f32::min()/max() not inlined

[kornelski]

fn test(x: f32) -> f32 {
    x.max(0.)
}

compiles to (with optimizations):

jmp	fmaxf@PLT

but I'd expect it to be inlined and generate code that is exactly the same as if x > 0. {x} else {0.}, which compiles to maxss inline. I've checked that even LTO doesn't inline the calls.

[kennytm]

We should reimplement fNN::max and min to not rely on cmath?

fn fmax(a: f32, b: f32) -> f32 {
    if b.is_nan() || b <= a { a } else { b }
}
fn fmin(a: f32, b: f32) -> f32 {
    if b.is_nan() || b >= a { a } else { b }
}

The resulting ASM looks like

#[no_mangle] #[inline(never)] pub fn better(x: f32) -> f32 {
    fmin(fmax(x, 0.), 1.)
}

=>

better:
	.cfi_startproc
	xorps	xmm1, xmm1
	cmpless	xmm1, xmm0
	andps	xmm0, xmm1
	minss	xmm0, dword ptr [rip + .LCPI0_0]
	ret

---

Details

I compared the equivalent C program

__attribute__((noinline))
float clamp(float x) {
    return fmin(fmax(x, 0), 1);
}

In gcc-7.1 -x c -std=c11 -O3 -march=native this compiled to

clamp:
        sub     rsp, 8
        vxorpd  xmm1, xmm1, xmm1
        vcvtss2sd       xmm0, xmm0, xmm0
        call    fmax
        vmovsd  xmm1, QWORD PTR .LC1[rip]
        call    fmin
        add     rsp, 8
        vcvtsd2ss       xmm0, xmm0, xmm0
        ret

while in gcc-7.1 -x c -std=c11 -O3 -march=native -ffast-math this becomes

clamp:
        vcvtss2sd       xmm0, xmm0, xmm0
        vmaxsd  xmm0, xmm0, QWORD PTR .LC0[rip]
        vminsd  xmm0, xmm0, QWORD PTR .LC1[rip]
        vcvtsd2ss       xmm0, xmm0, xmm0
        ret

The result is similar in clang 4.

Without -ffast-math

clamp:                                  # @clamp
        vxorps  xmm1, xmm1, xmm1
        vmaxss  xmm1, xmm1, xmm0
        vcmpunordss     xmm0, xmm0, xmm0
        vandnps xmm0, xmm0, xmm1
        vmovss  xmm1, dword ptr [rip + .LCPI0_0] # xmm1 = mem[0],zero,zero,zero
        vminss  xmm2, xmm1, xmm0
        vcmpunordss     xmm0, xmm0, xmm0
        vandnps xmm2, xmm0, xmm2
        vandps  xmm0, xmm0, xmm1
        vorps   xmm0, xmm2, xmm0
        ret

With -ffast-math

clamp:                                  # @clamp
        vxorps  xmm1, xmm1, xmm1
        vmaxss  xmm0, xmm0, xmm1
        vminss  xmm0, xmm0, dword ptr [rip + .LCPI0_0]
        ret

ICC 17 refuses to inline fmin or fmax with or without -ffast-math.

To be precise, the optimization is done with the -ffinite-math-only flag, but I don't see any problem in the Rust code when ±∞, ±0 or quiet NaNs are involved. -fno-signaling-nans does nothing so I don't think signaling NaNs is a concern for GCC.

---

Test of equivalence and showing considerate speed improvement:

Program

#![feature(test)]
#![feature(dec2flt)]

extern crate test;
extern crate core;

use test::{Bencher, black_box};
use core::num::dec2flt::rawfp::RawFloat;
use std::f32;

fn fmax(a: f32, b: f32) -> f32 {
    if b.is_nan() || b <= a { a } else { b }
}
fn fmin(a: f32, b: f32) -> f32 {
    if b.is_nan() || b >= a { a } else { b }
}

const NUMBERS: &[f32] = &[
    f32::INFINITY,
    f32::MAX,
    10.0,
    1.0,
    0.1,
    f32::MIN_POSITIVE,
    0.0,
    -0.0,
    -f32::MIN_POSITIVE,
    -0.1,
    -1.0
    -10.0,
    -f32::MAX,
    -f32::INFINITY,
    f32::NAN,
];

#[test]
fn test_equivalent() {
    for a in NUMBERS {
        for b in NUMBERS {
            let c = fmax(*a, *b);
            let d = a.max(*b);
            assert_eq!(c.integer_decode(), d.integer_decode(), "max({:?}, {:?}) => wrong:{:?}, correct:{:?}", a, b, c, d);

            let e = fmin(*a, *b);
            let f = a.min(*b);
            assert_eq!(e.integer_decode(), f.integer_decode(), "min({:?}, {:?}) => wrong:{:?}, correct:{:?}", a, b, e, f);
        }
    }
}

#[bench]
fn test_max_std(bencher: &mut Bencher) {
    bencher.iter(|| {
        for a in NUMBERS {
            for b in NUMBERS {
                black_box(a.max(*b));
            }
        }
    })
}

#[bench]
fn test_max_new(bencher: &mut Bencher) {
    bencher.iter(|| {
        for a in NUMBERS {
            for b in NUMBERS {
                black_box(fmax(*a, *b));
            }
        }
    })
}

#[bench]
fn test_min_std(bencher: &mut Bencher) {
    bencher.iter(|| {
        for a in NUMBERS {
            for b in NUMBERS {
                black_box(a.min(*b));
            }
        }
    })
}

#[bench]
fn test_min_new(bencher: &mut Bencher) {
    bencher.iter(|| {
        for a in NUMBERS {
            for b in NUMBERS {
                black_box(fmin(*a, *b));
            }
        }
    })
}

$ ./1 --test --bench

running 5 tests
test test_equivalent ... ok
test test_max_new ... bench:          66 ns/iter (+/- 19)
test test_max_std ... bench:         504 ns/iter (+/- 64)
test test_min_new ... bench:          68 ns/iter (+/- 30)
test test_min_std ... bench:         509 ns/iter (+/- 80)

test result: ok. 1 passed; 0 failed; 0 ignored; 4 measured; 0 filtered out

[kennytm]

There is an LLVM intrinsic llvm.minnum.fNN. When compared with the pure Rust implementation, the generated assembly is generally worse on x86_64 and i686, and better on ARM and AArch64 (Both won't call fmaxf/fminf unless there is no hard-float support). I haven't checked other targets.

Check program

// -Copt-level=3 -Ctarget-cpu=native -Cpanic=abort 

#![crate_type="lib"]
#![feature(link_llvm_intrinsics)]

extern {
    #[link_name = "llvm.minnum.f32"]
    pub fn minnum_f32(a: f32, b: f32) -> f32;
    #[link_name = "llvm.maxnum.f32"]
    pub fn maxnum_f32(a: f32, b: f32) -> f32;
}

pub trait MinMaxTest: Sized {
    fn min_llvm(self, other: Self) -> Self;
    fn min_rust(self, other: Self) -> Self;
    fn max_llvm(self, other: Self) -> Self;
    fn max_rust(self, other: Self) -> Self;
}

impl MinMaxTest for f32 {
    /*
    x86_64:
        _<f32 as 2::MinMaxTest>::min_llvm::hd76590e5233a75ba:
            pushq	%rbp
            movq	%rsp, %rbp
            vminss	%xmm1, %xmm0, %xmm2
            vcmpunordss	%xmm1, %xmm1, %xmm1
            vblendvps	%xmm1, %xmm0, %xmm2, %xmm0
            popq	%rbp
            retq
    
    aarch64:
        _<f32 as 2::MinMaxTest>::min_llvm::hd76590e5233a75ba:
            fminnm	s0, s1, s0
            ret
    */
    fn min_llvm(self, other: Self) -> Self {
        unsafe { minnum_f32(other, self) }     
    }

    /*
    x86_64:
        _<f32 as 2::MinMaxTest>::min_rust::h6a6994ac4726b5ad:
            pushq	%rbp
            movq	%rsp, %rbp
            vminss	%xmm1, %xmm0, %xmm2
            vcmpunordss	%xmm1, %xmm1, %xmm1
            vblendvps	%xmm1, %xmm0, %xmm2, %xmm0
            popq	%rbp
            retq
    
    aarch64:
        _<f32 as 2::MinMaxTest>::min_rust::h6a6994ac4726b5ad:
            fcmp	s1, s0
            fccmp	s1, s1, #1, le
            fcsel	s0, s0, s1, vs
            ret
    */
    fn min_rust(self, other: Self) -> Self {
        if other.is_nan() || other > self { self } else { other }
    }

    /*
    x86_64:
        _<f32 as 2::MinMaxTest>::max_llvm::h8b7b369f1c3662de:
            pushq	%rbp
            movq	%rsp, %rbp
            vmaxss	%xmm1, %xmm0, %xmm2
            vcmpunordss	%xmm1, %xmm1, %xmm1
            vblendvps	%xmm1, %xmm0, %xmm2, %xmm0
            popq	%rbp
            retq

    aarch64:
        _<f32 as 2::MinMaxTest>::max_llvm::h8b7b369f1c3662de:
            fmaxnm	s0, s1, s0
            ret
    */
    fn max_llvm(self, other: Self) -> Self {
        unsafe { maxnum_f32(other, self) }
    }

    /*
    x86_64:
        _<f32 as 2::MinMaxTest>::max_rust::hb29df4916b2f44f3:
            pushq	%rbp
            movq	%rsp, %rbp
            vmaxss	%xmm1, %xmm0, %xmm2
            vcmpunordss	%xmm1, %xmm1, %xmm1
            vblendvps	%xmm1, %xmm0, %xmm2, %xmm0
            popq	%rbp
            retq

    aarch64:
        _<f32 as 2::MinMaxTest>::max_rust::hb29df4916b2f44f3:
            fcmp	s1, s0
            fccmp	s1, s1, #1, pl
            fcsel	s0, s0, s1, vs
            ret
    */
    fn max_rust(self, other: Self) -> Self {
        if other.is_nan() || other < self { self } else { other }
    }

}

/*
x86_64:
    __2::clamp01_rfc1961::hf9aa1066c6c8cd7e:
        pushq	%rbp
        movq	%rsp, %rbp
        vxorps	%xmm1, %xmm1, %xmm1
        vmaxss	%xmm0, %xmm1, %xmm0
        vmovss	LCPI4_0(%rip), %xmm1
        vminss	%xmm0, %xmm1, %xmm0
        popq	%rbp
        retq

aarch64:
    __2::clamp01_rfc1961::hf9aa1066c6c8cd7e:
        movi.2d	v1, #0000000000000000
        fmax	s0, s0, s1
        fmov	s1, #1.00000000
        fmin	s0, s0, s1
        ret
*/
pub fn clamp01_rfc1961(mut x: f32) -> f32 {
    if x < 0.0 { x = 0.0; }
    if x > 1.0 { x = 1.0; }
    x
}

/*
x86_64:
    __2::clamp01_llvm::h30ee56b2e9fb6a19:
        pushq	%rbp
        movq	%rsp, %rbp
        vxorps	%xmm1, %xmm1, %xmm1
        vmaxss	%xmm0, %xmm1, %xmm1
        vcmpunordss	%xmm0, %xmm0, %xmm0
        vandnps	%xmm1, %xmm0, %xmm0
        vmovss	LCPI5_0(%rip), %xmm1
        vminss	%xmm0, %xmm1, %xmm2
        vcmpunordss	%xmm0, %xmm0, %xmm0
        vandnps	%xmm2, %xmm0, %xmm2
        vandps	%xmm1, %xmm0, %xmm0
        vorps	%xmm0, %xmm2, %xmm0
        popq	%rbp
        retq

aarch64:
    __2::clamp01_llvm::h30ee56b2e9fb6a19:
        movi.2d	v1, #0000000000000000
        fmaxnm	s0, s0, s1
        fmov	s1, #1.00000000
        fminnm	s0, s0, s1
        ret
*/
pub fn clamp01_llvm(x: f32) -> f32 {
    x.max_llvm(0.0).min_llvm(1.0)
}

/*
x86_64:
    __2::clamp01_rust::h324077e72816ec64:
        pushq	%rbp
        movq	%rsp, %rbp
        vxorps	%xmm1, %xmm1, %xmm1
        vmaxss	%xmm1, %xmm0, %xmm0
        vminss	LCPI6_0(%rip), %xmm0, %xmm0
        popq	%rbp
        retq

aarch64:
    __2::clamp01_rust::h324077e72816ec64:
        movi.2d	v1, #0000000000000000
        fmaxnm	s0, s0, s1
        fmov	s1, #1.00000000
        fminnm	s0, s0, s1
        ret
*/
pub fn clamp01_rust(x: f32) -> f32 {
    x.max_rust(0.0).min_rust(1.0)
}

/*
x86_64:
    __2::clamp01_std::h5df114f63023c56b:
        pushq	%rbp
        movq	%rsp, %rbp
        vxorps	%xmm1, %xmm1, %xmm1
        callq	_fmaxf
        vmovss	LCPI7_0(%rip), %xmm1
        popq	%rbp
        jmp	_fminf

aarch64:
    __2::clamp01_std::h5df114f63023c56b:
        stp	x29, x30, [sp, #-16]!
        mov	 x29, sp
        movi.2d	v1, #0000000000000000
        bl	_fmaxf
        fmov	s1, #1.00000000
        ldp	x29, x30, [sp], #16
        b	_fminf
*/
pub fn clamp01_std(x: f32) -> f32 {
    x.max(0.0).min(1.0)
}

[Mark-Simulacrum]

Seems fixed. Closing.

bad:
	xorps	%xmm1, %xmm1
	cmpless	%xmm0, %xmm1
	andps	%xmm1, %xmm0
	minss	.LCPI0_0(%rip), %xmm0
	retq

good:
	xorps	%xmm1, %xmm1
	cmpnless	%xmm0, %xmm1
	minss	.LCPI1_0(%rip), %xmm0
	andnps	%xmm0, %xmm1
	movaps	%xmm1, %xmm0
	retq