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URI Pass-by-Value Overhead
jklowden wrote 5 hours 40 min ago:
There is no pass-by-value overhead. There are only implementation
decisions.
Pass by value describes the semantics of a function call, not
implementation. Passing a const reference in C++ is pass-by-value. If
the user opts to pass "a copy" instead, nothing requires the compiler
to actually copy the data. The compiler is required only to supply the
actual parameter as if it was copied.
mattnewport wrote 5 hours 0 min ago:
This might be true in the abstract but it's not true of actual
compilers dealing with real world calling conventions. Absent
inlining or whole program optimization, calling conventions across
translation units don't leave much room for flexibility.
The semantics of pass by const reference are also not exactly the
same as pass by value in C++. The compiler can't in general assume a
const reference doesn't alias other arguments or global variables and
so has to be more conservative with certain optimizations than with
pass by value.
duped wrote 5 hours 6 min ago:
Unfortunately "the compiler is required to supply the actual
parameter as if it was copied" is leaky with respect to the ABI and
linker. In C and C++ you cannot fully abstract it.
codedokode wrote 7 hours 53 min ago:
I usually use ChatGPT for such microbenchmarks (of course I design it
myself and use LLM only as dumb code generator, so I don't need to
remember how to measure time with nanosecond precision. I still have to
add workarounds to prevent compiler over-optimizing the code). It's
amazing, that when you get curious (for example, what is the fastest
way to find an int in a small sorted array: using linear, binary search
or branchless full scan?) you can get the answer in a couple minutes
instead of spending 20-30 minutes writing the code manually.
By the way, the fastest way was branchless linear scan up to 32-64
elements, as far as I remember.
anonymous908213 wrote 9 hours 15 min ago:
> Donât pass around data of size 4046-4080 bytes or 8161-8176 bytes,
by value (at least not on an AMD Ryzen 3900X).
What a fascinating CPU bug. I am quite curious as to how that came to
pass.
srcmax wrote 2 hours 56 min ago:
That's called 4k aliasing.
4K aliasing occurs when you store one memory location, then load from
another memory location which is 4KB offset from original.
jasonthorsness wrote 5 hours 47 min ago:
Apparently some sizes are cursed!
It would be great to repeat the authorâs tests on other CPU models
TuxSH wrote 3 hours 39 min ago:
I wonder what the page size is on his system (and what effective
alignment his pointers have). If it's 4K, the sizes look really
close to 0x1000 and 0x2000 - maybe crossing page boundaries?
sgarland wrote 6 hours 49 min ago:
Me too, and I hope this article gets more traction.
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