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From: "yyc1992 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/106327] New: side-effect-free _x variance not optimized
 to unpredicated instruction
Date: Sat, 16 Jul 2022 20:11:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106327

            Bug ID: 106327
           Summary: side-effect-free _x variance not optimized to
                    unpredicated instruction
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yyc1992 at gmail dot com
  Target Milestone: ---

Related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106326 .

According to the Arm C Language Extension for SVE, when the _x predicate is
used,

> The compiler can then pick whichever form of instruction seems to give th=
e best code. This includes using unpredicated instructions, where available=
 and suitable

Because of this, I'm expecting the following to be optimized to a single add
instruction, as if a `svptrue_b64()` predicate is used.

```
svfloat64_t add(svfloat64_t a, svfloat64_t b)
{
    auto und_ok =3D svcmpge(svptrue_b64(), a, b);
    return svadd_x(und_ok, a, b);
}
```

However, gcc compiles this as _m and generates

```
        ptrue   p0.b, all
        fcmge   p0.d, p0/z, z0.d, z1.d
        fadd    z0.d, p0/m, z0.d, z1.d
```

In general, is there any reason not to treat an `add_x` (also other
side-effect-free functions) with an unknown predicate as unpredicated one?=