Compile-time Functionality

StatsLib is designed to operate equally well as a compile-time computation engine. Compile-time computation allows the compiler to replace function calls (e.g., dnorm(0,0,1)) with static values in the source code. That is, functions are evaluated during the compilation process, rather than at run-time. This capability is made possible due to the templated constexpr design of the library and can be verified by inspecting the assembly code generated by the compiler.

The compile-time features are enabled using the constexpr specifier. The example below computes the pdf, cdf, and quantile function of the Laplace distribution.

#include "stats.hpp"

int main()
{

    constexpr double dens_1  = stats::dlaplace(1.0,1.0,2.0); // answer = 0.25
    constexpr double prob_1  = stats::plaplace(1.0,1.0,2.0); // answer = 0.5
    constexpr double quant_1 = stats::qlaplace(0.1,1.0,2.0); // answer = -2.218875...

    return 0;
}

Inspecting the assembly code generated by Clang (without any optimization):

LCPI0_0:
    .quad   -4611193153885729483    ## double -2.2188758248682015
LCPI0_1:
    .quad   4602678819172646912     ## double 0.5
LCPI0_2:
    .quad   4598175219545276417     ## double 0.25000000000000006
    .section        __TEXT,__text,regular,pure_instructions
    .globl  _main
    .p2align        4, 0x90
_main:                                  ## @main
    push    rbp
    mov     rbp, rsp
    xor     eax, eax
    movsd   xmm0, qword ptr [rip + LCPI0_0] ## xmm0 = mem[0],zero
    movsd   xmm1, qword ptr [rip + LCPI0_1] ## xmm1 = mem[0],zero
    movsd   xmm2, qword ptr [rip + LCPI0_2] ## xmm2 = mem[0],zero
    mov     dword ptr [rbp - 4], 0
    movsd   qword ptr [rbp - 16], xmm2
    movsd   qword ptr [rbp - 24], xmm1
    movsd   qword ptr [rbp - 32], xmm0
    pop     rbp
    ret

We see that functions call have been replaced by numeric values.