C++ view types: pass by const& or by value?
When in doubt, pass by value.
Now, you should only rarely be in doubt.
Often values are expensive to pass and give little benefit. Sometimes you actually want a reference to a possibly mutating value stored elsewhere. Often, in generic code, you don't know if copying is an expensive operation, so you err on the side of not.
The reason why you should pass by value when in doubt is because values are easier to reason about. A reference (even a const one) to external data could mutate in the middle of an algorithm when you call a function callback or what have you, rendering what seems to be a simple function into a complex mess.
In this case, you already have an implicit reference bind (to the contents of the container you are viewing). Adding another implicit reference bind (to the view object that looks into the container) is no less bad because there are already complications.
Finally, compilers can reason about values better than they can about references to values. If you leave the locally analyzed scope (via a function pointer callback), the compiler has to presume the value stored in the const reference may have completely changed (if it cannot prove the contrary). A value in automatic storage with nobody taking a pointer to it can be assumed to not modify in a similar way -- there is no defined way to access it and change it from an external scope, so such modifications can be presumed to not-happen.
Embrace the simplicity when you have an opportunity to pass a value as a value. It only happens rarely.
EDIT: Code is available here: https://github.com/acmorrow/stringview_param
I've created some example code which appears to demonstrate that pass-by-value for string_view like objects results in better code for both callers and function definitions on at least one platform.
First, we define a fake string_view class (I didn't have the real thing handy) in string_view.h:
#pragma once
#include <string>
class string_view {
public:
string_view()
: _data(nullptr)
, _len(0) {
}
string_view(const char* data)
: _data(data)
, _len(strlen(data)) {
}
string_view(const std::string& data)
: _data(data.data())
, _len(data.length()) {
}
const char* data() const {
return _data;
}
std::size_t len() const {
return _len;
}
private:
const char* _data;
size_t _len;
};
Now, lets define some functions that consume a string_view, either by value or by reference. Here are the signatures in example.hpp:
#pragma once
class string_view;
void __attribute__((visibility("default"))) use_as_value(string_view view);
void __attribute__((visibility("default"))) use_as_const_ref(const string_view& view);
The bodies of these functions are defined as follows, in example.cpp:
#include "example.hpp"
#include <cstdio>
#include "do_something_else.hpp"
#include "string_view.hpp"
void use_as_value(string_view view) {
printf("%ld %ld %zu\n", strchr(view.data(), 'a') - view.data(), view.len(), strlen(view.data()));
do_something_else();
printf("%ld %ld %zu\n", strchr(view.data(), 'a') - view.data(), view.len(), strlen(view.data()));
}
void use_as_const_ref(const string_view& view) {
printf("%ld %ld %zu\n", strchr(view.data(), 'a') - view.data(), view.len(), strlen(view.data()));
do_something_else();
printf("%ld %ld %zu\n", strchr(view.data(), 'a') - view.data(), view.len(), strlen(view.data()));
}
The do_something_else function is here is a stand-in for arbitrary calls to functions that the compiler does not have insight about (e.g. functions from other dynamic objects, etc.). The declaration is in do_something_else.hpp:
#pragma once
void __attribute__((visibility("default"))) do_something_else();
And the trivial definition is in do_something_else.cpp:
#include "do_something_else.hpp"
#include <cstdio>
void do_something_else() {
std::printf("Doing something\n");
}
We now compile do_something_else.cpp and example.cpp into individual dynamic libraries. Compiler here is XCode 6 clang on OS X Yosemite 10.10.1:
clang++ -mmacosx-version-min=10.10 --stdlib=libc++ -O3 -flto -march=native -fvisibility-inlines-hidden -fvisibility=hidden --std=c++11 ./do_something_else.cpp -fPIC -shared -o libdo_something_else.dylib
clang++ -mmacosx-version-min=10.10 --stdlib=libc++ -O3 -flto -march=native -fvisibility-inlines-hidden -fvisibility=hidden --std=c++11 ./example.cpp -fPIC -shared -o libexample.dylib -L. -ldo_something_else
Now, we disassemble libexample.dylib:
> otool -tVq ./libexample.dylib
./libexample.dylib:
(__TEXT,__text) section
__Z12use_as_value11string_view:
0000000000000d80 pushq %rbp
0000000000000d81 movq %rsp, %rbp
0000000000000d84 pushq %r15
0000000000000d86 pushq %r14
0000000000000d88 pushq %r12
0000000000000d8a pushq %rbx
0000000000000d8b movq %rsi, %r14
0000000000000d8e movq %rdi, %rbx
0000000000000d91 movl $0x61, %esi
0000000000000d96 callq 0xf42 ## symbol stub for: _strchr
0000000000000d9b movq %rax, %r15
0000000000000d9e subq %rbx, %r15
0000000000000da1 movq %rbx, %rdi
0000000000000da4 callq 0xf48 ## symbol stub for: _strlen
0000000000000da9 movq %rax, %rcx
0000000000000dac leaq 0x1d5(%rip), %r12 ## literal pool for: "%ld %ld %zu\n"
0000000000000db3 xorl %eax, %eax
0000000000000db5 movq %r12, %rdi
0000000000000db8 movq %r15, %rsi
0000000000000dbb movq %r14, %rdx
0000000000000dbe callq 0xf3c ## symbol stub for: _printf
0000000000000dc3 callq 0xf36 ## symbol stub for: __Z17do_something_elsev
0000000000000dc8 movl $0x61, %esi
0000000000000dcd movq %rbx, %rdi
0000000000000dd0 callq 0xf42 ## symbol stub for: _strchr
0000000000000dd5 movq %rax, %r15
0000000000000dd8 subq %rbx, %r15
0000000000000ddb movq %rbx, %rdi
0000000000000dde callq 0xf48 ## symbol stub for: _strlen
0000000000000de3 movq %rax, %rcx
0000000000000de6 xorl %eax, %eax
0000000000000de8 movq %r12, %rdi
0000000000000deb movq %r15, %rsi
0000000000000dee movq %r14, %rdx
0000000000000df1 popq %rbx
0000000000000df2 popq %r12
0000000000000df4 popq %r14
0000000000000df6 popq %r15
0000000000000df8 popq %rbp
0000000000000df9 jmp 0xf3c ## symbol stub for: _printf
0000000000000dfe nop
__Z16use_as_const_refRK11string_view:
0000000000000e00 pushq %rbp
0000000000000e01 movq %rsp, %rbp
0000000000000e04 pushq %r15
0000000000000e06 pushq %r14
0000000000000e08 pushq %r13
0000000000000e0a pushq %r12
0000000000000e0c pushq %rbx
0000000000000e0d pushq %rax
0000000000000e0e movq %rdi, %r14
0000000000000e11 movq (%r14), %rbx
0000000000000e14 movl $0x61, %esi
0000000000000e19 movq %rbx, %rdi
0000000000000e1c callq 0xf42 ## symbol stub for: _strchr
0000000000000e21 movq %rax, %r15
0000000000000e24 subq %rbx, %r15
0000000000000e27 movq 0x8(%r14), %r12
0000000000000e2b movq %rbx, %rdi
0000000000000e2e callq 0xf48 ## symbol stub for: _strlen
0000000000000e33 movq %rax, %rcx
0000000000000e36 leaq 0x14b(%rip), %r13 ## literal pool for: "%ld %ld %zu\n"
0000000000000e3d xorl %eax, %eax
0000000000000e3f movq %r13, %rdi
0000000000000e42 movq %r15, %rsi
0000000000000e45 movq %r12, %rdx
0000000000000e48 callq 0xf3c ## symbol stub for: _printf
0000000000000e4d callq 0xf36 ## symbol stub for: __Z17do_something_elsev
0000000000000e52 movq (%r14), %rbx
0000000000000e55 movl $0x61, %esi
0000000000000e5a movq %rbx, %rdi
0000000000000e5d callq 0xf42 ## symbol stub for: _strchr
0000000000000e62 movq %rax, %r15
0000000000000e65 subq %rbx, %r15
0000000000000e68 movq 0x8(%r14), %r14
0000000000000e6c movq %rbx, %rdi
0000000000000e6f callq 0xf48 ## symbol stub for: _strlen
0000000000000e74 movq %rax, %rcx
0000000000000e77 xorl %eax, %eax
0000000000000e79 movq %r13, %rdi
0000000000000e7c movq %r15, %rsi
0000000000000e7f movq %r14, %rdx
0000000000000e82 addq $0x8, %rsp
0000000000000e86 popq %rbx
0000000000000e87 popq %r12
0000000000000e89 popq %r13
0000000000000e8b popq %r14
0000000000000e8d popq %r15
0000000000000e8f popq %rbp
0000000000000e90 jmp 0xf3c ## symbol stub for: _printf
0000000000000e95 nopw %cs:(%rax,%rax)
Interestingly, the by-value version is several instructions shorter. But that is only the function bodies. What about callers?
We will define some functions that invoke these two overloads, forwarding a const std::string&, in example_users.hpp:
#pragma once
#include <string>
void __attribute__((visibility("default"))) forward_to_use_as_value(const std::string& str);
void __attribute__((visibility("default"))) forward_to_use_as_const_ref(const std::string& str);
And define them in example_users.cpp:
#include "example_users.hpp"
#include "example.hpp"
#include "string_view.hpp"
void forward_to_use_as_value(const std::string& str) {
use_as_value(str);
}
void forward_to_use_as_const_ref(const std::string& str) {
use_as_const_ref(str);
}
Again, we compile example_users.cpp to a shared library:
clang++ -mmacosx-version-min=10.10 --stdlib=libc++ -O3 -flto -march=native -fvisibility-inlines-hidden -fvisibility=hidden --std=c++11 ./example_users.cpp -fPIC -shared -o libexample_users.dylib -L. -lexample
And, again, we look at the generated code:
> otool -tVq ./libexample_users.dylib
./libexample_users.dylib:
(__TEXT,__text) section
__Z23forward_to_use_as_valueRKNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE:
0000000000000e70 pushq %rbp
0000000000000e71 movq %rsp, %rbp
0000000000000e74 movzbl (%rdi), %esi
0000000000000e77 testb $0x1, %sil
0000000000000e7b je 0xe8b
0000000000000e7d movq 0x8(%rdi), %rsi
0000000000000e81 movq 0x10(%rdi), %rdi
0000000000000e85 popq %rbp
0000000000000e86 jmp 0xf60 ## symbol stub for: __Z12use_as_value11string_view
0000000000000e8b incq %rdi
0000000000000e8e shrq %rsi
0000000000000e91 popq %rbp
0000000000000e92 jmp 0xf60 ## symbol stub for: __Z12use_as_value11string_view
0000000000000e97 nopw (%rax,%rax)
__Z27forward_to_use_as_const_refRKNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE:
0000000000000ea0 pushq %rbp
0000000000000ea1 movq %rsp, %rbp
0000000000000ea4 subq $0x10, %rsp
0000000000000ea8 movzbl (%rdi), %eax
0000000000000eab testb $0x1, %al
0000000000000ead je 0xebd
0000000000000eaf movq 0x10(%rdi), %rax
0000000000000eb3 movq %rax, -0x10(%rbp)
0000000000000eb7 movq 0x8(%rdi), %rax
0000000000000ebb jmp 0xec7
0000000000000ebd incq %rdi
0000000000000ec0 movq %rdi, -0x10(%rbp)
0000000000000ec4 shrq %rax
0000000000000ec7 movq %rax, -0x8(%rbp)
0000000000000ecb leaq -0x10(%rbp), %rdi
0000000000000ecf callq 0xf66 ## symbol stub for: __Z16use_as_const_refRK11string_view
0000000000000ed4 addq $0x10, %rsp
0000000000000ed8 popq %rbp
0000000000000ed9 retq
0000000000000eda nopw (%rax,%rax)
And, again, the by-value version is several instructions shorter.
It appears to me that, at least by the coarse metric of instruction count, that the by-value version produces better code for both callers and for generated function bodies.
I'm of course open to suggestions to how to improve this test. Obviously a next step would be to refactor this into something where I could benchmark it meaningfully. I will try to do that soon.
I will post the example code to github with some sort of build script so others can test on their systems.
But based on the discussion above, and the results of inspecting the generated code, my conclusion is that pass-by-value is the way to go for view types.
Putting aside philosophical questions about the signaling value of const&-ness versus value-ness as function parameters, we can take a look at some ABI implications on various architectures.
http://www.macieira.org/blog/2012/02/the-value-of-passing-by-value/ lays out some decision making and testing done by some QT folks on x86-64, ARMv7 hard-float, MIPS hard-float (o32) and IA-64. Mostly, it checks whether functions can pass various structs through registers. Not surprisingly, it appears that each platform can manage 2 pointers by register. And given that sizeof(size_t) is generally sizeof(void*), there's little reason to believe that we'll be spilling to memory here.
We can find more wood for the fire, considering suggestions like: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3538.html. Note that const ref has some downsides, namely the risk of aliasing, which can prevent important optimizations and require extra thought for the programmer. In the absence of C++ support for C99's restrict, passing by value can improve performance and lower cognitive load.
I suppose then that I'm synthesizing two arguments in favor of pass by value:
- 32 bit platforms often lacked the capacity to pass two word structs by register. This no longer appears to be an issue.
- const references are quantitatively and qualitatively worse than values in that they can alias.
All of which would lead me to favor pass by value for <16 byte structures of integral types. Obviously your mileage may vary and testing should always be done where performance is an issue, but values do seem a little nicer for very small types.
In addition to what have already been said here in favour of passing by value, modern C++ optimizers struggle with reference arguments.
When the body of the callee is not available in the translation unit (the function resides in a shared library or in another translation unit and link-time optimization is not available) the following things happen:
- The optimizer assumes that the arguments passed by reference or reference to const can be changed (
constdoes not matter because ofconst_cast) or referred to by a global pointer, or changed by another thread. Basically, the arguments passed by reference become poisoned values in the call site, which the optimizer cannot apply many optimizations to any more. - In the callee if there are several reference/pointer arguments of the same base type, the optimizer assumes that they alias with something else and that again precludes many optimizations.
- Moreover, all
chartype arrays can alias values of any other type, so that modifying anystd::stringobject implies modifying any and all other objects, resulting in following machine code having to reload all objects from memory.restrictkeyword was added toCto solve exactly this inefficiency. Different addresses may still alias because one can map one page frame multiple times into one virtual address space (and that's a popular trick for 0-copy receive ring buffers), that's why the compiler cannot assume no aliasing for different addresses unlessrestrictkeyword is used.
From the optimizer standpoint of view passing and returning by value is the best because that obviates the need for alias analysis: the caller and the callee own their copies of values exclusively so that these values cannot be modified from anywhere else.
For a detailed treatment of the subject I cannot recommend enough Chandler Carruth: Optimizing the Emergent Structures of C++. The punchline of the talk is "people need to change their heads about passing by value... the register model of passing arguments is obsolete."
Here are my rules of thumb for passing variables to functions:
- If the variable can fit inside the processor's register and will not be modified, pass by value.
- If the variable will be modified, pass by reference.
- If the variable is larger than the processor's register and will not be modified, pass by constant reference.
- If you need to use pointers, pass by smart pointer.
Hope that helps.