Clamp Function for f32 and f64

[Alex-Velez]

I thought the clamp function could use a little improvement for readability purposes. The function now returns early in order to skip the extra bound checks.

If there was a reason for binding self to x or if this code is incorrect, please correct me :)

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @scottmcm (or someone else) soon.

Please see the contribution instructions for more information.

[scottmcm]

These are very susceptible to getting worse for different implementations, since they were defined in the RFC very intentionally to be able to be implemented as minss+maxss, rather than needing branching.

But I think we don't have a test for that right now. So could you please add a https://github.com/rust-lang/rust/tree/master/src/test/assembly test for it? I think this should work:

// Floating-point clamp is designed to be implementable as max+min,
// so check to make sure that's what it's actually emitting.

// assembly-output: emit-asm
// compile-flags: -O -C "llvm-args=-x86-asm-syntax=intel"
// only-x86_64

// CHECK-LABEL: clamp_demo:
#[no_mangle]
pub fn clamp_demo(a: f32, x: f32, y: f32) -> f32 {
    // CHECK: maxss
    // CHECK: minss
    a.clamp(x, y)
}

// CHECK-LABEL: clamp12_demo:
#[no_mangle]
pub fn clamp12_demo(a: f32) -> f32 {
    // CHECK-NEXT: movss   xmm1
    // CHECK-NEXT: maxss   xmm1, xmm0
    // CHECK-NEXT: movss   xmm0
    // CHECK-NEXT: minss   xmm0, xmm1
    // CHECK-NEXT: ret
    a.clamp(1.0, 2.0)
}

Then if the new implementation of the method still passes (or produces similarly good ASM), then I'd be happy to use it!

@rustbot author

[5225225]

I checked the codegen in godbolt

pub fn clamp_old(this: f32, min: f32, max: f32) -> f32 {
    assert!(min <= max);
    let mut x = this;
    if x < min {
        x = min;
    }
    if x > max {
        x = max;
    }
    x
}

pub fn clamp_new(this: f32, min: f32, max: f32) -> f32 {
    assert!(min <= max);
    if this <= min {
        return min;
    }
    if this > max {
        return max;
    }
    this
}

example::clamp_old:
        ucomiss xmm2, xmm1
        jb      .LBB0_2
        maxss   xmm1, xmm0
        minss   xmm2, xmm1
        movaps  xmm0, xmm2
        ret
.LBB0_2:
        push    rax
        lea     rdi, [rip + .L__unnamed_1]
        lea     rdx, [rip + .L__unnamed_2]
        mov     esi, 28
        call    qword ptr [rip + core::panicking::panic@GOTPCREL]
        ud2

example::clamp_new:
        ucomiss xmm2, xmm1
        jb      .LBB1_2
        minss   xmm2, xmm0
        cmpnless        xmm0, xmm1
        andps   xmm2, xmm0
        andnps  xmm0, xmm1
        orps    xmm0, xmm2
        ret
.LBB1_2:
        push    rax
        lea     rdi, [rip + .L__unnamed_1]
        lea     rdx, [rip + .L__unnamed_3]
        mov     esi, 28
        call    qword ptr [rip + core::panicking::panic@GOTPCREL]
        ud2

(https://godbolt.org/z/3hd33cfKW)

[scottmcm]

        minss   xmm2, xmm0
        cmpnless        xmm0, xmm1
        andps   xmm2, xmm0
        andnps  xmm0, xmm1
        orps    xmm0, xmm2
        ret

Thanks, this is what I was concerned about.

For example, on Zen2 https://www.agner.org/optimize/instruction_tables.pdf lists maxss as latency 1, RThroughput ½ -- with {and(n)|or}ps as latency 1, RThroughput ¼. So it looks like this implementation is slightly more expensive, in addition to being more instructions.

Targeting that architecture specifically doesn't fix it either: https://godbolt.org/z/ET5sEff8q. That can vblendvps instead of and+andn+or, but that's Latency 1 RThroughput ½ just like maxss, and it still needs a vcmpltss first so it's slower.

(In general, down at the single instruction level, trying to branch to skip things is often slower than just running them.)

So I think you could update this to use a mut self if you prefer instead of the separate mut x, but the return approach seems to be suboptimal right now.

[scottmcm]

(closing and immediately reopening to try to re-kick the PR build, since only 1 of the three jobs ran.)

[Alex-Velez]

@scottmcm Thank you for your advice! I attempted to change the clamp function using mut self, however, it seems to produce this code according to godbolt.

example::clamp_old:
        ucomiss xmm2, xmm1
        jb      .LBB0_2
        maxss   xmm1, xmm0
        minss   xmm2, xmm1
        movaps  xmm0, xmm2
        ret
.LBB0_2:
        push    rax
        lea     rdi, [rip + .L__unnamed_1]
        lea     rdx, [rip + .L__unnamed_2]
        mov     esi, 28
        call    qword ptr [rip + core::panicking::panic@GOTPCREL]
        ud2

example::clamp_new:
        ucomiss xmm2, xmm1
        jb      .LBB1_5
        ucomiss xmm1, xmm0
        jae     .LBB1_4
        ucomiss xmm0, xmm2
        movaps  xmm1, xmm0
        jbe     .LBB1_4
        movaps  xmm1, xmm2
.LBB1_4:
        movaps  xmm0, xmm1
        ret
.LBB1_5:
        push    rax
        lea     rdi, [rip + .L__unnamed_1]
        lea     rdx, [rip + .L__unnamed_3]
        mov     esi, 28
        call    qword ptr [rip + core::panicking::panic@GOTPCREL]
        ud2

This new clamp also has slightly different functionality than the original one, since I think it mutates the original value? and returns a float, as opposed to just returning a new value.

fn clamp(mut self, min: f32, max: f32) -> f32 {
    assert!(min <= max);
    if self <= min {
        self = min;
    } else if self > max {
        self = max;
    }
    self
}

[scottmcm]

Hopefully just the one more change and things'll be good 🤞

If you could also squash all the commits, I'd appreciate it.

[scottmcm]

Huzzah! The PR build finally stopped resisting. Thanks for your persistence here -- it's great to get an assembly test for this to record the work from the RFC in a way that's actually checked.

@bors r+

[bors]

📌 Commit 302689f has been approved by scottmcm

It is now in the queue for this repository.

[matthiaskrgr]

@bors r- failed in a rollup #100664 (comment)

[scottmcm]

@bors r+

[bors]

📌 Commit 0314647 has been approved by scottmcm

It is now in the queue for this repository.