Mex function faster than identical C++ code - c++

Question

When I run a program in C++, it runs slower than the identical program called by MATLAB using Mex functions.
I tried some sample code to test this, which confirmed my suspicion:
Using C++:
#include <stdio.h>
#include <ctime>
void process(int a[10000], int b[10000]) {
const int dim[2] = {1, 10000};
int barData[20000];
clock_t begin = clock();
for (int i = 0; i < dim[1]; i++) {
for (int j = 0; j < i; j++) {
barData[j] = a[i];
barData[j] = b[i];
}
}
clock_t end = clock();
double elapsed_secs = double(end - begin) / CLOCKS_PER_SEC;
printf("%f\n", elapsed_secs);
}
int main() {
int a[10000], b[10000];
process(a,b);
return 0;
}
Using Mex functions:
#include <stdio.h>
#include "mex.h"
void process(const mxArray *first, const mxArray *second) {
int* a = (int *)mxGetData(first);
int* b = (int *)mxGetData(second);
const int *dim = mxGetDimensions(first);
const int dims[2] = {1,dim[1]*2};
mxArray* bar = mxCreateNumericArray(2, dims, mxINT64_CLASS, mxREAL);
int* barData = (int*)mxGetData(bar);
clock_t begin = clock();
for (int i = 0; i < dim[1]; i++) {
for (int j = 0; j < i; j++) {
barData[j] = a[i];
barData[j] = b[i];
}
}
clock_t end = clock();
double elapsed_secs = double(end - begin) / CLOCKS_PER_SEC;
printf("%f\n", elapsed_secs);
}
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) {
process(prhs[0], prhs[1]);
}
And calling it from MATLAB as follows:
mex test.cpp -output foo
foo(rand(1,10000), rand(1,10000))
Mex function gives ~0.012s while C++ code gives 0.108s. The trends scale for larger array sizes too. Why is this, and is there a way to make the C++ code run with the Mex function speed?

Answer 1

As #Praetorian states in a comment above, you are probably not doing optimization on the C++ code.
Here is what the LLVMIR (pseudo-assembly) of your code is without optimization:
; ModuleID = 'test.cpp'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
#_ZZ7processPiS_E3dim = internal constant [2 x i32] [i32 1, i32 10000], align 4
#.str = private unnamed_addr constant [4 x i8] c"%f\0A\00", align 1
; Function Attrs: uwtable
define void #_Z7processPiS_(i32* %a, i32* %b) #0 {
%1 = alloca i32*, align 8
%2 = alloca i32*, align 8
%barData = alloca [20000 x i32], align 16
%begin = alloca i64, align 8
%i = alloca i32, align 4
%j = alloca i32, align 4
%end = alloca i64, align 8
%elapsed_secs = alloca double, align 8
store i32* %a, i32** %1, align 8
store i32* %b, i32** %2, align 8
%3 = call i64 #clock() #3
store i64 %3, i64* %begin, align 8
store i32 0, i32* %i, align 4
br label %4
; <label>:4 ; preds = %34, %0
%5 = load i32* %i, align 4
%6 = load i32* getelementptr inbounds ([2 x i32]* #_ZZ7processPiS_E3dim, i32 0, i64 1), align 4
%7 = icmp slt i32 %5, %6
br i1 %7, label %8, label %37
; <label>:8 ; preds = %4
store i32 0, i32* %j, align 4
br label %9
; <label>:9 ; preds = %30, %8
%10 = load i32* %j, align 4
%11 = load i32* %i, align 4
%12 = icmp slt i32 %10, %11
br i1 %12, label %13, label %33
; <label>:13 ; preds = %9
%14 = load i32* %i, align 4
%15 = sext i32 %14 to i64
%16 = load i32** %1, align 8
%17 = getelementptr inbounds i32* %16, i64 %15
%18 = load i32* %17, align 4
%19 = load i32* %j, align 4
%20 = sext i32 %19 to i64
%21 = getelementptr inbounds [20000 x i32]* %barData, i32 0, i64 %20
store i32 %18, i32* %21, align 4
%22 = load i32* %i, align 4
%23 = sext i32 %22 to i64
%24 = load i32** %2, align 8
%25 = getelementptr inbounds i32* %24, i64 %23
%26 = load i32* %25, align 4
%27 = load i32* %j, align 4
%28 = sext i32 %27 to i64
%29 = getelementptr inbounds [20000 x i32]* %barData, i32 0, i64 %28
store i32 %26, i32* %29, align 4
br label %30
; <label>:30 ; preds = %13
%31 = load i32* %j, align 4
%32 = add nsw i32 %31, 1
store i32 %32, i32* %j, align 4
br label %9
; <label>:33 ; preds = %9
br label %34
; <label>:34 ; preds = %33
%35 = load i32* %i, align 4
%36 = add nsw i32 %35, 1
store i32 %36, i32* %i, align 4
br label %4
; <label>:37 ; preds = %4
%38 = call i64 #clock() #3
store i64 %38, i64* %end, align 8
%39 = load i64* %end, align 8
%40 = load i64* %begin, align 8
%41 = sub nsw i64 %39, %40
%42 = sitofp i64 %41 to double
%43 = fdiv double %42, 1.000000e+06
store double %43, double* %elapsed_secs, align 8
%44 = load double* %elapsed_secs, align 8
%45 = call i32 (i8*, ...)* #printf(i8* getelementptr inbounds ([4 x i8]* #.str, i32 0, i32 0), double %44)
ret void
}
; Function Attrs: nounwind
declare i64 #clock() #1
declare i32 #printf(i8*, ...) #2
; Function Attrs: uwtable
define i32 #main() #0 {
%1 = alloca i32, align 4
%a = alloca [10000 x i32], align 16
%b = alloca [10000 x i32], align 16
store i32 0, i32* %1
%2 = getelementptr inbounds [10000 x i32]* %a, i32 0, i32 0
%3 = getelementptr inbounds [10000 x i32]* %b, i32 0, i32 0
call void #_Z7processPiS_(i32* %2, i32* %3)
ret i32 0
}
attributes #0 = { uwtable "less-precise-fpmad"="false" "no-frame- pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp- math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { nounwind "less-precise-fpmad"="false" "no-frame- pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp- math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #2 = { "less-precise-fpmad"="false" "no-frame-pointer- elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no- nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp- math"="false" "use-soft-float"="false" }
attributes #3 = { nounwind }
!llvm.ident = !{!0}
!0 = !{!"clang version 3.6.2 (tags/RELEASE_362/final)"}
Notice that Z7processPiS is very long.
Here is with optimization -O3 (which is generally safe in C++ nowadays):
; ModuleID = 'test.cpp'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
#.str = private unnamed_addr constant [4 x i8] c"%f\0A\00", align 1
; Function Attrs: nounwind uwtable
define void #_Z7processPiS_(i32* nocapture readnone %a, i32* nocapture readnone %b) #0 {
%1 = tail call i64 #clock() #2
%2 = tail call i64 #clock() #2
%3 = sub nsw i64 %2, %1
%4 = sitofp i64 %3 to double
%5 = fdiv double %4, 1.000000e+06
%6 = tail call i32 (i8*, ...)* #printf(i8* getelementptr inbounds ([4 x i8]* #.str, i64 0, i64 0), double %5)
ret void
}
; Function Attrs: nounwind
declare i64 #clock() #1
; Function Attrs: nounwind
declare i32 #printf(i8* nocapture readonly, ...) #1
; Function Attrs: nounwind uwtable
define i32 #main() #0 {
%1 = tail call i64 #clock() #2
%2 = tail call i64 #clock() #2
%3 = sub nsw i64 %2, %1
%4 = sitofp i64 %3 to double
%5 = fdiv double %4, 1.000000e+06
%6 = tail call i32 (i8*, ...)* #printf(i8* getelementptr inbounds ([4 x i8]* #.str, i64 0, i64 0), double %5) #2
ret i32 0
}
attributes #0 = { nounwind uwtable "less-precise-fpmad"="false" "no-frame- pointer-elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft- float"="false" }
attributes #1 = { nounwind "less-precise-fpmad"="false" "no-frame-pointer- elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack- protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #2 = { nounwind }
!llvm.ident = !{!0}
!0 = !{!"clang version 3.6.2 (tags/RELEASE_362/final)"}
P.S.: It would be more idiomatic to write:
#include <iostream>
#include <vector>
#include <ctime>
using std::vector;
void process(vector<int> a, vector<int> b) {
const pair<int,int> dim = {1, 10000};
vector<int> barData(20000,0);
clock_t begin = clock();
for (int i = 0; i < dim.second; i++) {
for (int j = 0; j < i; j++) {
barData[j] = a[i];
barData[j] = b[i];
}
}
clock_t end = clock();
std::cout << double(end-begin)/CLOCKS_PER_SEC << '\n';
}
int main() {
vector<int> a(10000, 0), b(10000,0);
process(a,b);
return 0;
}