llvm dev - Dec 2016 - [cfe-dev] Modernizing LLVM Coding Style Guide and enforcing Clang-tidy

> On Dec 29, 2016, at 5:54 AM, Daniel Berlin <dberlin at dberlin.org>
wrote:
> 
> 
> From yesterday discussion, Daniel Berlin proposed using emplace_back
everywhere to make code easier to maintain. I think it is valid argument, but I
believe it would reduce readability.
> 
> Just to be clear; I proposed not trying to switch uses back and forth
without real data, and to come to some agreement about what should be written in
the first place, preferably based on real data, and then, switching in some
organized fashion, not just randomly :)
> 
> IE Either have a clear coding standard and enforce it, or leave uses alone
one way or the other without some demonstration that it actually matters.
> 
> I would *personally* prefer to just use emplace_back everywhere, and not
try to switch between the two, without demonstration of real benefit.
> 
> (i'd also accept the other way, use push_back everywhere, and not try
to switch between the two, without demonstration of real benefit).
> 
> This preference is based on a simple truth: People suck at deciding when to
use one or the other. Yes, i know how to use it and when to use it.  Lots of our
types are cheaply movable, etc, so you probably won't see any performance
difference even when using them "right".  People have enough style/etc
issues without having to try to think hard about this.
I agree that “people suck at deciding”, me in the first place in the line, and
that’s why I like clear and good guideline and stick to it unless
"demonstration of real benefit”.
I also think we can have clear guideline that are different from “always use
emplace_back” or “always use push_back”, like Piotr is suggesting.

That said, I’m not convinced the relative “ugliness" of emplace_back  (I’m
not sure what’s ugly there, I only see it as a habit to take)  vs push_back is
enough to justify a set of rules to decide between the two, I’d be fine with
“always using emplace_back” in the name of “simple rule is better when
possible".
> 
> So that isn't what I would necessarily propose for LLVM.
> 
> For LLVM, my answer would be "either make tool do what we want, force
use of tool, or be consistent and obvious with what we want and then fix it if
it leads to performance issue”
+1
Ultimately whatever we do is not practical if it can’t be done by a tool.

> 
> 
>  
> There is also other reason - emplace_back can't take arguments of some
types like bitfields, NULL, static member. Using emplace can also lead to memory
leak in case of smart ptrs
> "
> auto *ptr = new int(1);
> v.push_back(std::unique_ptr<int>(ptr));
> This is because replacing it with emplace_back could cause a leak of this
pointer if emplace_back would throw exception before emplacement (e.g. not
enough memory to add new element).".
> 
> 
> This seems, IMHO, not a useful argument for LLVM since it does not try to
use exception based error handling to recover.
This, and also we should not write code like that (naked pointers, calling new),
using make_unique<> for instance would prevent any such situation. Passing
a raw pointer to a container of unique_ptr can be flagged by a tool.


— 
Mehdi
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On Thu, Dec 29, 2016 at 10:04 AM Mehdi Amini via cfe-dev <
cfe-dev at lists.llvm.org> wrote:
> On Dec 29, 2016, at 5:54 AM, Daniel Berlin <dberlin at dberlin.org>
wrote:
>
>
> From yesterday discussion, Daniel Berlin proposed using emplace_back
> everywhere to make code easier to maintain. I think it is valid argument,
> but I believe it would reduce readability.
>
>
> Just to be clear; I proposed not trying to switch uses back and forth
> without real data, and to come to some agreement about what should be
> written in the first place, preferably based on real data, and then,
> switching in some organized fashion, not just randomly :)
>
> IE Either have a clear coding standard and enforce it, or leave uses alone
> one way or the other without some demonstration that it actually matters.
>
> I would *personally* prefer to just use emplace_back everywhere, and not
> try to switch between the two, without demonstration of real benefit.
>
> (i'd also accept the other way, use push_back everywhere, and not try
to
> switch between the two, without demonstration of real benefit).
>
> This preference is based on a simple truth: People suck at deciding when
> to use one or the other. Yes, i know how to use it and when to use it.
> Lots of our types are cheaply movable, etc, so you probably won't see
any
> performance difference even when using them "right".  People have
enough
> style/etc issues without having to try to think hard about this.
>
>
> I agree that “people suck at deciding”, me in the first place in the line,
> and that’s why I like clear and good guideline and stick to it unless
> "demonstration of real benefit”.
> I also think we can have clear guideline that are different from “always
> use emplace_back” or “always use push_back”, like Piotr is suggesting.
>
I don't think Piotr is suggesting "always use push_back" but
"always use
push_back when it's valid" & I'd second this.

To show a simpler example:

std::unique_ptr<T> u(v);

std::unique_ptr<T> u = v;

I'd generally advocate for using copy init (the second form - it doesn't
copy in this case, it moves) over direct init (the first form) because it's
more legible - copy init can only execute implicit ctor operations, whereas
direct init can invoke implicit and explicit operations. So from a
readability perspective seeing the latter is easier than seing the former -
I know the operation is further constrained to simpler/less interesting
operations (eg: 'v' can't be a raw pointer in the second form, it
might be
(& then I have to worry about whether that's an ownership transfer
that's
intended), etc)

& push_back V emplace_back is the same choice - push_back means only
implicit ops are used and so it's more readable, emplace_back requires more
careful attention.

I think this is a reasonably good stylistic rule - but I'm happy leaving an
open to judgment for sure - there might be readability reasons that an
emplace_back may work better in some cases.

I'd think of this like "else after return" - we don't have any
enforcement,
sometimes we make judgment about it being better (for consistency -
sometimes there's a pattern such that else makes the code more readable),
we haven't gone back to do any cleanup of the codebase, but it's pretty
consistently applied/desired in code review.

All that said - sure, I'd like to have tools to help me get this right
(else after return and "prefer copy init style over direct init")
& make
that really easy to do.

- Dave

>
> That said, I’m not convinced the relative “ugliness" of emplace_back 
(I’m
> not sure what’s ugly there, I only see it as a habit to take)  vs push_back
> is enough to justify a set of rules to decide between the two, I’d be fine
> with “always using emplace_back” in the name of “simple rule is better when
> possible".
>
>
> So that isn't what I would necessarily propose for LLVM.
>
> For LLVM, my answer would be "either make tool do what we want, force
use
> of tool, or be consistent and obvious with what we want and then fix it if
> it leads to performance issue”
>
>
> +1
> Ultimately whatever we do is not practical if it can’t be done by a tool.
>
Not sure I follow this - we have lots of things we don't currently do with
a tool that's still part of our style & stuff we try to catch with code
review, etc. (else after return is my best example)

>
>
>
>
>
>
> There is also other reason - emplace_back can't take arguments of some
> types like bitfields, NULL, static member. Using emplace can also lead to
> memory leak in case of smart ptrs
> "
>
> auto *ptr = new int(1);v.push_back(std::unique_ptr<int>(ptr));
>
> This is because replacing it with emplace_back could cause a leak of this
> pointer if emplace_back would throw exception before emplacement (e.g.
> not enough memory to add new element).".
>
>
> This seems, IMHO, not a useful argument for LLVM since it does not try to
> use exception based error handling to recover.
>
>
> This, and also we should not write code like that (naked pointers, calling
> new), using make_unique<> for instance would prevent any such
situation.
> Passing a raw pointer to a container of unique_ptr can be flagged by a
tool.
>
Sure - but it generalizes further than that, it's just a nice simple
example.

std::vector<int> v(i);

'i' could be an int & this would make a vector of that many ints, or
it
could be another vector & this is making a copy, etc.

std::vector<int> v = i;

I know taht's not creating a vector of 'i' many ints.

Yeah, not a great example either - but the general idea applies, the latter
form is less powerful & so is easier to read because you don't have to
worry/think about it as much.

I think we mostly do this sort of thing out of habit anyway - but codifying
it (& if possible, tools to help) could be nice.


>
>
> —
> Mehdi
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
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> On Dec 29, 2016, at 11:20 AM, David Blaikie <dblaikie at gmail.com>
wrote:
> 
> 
> 
> On Thu, Dec 29, 2016 at 10:04 AM Mehdi Amini via cfe-dev <cfe-dev at
lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>> On Dec 29, 2016, at 5:54 AM, Daniel Berlin <dberlin at dberlin.org
<mailto:dberlin at dberlin.org>> wrote:
>> 
>> 
>> From yesterday discussion, Daniel Berlin proposed using emplace_back
everywhere to make code easier to maintain. I think it is valid argument, but I
believe it would reduce readability.
>> 
>> Just to be clear; I proposed not trying to switch uses back and forth
without real data, and to come to some agreement about what should be written in
the first place, preferably based on real data, and then, switching in some
organized fashion, not just randomly :)
>> 
>> IE Either have a clear coding standard and enforce it, or leave uses
alone one way or the other without some demonstration that it actually matters.
>> 
>> I would *personally* prefer to just use emplace_back everywhere, and
not try to switch between the two, without demonstration of real benefit.
>> 
>> (i'd also accept the other way, use push_back everywhere, and not
try to switch between the two, without demonstration of real benefit).
>> 
>> This preference is based on a simple truth: People suck at deciding
when to use one or the other. Yes, i know how to use it and when to use it. 
Lots of our types are cheaply movable, etc, so you probably won't see any
performance difference even when using them "right".  People have
enough style/etc issues without having to try to think hard about this.
> 
> I agree that “people suck at deciding”, me in the first place in the line,
and that’s why I like clear and good guideline and stick to it unless
"demonstration of real benefit”.
> I also think we can have clear guideline that are different from “always
use emplace_back” or “always use push_back”, like Piotr is suggesting.
> 
> I don't think Piotr is suggesting "always use push_back" but
"always use push_back when it's valid" & I'd second this.
Define “valid”? Just that it will compile?
> To show a simpler example:
> 
> std::unique_ptr<T> u(v);
> 
> std::unique_ptr<T> u = v;
> 
> I'd generally advocate for using copy init (the second form - it
doesn't copy in this case, it moves) over direct init (the first form)
because it's more legible - copy init can only execute implicit ctor
operations, whereas direct init can invoke implicit and explicit operations. So
from a readability perspective seeing the latter is easier than seing the former
- I know the operation is further constrained to simpler/less interesting
operations (eg: 'v' can't be a raw pointer in the second form, it
might be (& then I have to worry about whether that's an ownership
transfer that's intended), etc)
> 
> & push_back V emplace_back is the same choice - push_back means only
implicit ops are used and so it's more readable
I don’t see what’s more readable about “only implicit ctor”. Emplace is very
explicit that we intended to construct an object, I don’t understand what hurts
readability here?

> , emplace_back requires more careful attention.
> 
> I think this is a reasonably good stylistic rule - but I'm happy
leaving an open to judgment for sure - there might be readability reasons that
an emplace_back may work better in some cases.
> 
> I'd think of this like "else after return" - we don't
have any enforcement, sometimes we make judgment about it being better (for
consistency - sometimes there's a pattern such that else makes the code more
readable), we haven't gone back to do any cleanup of the codebase, but
it's pretty consistently applied/desired in code review.
I have a different impression: we are actively cleaning the codebase with
tidy-checks. For instance look for `git log --author=Zelenko`.
Another example is that a few months ago the entire LLDB codebase has been
formatted with clang-format, after marking the specific places where it was not
desirable (tables for instance) with comment to disable clang-format.

— 
Mehdi

> 
> All that said - sure, I'd like to have tools to help me get this right
(else after return and "prefer copy init style over direct init")
& make that really easy to do.
> 
> - Dave
>  
> 
> That said, I’m not convinced the relative “ugliness" of emplace_back 
(I’m not sure what’s ugly there, I only see it as a habit to take)  vs push_back
is enough to justify a set of rules to decide between the two, I’d be fine with
“always using emplace_back” in the name of “simple rule is better when
possible".
> 
> 
>> 
>> So that isn't what I would necessarily propose for LLVM.
>> 
> 
>> For LLVM, my answer would be "either make tool do what we want,
force use of tool, or be consistent and obvious with what we want and then fix
it if it leads to performance issue”
> 
> +1
> Ultimately whatever we do is not practical if it can’t be done by a tool.
> 
> Not sure I follow this - we have lots of things we don't currently do
with a tool that's still part of our style & stuff we try to catch with
code review, etc. (else after return is my best example)
>  
> 
> 
>> 
>> 
>>  
>> There is also other reason - emplace_back can't take arguments of
some types like bitfields, NULL, static member. Using emplace can also lead to
memory leak in case of smart ptrs
>> "
>> auto *ptr = new int(1);
>> v.push_back(std::unique_ptr<int>(ptr));
>> This is because replacing it with emplace_back could cause a leak of
this pointer if emplace_back would throw exception before emplacement (e.g. not
enough memory to add new element).".
>> 
>> 
>> This seems, IMHO, not a useful argument for LLVM since it does not try
to use exception based error handling to recover.
> 
> 
> This, and also we should not write code like that (naked pointers, calling
new), using make_unique<> for instance would prevent any such situation.
Passing a raw pointer to a container of unique_ptr can be flagged by a tool.
> 
> Sure - but it generalizes further than that, it's just a nice simple
example.
> 
> std::vector<int> v(i);
> 
> 'i' could be an int & this would make a vector of that many
ints, or it could be another vector & this is making a copy, etc.
> 
> std::vector<int> v = i;
> 
> I know taht's not creating a vector of 'i' many ints.
> 
> Yeah, not a great example either - but the general idea applies, the latter
form is less powerful & so is easier to read because you don't have to
worry/think about it as much.
> 
> I think we mostly do this sort of thing out of habit anyway - but codifying
it (& if possible, tools to help) could be nice.
> 
>  
> 
> 
> — 
> Mehdi
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev>
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