtins : __builtin_expect;
    if (__builtin_expect(p =3D=3D 0, false))  // 1,2
        return 1;
    if (__builtin_expect(p =3D=3D 1, false))  // 3,4
        return x;

    double s =3D x;
    double v =3D 1;
    for (ulong i =3D p; i > 1; i >>=3D 1)     // 5
    {
        v =3D (i & 0x1) ? s * v : v;
        s =3D s * s;
    }
    return v * s;
}
---

The first 5 lines of assembly for the above function (corresponding souce l=
ine
marked in comments)
---
double example.pow(in double, in ulong):
        test    rdi, rdi
        je      .L6
        cmp     rdi, 1
        je      .L1
        jbe     .L1
---

The jbe condition looks like it is either redundant, or suboptimal.

This @safe feature could be restricted to just when reading the value of a =
bool
field that comes from a union.=