True, but not only inlined functions.

It can make compiler jump to the excepted branch without redundant check-ups. We can even use 1 function and many tag.

Like this:

_func:

; fastcall

; rcx = int para1

; rdx = bool para2

cmp rdx, 0

jne _func_para2_true

cmp rcx, 0

jl _func_para2_true_para1_l0

_func_para2_false_para1_geq0

; ...

ret

_func_para2_true:

; ...

ret

_func_para2_true_para1_l0:

; ...

ret

In fact, even if compiler not inlined the function, it still works.

For example, _func(i assure(i >= 0, b assure(!b)) will be:

    call _func_para2_false_para1_geq0

And 2 branches will be skipped.

I think compiler couldn't do above well.

---

发件人: Std-Proposals <std-proposals-bounces@lists.isocpp.org> 代表 Sebastian Wittmeier via Std-Proposals <std-proposals@lists.isocpp.org>
发送时间: 2025年6月1日 21:25
收件人: std-proposals@lists.isocpp.org <std-proposals@lists.isocpp.org>
抄送: Sebastian Wittmeier <wittmeier@projectalpha.org>
主题: Re: [std-proposals] A Proposal about A New Keyword assure

 

Do I understand that correctly that you are talking about getting gains with inlined functions by giving additional static (constexpr expression) constraints at the call-site?

 

Why is it better (except syntax) compared to existing facilities?
 

-----Ursprüngliche Nachricht-----
Von: SD SH via Std-Proposals <std-proposals@lists.isocpp.org>
Gesendet: So 01.06.2025 15:19
Betreff: [std-proposals] A Proposal about A New Keyword assure
An: std-proposals@lists.isocpp.org;
CC: SD SH <Z5515zwy@outlook.com>;

This proposal introduces a new contextual-keyword assure that allows callers to declare parament Constraints at function call sites. can utilizing these Constraints to optimize function calling. (e.g. removing branches) Violations result in undefined behavior. Analogous to [[assume]].

 

Motivation

        Check-ups of function definition sites is unnessary at some call sites, but compilers may preserve check-ups for them. Sometimes we need more performance optimizations but we can't do many optimizations such as built-in functions and third-party libraries.

        e.g. Lighting computing will call sqrt many times.

        If sqrt(double _X) like this:

            double __fastcall sqrt(double _X)

            {

                if(_X < 0.0)

                {

                    return std::numeric_limits<double>::infinity();

                }

                else

                {

                    // ...

                }

            }

            // asm:

            /*

                _sqrt:

                    pxor xmm1, xmm1

                    ucomisd xmm0, xmm1

                    jl _sqrt__Xl0

                    ; ...

                _sqrt__Xl0:

                    movsd xmm0, 0x7FF8000000000000

                    ret

            */

        In lighting computing, _X is always equals zero or greater than zero, so branch of if(_X < 0.0) is never performed there, but compiler may not remove it.
        assure will slove this issue.

        sqrt(d assure(d >= 0)) may generates

            _sqrt:

                pxor xmm1, xmm1

                ucomisd xmm0, xmm1

                jl _sqrt__Xl0

            _sqrt_sometime:

                ; ...

                ret

            _sqrt__Xl0:

                movsd xmm0, 0x7FF8000000000000

                ret

            ;...

            movsd xmm0, [somewhere]

            call _sqrt_simetime

        or inline call or a new function of sqrt, we needn't program another sqrt for this call site and compiler can optimize program better.

Design Decisions

    Syntax

        func(para1 assure(const-bool-expr(INPUT: { para1, const paras... })), paras...)

 

        Rejected Alternatives:

            func(paras...) assure(const-bool-expr(INPUT: const paras...))

                makes compiler implementation difficult.

        Semantics:

            Constraints are purely caller-to-compiler optimization hints.

            Expressions must be side-effect-free and dependent only on caller-visible state.

            Expressions should be true, or function calling has undefined behavior

        Alternative Considered:

            Runtime assertions (for security) rejected due to performance impact.

 

    Ecosystem Impact

        User:           Performance gains with constraints

        Implementers:   Medium or low implementation effort

        STL:            Greater room for optimization

 

View the full text (R0) on GitHub: CPP-Proposals/assure.txt

 

Thank you!

 

E. S. Himers <Z5515zwy@outlook.com>

-- 
 Std-Proposals mailing list
 Std-Proposals@lists.isocpp.org
 https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
 

--
Std-Proposals mailing list
Std-Proposals@lists.isocpp.org
https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals