Date: Sun, 1 Jun 2025 15:25:10 +0200
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?
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Von:SD SH via Std-Proposals <std-proposals_at_[hidden]>
Gesendet:So 01.06.2025 15:19
Betreff:[std-proposals] A Proposal about A New Keyword assure
An:std-proposals_at_[hidden];
CC:SD SH <Z5515zwy_at_[hidden]>;
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_at_[hidden]>
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Received on 2025-06-01 13:33:08