modernize-loop-convert and auto
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Peter Kasting
Dec 27, 2024, 3:08:41 PM12/27/24
to cxx
Hi folks,
We currently enable the clang-tidy modernize-loop-convert pass, which converts iterating loops to range-for:
for (size_t i = 0; I < vec.size(); ++I) {
do(vec[i]);
}
==>
for (auto& elem : vec) {
do(elem);
}
Note that this transform uses auto& as the element type. AFAICT, the transform always does this, and there's no option to have it use the actual deduced type instead (sometimes or always). As a result, running this transform and accepting its fixes could introduce usage of "auto" that's arguably beyond what the style guide allows.
We should decide what we want to do here:
- Disable the transform until/unless someone fixes upstream to give configurability around this.
- Continue to run this, but (somewhere?) explicitly direct people to fix up its results. (Note: If auto& deduces to a pointer type, the Chromium style plugin will complain, and people will have to fix up manually to use anyway.)
- Not worry about the issue -- on balance the tradeoffs are best as they are.
- Attempt to advocate upstream for more leeway around auto, or add a Chromium style exception, or something.
I vote for #3; this is too trivial to worry about and we shouldn't direct people to fight the tools. If someone wants to add #1 I'm not opposed, but I don't want to block.
However, when this came up in a recent code review, my reviewer (reasonably) expressed concern about introducing violations, and whether this would be used as ammunition against clang-tidy in general, or this pass specifically, or the idea of applying automatic fixes.
PK
Alex Cooper
Dec 27, 2024, 8:51:12 PM12/27/24
to Peter Kasting, cxx
I would argue that in most cases it's just as clear what the type is as in the iterating loop (albeit sometimes with the iterating loop one of the first things done is to assign to a reference), and as such it at least doesn't make the situation worse.
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Adam Rice
Dec 27, 2024, 11:29:34 PM12/27/24
to Alex Cooper, Peter Kasting, cxx
I've seen modernize-loop-convert try to "fix" loops that actually needed the loop counter in the past. Fortunately I've never seen the transformed code successfully compile afterwards, but I'd be concerned about running it on everything unless we have reason to believe it has improved.
There is also the weird case where dereferencing the iterator returns an rvalue, so `auto &` won't compile. Does it handle that correctly?
Last time I looked, Clang couldn't optimize base::span's iterators correctly when compiling for ARM32 Android. I think that it didn't inline deep enough to see that it was dealing with a contiguous range. But if people are writing loops that subscript spans, they're probably failing to optimise already, so a performance degradation is unlikely.
The style violation doesn't bother me nearly as much as leaving out `const` when the loop isn't intended to mutate the range. In an ideal world I'd like our code to model good practices, and `const auto&` is right more often than `auto &`.
Provided we have confidence that the transform will not change the meaning of the code, I favour option 3.
Avi Drissman
Dec 27, 2024, 11:36:35 PM12/27/24
to Adam Rice, Alex Cooper, Peter Kasting, cxx
I have never seen modernize-loop-convert try to "fix" loops that actually needed the loop counter. In general, while I do like its suggestion to modernize loops, I'm also a fan of humans taking that suggestion and applying their own judgement while doing so.
I'm happy with option 3.
Avi
Peter Kasting
Dec 28, 2024, 4:11:51 AM12/28/24
to cxx, Adam Rice, Peter Kasting, cxx, Alexander Cooper
On Friday, December 27, 2024 at 12:29:34 PM UTC-8 Adam Rice wrote:
I've seen modernize-loop-convert try to "fix" loops that actually needed the loop counter in the past. Fortunately I've never seen the transformed code successfully compile afterwards, but I'd be concerned about running it on everything unless we have reason to believe it has improved.
That sounds like its threshold was set to "risky", which can result in problematic conversions. We don't run with that though.
There is also the weird case where dereferencing the iterator returns an rvalue, so `auto &` won't compile. Does it handle that correctly?
Don't know.
I have run this transform over all violations in base/ and chrome/browser/ui (about 4000 files) and haven't run into any compile errors.
Last time I looked, Clang couldn't optimize base::span's iterators correctly when compiling for ARM32 Android. I think that it didn't inline deep enough to see that it was dealing with a contiguous range. But if people are writing loops that subscript spans, they're probably failing to optimise already, so a performance degradation is unlikely.
Yeah, I can't see how any counting loop will perform better in this specific case than a range-for.
The style violation doesn't bother me nearly as much as leaving out `const` when the loop isn't intended to mutate the range. In an ideal world I'd like our code to model good practices, and `const auto&` is right more often than `auto &`.
Agreed; I wish there was an option to use `const` where possible. I'm "option 3" with this one as well.
PK
Victor Vasiliev
Dec 28, 2024, 4:12:22 AM12/28/24
to Peter Kasting, cxx
Does it infer or omit the const modifier for auto?
I'm mildly biased towards 1, mostly because I'm not sure range-based loops are generally a big improvement over indexing explicitly.
--
Peter Kasting
Dec 28, 2024, 4:15:05 AM12/28/24
to cxx, Victor Vasiliev, cxx, Peter Kasting
On Friday, December 27, 2024 at 5:12:22 PM UTC-8 Victor Vasiliev wrote:
Does it infer or omit the const modifier for auto?
Omit, AFAICT. This won't actually change the deduced type, just how it reads.
I'm mildly biased towards 1, mostly because I'm not sure range-based loops are generally a big improvement over indexing explicitly.
For readability, safety, perf, ...?
PK
Roland McGrath
Dec 28, 2024, 4:26:43 AM12/28/24
to Peter Kasting, cxx, Victor Vasiliev
In fact, `for (const auto& elt : ...)` will produce a different type than `for (auto& elt : ...)` in many cases. That is, if the container/range's iterators don't already return const references, then with `auto&` then `elt` won't be `const` within the loop body. But when doing a loop intended to have read-only semantics, you probably want it to be so that any non-`const` method calls made in that loop get caught and explicitly thought through to make sure it's intended for the loop to possibly start having side effects it didn't before.
Really the best case scenario is that a clang-tidy check would see that `elt` could be `const` based on how it's actually being used, and so will suggest making it so, orthogonal to whether it was a human or another clang-tidy transformation that wrote `auto&` in the first place.
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Peter Kasting
Dec 28, 2024, 4:46:58 AM12/28/24
to Roland McGrath, cxx, Victor Vasiliev
On Fri, Dec 27, 2024, 5:26 PM Roland McGrath <mcgr...@chromium.org> wrote:
In fact, `for (const auto& elt : ...)` will produce a different type than `for (auto& elt : ...)` in many cases. That is, if the container/range's iterators don't already return const references, then with `auto&` then `elt` won't be `const` within the loop body.
Yes; this isn't about deduction, though, it's the same as any other not-locally-const variable. Note that the style guide does not require local const and most chrome code doesn't use it, so much as I personally like it, I think it would be unfair to treat this as a strike against this transform.
But when doing a loop intended to have read-only semantics, you probably want it to be so that any non-`const` method calls made in that loop get caught and explicitly thought through to make sure it's intended for the loop to possibly start having side effects it didn't before.
I don't think we're typically going to add unsafety here, given that operator[] normally returns non const refs.
Really the best case scenario is that a clang-tidy check would see that `elt` could be `const` based on how it's actually being used, and so will suggest making it so, orthogonal to whether it was a human or another clang-tidy transformation that wrote `auto&` in the first place.
That's probably possible. I think there might even be a transform (that we don't use) that tries to make more things local consts. I could be misremembering, though.
PK
Adam Rice
Dec 30, 2024, 12:56:50 AM12/30/24
to Victor Vasiliev, Peter Kasting, cxx
Part of the context is that we're trying to reduce usage of indexing overall with the memory safety effort. AFAIK most of the containers we use now do bounds checks in their subscript operators but it's not obvious from local inspection whether a subscript is going to be bounds-checked or not. And humans have found many ways to get indexed loops subtly wrong.
Peter Boström
Dec 30, 2024, 4:00:56 AM12/30/24
to Adam Rice, Victor Vasiliev, Peter Kasting, cxx
From the style guide we have "Use type deduction only if it makes the code clearer to readers who aren't familiar with the project, or if it makes the code safer. Do not use it merely to avoid the inconvenience of writing an explicit type."
I think this falls under "Do not use it merely to avoid the inconvenience of writing an explicit type", but maybe I'm reading/language-lawyering this wrongly. I don't think that auto makes the code clearer here, you could argue that it makes the code safer if it makes you apply this transformation at all. I.e. for (auto& foo : foos) is safer than for (size_t i ...) to me, but not safer than for (const Foo& foo : foos) if the latter is on the table.
I think this falls under "Do not use it merely to avoid the inconvenience of writing an explicit type", but maybe I'm reading/language-lawyering this wrongly. I don't think that auto makes the code clearer here, you could argue that it makes the code safer if it makes you apply this transformation at all. I.e. for (auto& foo : foos) is safer than for (size_t i ...) to me, but not safer than for (const Foo& foo : foos) if the latter is on the table.
Do you have any idea whether "ugh I'll just go through all the auto&s" is feasible or too toil heavy? If we're in a good steady state I'd be comfortable with people manually having to fix auto& instances (can we have this as a violation without autofixing it when supplying -fix?) when adding a handful of violations, less so with requiring you to fix thousands. Index-based for loops seem worse to me than auto& in loops if that's the real tradeoff we're making here.
Peter Kasting
Dec 30, 2024, 4:25:50 PM12/30/24
to cxx, Peter Boström, Victor Vasiliev, Peter Kasting, cxx, Adam Rice
On Sunday, December 29, 2024 at 5:00:56 PM UTC-8 Peter Boström wrote:
I don't think that auto makes the code clearer here, you could argue that it makes the code safer if it makes you apply this transformation at all. I.e. for (auto& foo : foos) is safer than for (size_t i ...) to me, but not safer than for (const Foo& foo : foos) if the latter is on the table.
No argument that in at least many cases, `const Foo&` might be a better type than `const auto&`, which in turn might be better than `auto&`.
I read your comments as saying that option (1) goes too far (better to have an imperfect transform than no transform), but you're not really comfortable with (3) and would rather have (2).
For context, I am thinking of this not only in the context of people running -fix locally, but in Gerrit/Tricium, where this will propose fixes and people can potentially auto-apply them. If we don't do (1), we're going to have this kind of suggestion with a well-lit path to Just Do It. I'm not sure (2) will be effective there even if I apply it locally to any fix CLs I personally write.
Do you have any idea whether "ugh I'll just go through all the auto&s" is feasible or too toil heavy?
It's borderline. It's probably ~500 or so files' worth of autofixes, maybe figure about 100 loop transforms to manually fix, assuming I have a script that finds them (which shouldn't be too hard to write). I would do it; I would grumble :)
But again, that only addresses this in the context of the CLs I'm writing at the moment; I'm more concerned about future use, especially in Gerrit.
PK
Peter Boström
Dec 31, 2024, 4:11:40 AM12/31/24
to Peter Kasting, cxx, Victor Vasiliev, Adam Rice
Is there any option to expose modernize-loop-convert in gerrit but disable the autofix option for that pass? I think the warning has more value than the autofix, but that's probably because I'm less comfortable putting auto everywhere (and I guess to an extent going against the styleguide).
Greg Thompson
Dec 31, 2024, 3:42:33 PM12/31/24
to Peter Boström, Adam Rice, Victor Vasiliev, Peter Kasting, cxx
IMO, this:
for (auto& item : container) {
item.UseIt();
is no more opaque than this:
for (size_t i = 0; i < container.size(); ++i) {
container[i].UseIt();
the latter tells the reader no more about the type of the thing being used. I prefer the former, and think auto is perfectly fine here.
Peter Boström
Jan 1, 2025, 9:55:31 PMJan 1
to Greg Thompson, Adam Rice, Victor Vasiliev, Peter Kasting, cxx
That's fair, since this transform is not a regression in the amount of types spelled out here I yield completely. :)
Peter Boström
Jan 2, 2025, 2:07:04 AMJan 2
to Greg Thompson, Adam Rice, Victor Vasiliev, Peter Kasting, cxx
Unless there are more or more loud objections to applying this I'll approve crrev.com/c/6120760 clang-format fixups in c/b/ui and this will possibly resume work in a lot of other directories.
If you really don't want this transform to happen, shout louder very soon (let's say 24h). Greg's point satisfies the style question sufficiently for me and I think range-based for loops are not controversial in src/ at this point. :)
Greg Thompson
Jan 2, 2025, 3:54:06 PMJan 2
to Peter Boström, Adam Rice, Victor Vasiliev, Peter Kasting, cxx
Another thing in favor of using range-based for loops is that `container.end()` will only be evaluated once. This alone can make a measurable difference for containers where `.end()` isn't free. In other cases, use of std algorithms can be even more efficient, as they may have magic powers to to bounds checking only during initialization rather than on each iteration. (I know that MS's implementation of the STL has historically done this. I'm not certain about the one we use.)
David Baron
Jan 3, 2025, 12:22:56 AMJan 3
to Greg Thompson, Peter Boström, Adam Rice, Victor Vasiliev, Peter Kasting, cxx
I'll just add a +1 to what seems like the emerging consensus here (which I think is Peter's original option #3). I think the clang-tidy pass here is generally an improvement to the code, fixing the results of that pass to avoid auto is also generally an improvement, and we shouldn't block the first improvement on also doing the second improvement (even though doing the first one alone does technically introduce style guide violations). I saw only a single example in the CL under discussion where there was a loss of explicit type information from this pass.
-David
-David
Peter Boström
Jan 3, 2025, 3:20:22 AMJan 3
to David Baron, Greg Thompson, Adam Rice, Victor Vasiliev, Peter Kasting, cxx
I've lgtm'd the change. :)
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