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Forcing inlining of callback (lambda) in C++17 in library


Forcing inlining of callback (lambda) in C++17 in library

By : rishabh
Date : October 17 2020, 06:10 AM
Hope that helps The whole point of std::function is to have a common type that can hold an arbitrary callable for a certain signature while, at the same time, allowing that arbitrary callable to be invoked through a common interface no matter what kind of thing the callable actually happened to be. Thus, if you think about it, std::function inherently requires an indirection of some sort. Which code needs to run for calling an std::function depends not just on the type, but on the particular value of the std::function. This makes std::function (at least the call to the stored callable) inherently not inlineable. The code generated for the function calling your callback has to be able to handle any std::function you may possibly throw at it. The only way a compiler could potentially offer something like inlining for std::function would be if it was somehow able to figure out that your function calling the callback is most of the time only going to be used with std::function objects holding a particular value and then generate a clone of the function calling the callback for that specific case. This would either require an almost unrealistically clairvoyant compiler to arrive at in general, or a lot of magic hardwired into the compiler just for std::function specifically. It's not completely impossible in theory. But I've never witnessed any compiler actually being able to do anything like that. In my experience, optimizers are just not really able to see through std::function. And I would not expect that to change anytime soon, as getting any meaningful optimization there would seem to require huge amounts of effort for a rather questionable benefit. std::function is just heavy machinery to begin with. You simply pay for what you use there. If you can't pay the price, don't use std::function…
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Why does compiler inlining produce slower code than manual inlining?

Why does compiler inlining produce slower code than manual inlining?


By : Pooja Sharma
Date : March 29 2020, 07:55 AM
wish helps you Background , Short Answer:
Your asd array is declared as this:
code :
int *asd=new int[16];
xor         eax,eax  

mov         edx,ecx  
and         edx,0Fh  
mov         dword ptr [ebp+edx*4],eax  
mov         eax,dword ptr [esp+1Ch]  
movss       xmm0,dword ptr [eax]  
movss       xmm1,dword ptr [edi]  
cvtps2pd    xmm0,xmm0  
cvtps2pd    xmm1,xmm1  
comisd      xmm1,xmm0  
xor         eax,eax
xor         eax,eax  
movzx       edx,al
movzx       edx,al  
mov         eax,ecx  //  False dependency on "eax".
asd[j%16] = a.test(b);
^^^^^^^^^   ^^^^^^^^^
 type int   type bool
Do C++ compilers make an independent decision about inlining a lambda function and its caller?

Do C++ compilers make an independent decision about inlining a lambda function and its caller?


By : appleonamadev
Date : March 29 2020, 07:55 AM
it fixes the issue The decision of inlining the call to fn inside printAll is independent of the decision of inlining printAll. In particular, fn has very high chances of being inlined in all compilers, as is also the case of the std::for_each, but because printAll contains a loop (the one in for_each) likeliness of inlining drop in some compilers (some of the heuristics for inlining include the complexity of the code, and the presence of loops).
From the list of options, the best match is:
Speed of lambda vs. inlining the function

Speed of lambda vs. inlining the function


By : Jónas Rafnsson
Date : March 29 2020, 07:55 AM
Hope that helps cbreak-work helped me on the #c++ freenode IRC channel. He suggested what "Kerrek SB" wrote above in the comments. I changed for_each_lit's declaration to:
code :
template<class Function>
void for_each_lit(
    const OccurClause& cl
    ,  Function func
);
Impact of Intrinsics and inlining on Lambda's performance?

Impact of Intrinsics and inlining on Lambda's performance?


By : user3729962
Date : March 29 2020, 07:55 AM
Does that help The reason why you haven't noticed an expected performance effect is poorly written benchmark.
I rewrote the benchmark using JMH and the things finally got right.
code :
package lambdademo;

import org.openjdk.jmh.annotations.*;

import java.util.List;

@State(Scope.Benchmark)
public class LambdaBenchmark {
    @Param("100")
    private static int loopCount;

    private static double identity(double val) {
        double result = 0;
        for (int i=0; i < loopCount; i++) {
            result += Math.sqrt(Math.abs(Math.pow(val, 2)));    
        }
        return result / loopCount;
    }

    private List<EmployeeRec> employeeList = new EmployeeFile().loadEmployeeList();

    @Benchmark
    public double streamAverage() {
        return streamAverageNoInline();
    }

    @Benchmark
    @Fork(jvmArgs = "-XX:-Inline")
    public double streamAverageNoInline() {
        return employeeList.stream()
                .filter(s -> s.getGender().equals("M"))
                .mapToDouble(s -> s.getAge())
                .average()
                .getAsDouble();
    }

    @Benchmark
    public double streamMath() {
        return streamMathNoIntrinsic();
    }

    @Benchmark
    @Fork(jvmArgs = {"-XX:+UnlockDiagnosticVMOptions", "-XX:DisableIntrinsic=_dpow,_dabs,_dsqrt"})
    public double streamMathNoIntrinsic() {
        return employeeList.stream()
                .filter(s -> s.getGender().equals("M"))
                .mapToDouble(s -> identity(s.getAge()))
                .average()
                .getAsDouble();
    }
}
Benchmark                              Mode  Cnt     Score    Error  Units
LambdaBenchmark.streamAverage          avgt    5    71,490 ±  0,770  ms/op
LambdaBenchmark.streamAverageNoInline  avgt    5   122,740 ±  0,576  ms/op
LambdaBenchmark.streamMath             avgt    5    92,672 ±  1,538  ms/op
LambdaBenchmark.streamMathNoIntrinsic  avgt    5  5747,007 ± 20,387  ms/op
C++11 Performance: Lambda inlining vs Function template specialization

C++11 Performance: Lambda inlining vs Function template specialization


By : sontungdt7
Date : March 29 2020, 07:55 AM
I hope this helps you . If the compiler can track "this function pointer points to this function", the compiler can inline the call through the function pointer.
Sometimes compilers can do this. Sometimes they cannot.
code :
template <int func(int, int)>
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