How to post my issue to https://groups.google.com/d/forum/benchmark-discuss

[chengm349]

I joined the group but I can't post my question there. Please guide.

I have a class MyClass and I want to measure its ctor performance like this:

static void BM_decimal_ctor_dbl(benchmark::State& state)
{
for (auto _ : state)
{
const MyClass d{123.67};
benchmark::DoNotOptimize(d);
}
}

2024-06-07T13:50:06+08:00
Running ./AS7_DEC/decimalBM
Run on (12 X 2194.84 MHz CPU s)
CPU Caches:
L1 Data 48 KiB (x12)
L1 Instruction 32 KiB (x12)
L2 Unified 1280 KiB (x12)
L3 Unified 49152 KiB (x2)
Load Average: 0.10, 0.17, 0.18

Benchmark Time CPU Iterations

BM_decimal_ctor_dbl 0.364 ns 0.364 ns 1937088812

I doubted my class performance too good. Is my benchmark code correct?

[dmah42]

depending on what you're doing the compiler just might be that good.

https://godbolt.org/z/rT6E3bf9b shows that the compiler reduces the code quite a bit.

[chengm349]

@dmah42
Question 1: So my code of using the lib is correct?
static void BM_decimal_ctor_dbl(benchmark::State& state) {
for (auto _ : state) {
const MyClass d{123.67};
benchmark::DoNotOptimize(&d);
}
}

Question 2: which assembly lines are for the MyClass ctor? These?
push %rbp
push %r14
push %rbx
sub $0x10,%rsp
mov %rdi,%rbx
mov 0x1c(%rdi),%ebp
mov 0x10(%rdi),%r14

[chengm349]

I want to move similar questions https://groups.google.com/d/forum/benchmark-discuss, but I can't post any question there.

[dmah42]

I can't identify the assembly without seeing what your class does. but godbolt does a good job of mapping the code to the assembly so you should be able to see it there.

[chengm349]

I meant by using your MyClass impl, where is the assembly code?

[dmah42]

ah, line 30 ( mov %rax,0x8(%rsp) )

but there's also a couple of lines moving the value to the stack:
movabs $0x40263dcc937ad959,%rax
lea 0x8(%rsp),%rcx

you can see this in the tool with the color coding.