Add the `formatter_len_hint` method to Show and use it in `to_string` #16544

pczarn · 2014-08-17T00:36:20Z

I'm trying to balance potential code bloat and precision of the upper bound.

Can size_hint default to 0 instead of None?
How costly is occasional inaccuracy of the upper bound?
How to implement size_hint for floats? I've yet to think about it.
Is size_hint also that useful for the general case of format!, as implemented in this PR, not only to_string?

~~UFCS would make this addition prettier.~~

Examples

18446744073709551615u64.to_string().byte_capacity() // => 32
123u.to_string().byte_capacity() // => 3
12.34f32.to_string().byte_capacity() // => 9
"foobar".to_string().byte_capacity() // => 6

Fixes #16415

huonw · 2014-08-17T00:41:09Z

src/libcore/fmt/mod.rs

@@ -148,113 +171,159 @@ impl<'a> Show for Arguments<'a> {
 pub trait Show {
    /// Formats the value using the given formatter.
    fn fmt(&self, &mut Formatter) -> Result;
+
+    /// Returns an upper bound on the size of the formatted string.


I think this should actually be a lower bound, or just a loosely defined "conservative estimate" (since a true upper bound can be much larger than the real value, leading to the same overallocation problems). In this case, returning 0 by default makes sense.

alexcrichton · 2014-08-17T02:55:30Z

In general, can you provide data to support a change such as this? The issue only claims that it may be an issue, I don't see any concrete numbers one way or the other. I'd like to see some analysis which compares the heap usage before and afterwards of projects like rustc (and perhaps servo) to see if a change such as this reaps the theoretical benefits.

In the past the code size generated by format! has often been the source of many problems in both servo, rustc, and benchmarks. Could you do some analysis to see what the impact is of passing size hints for all invocations of format_args!. This is inflating the amount of code generated for each invocation, as well as stack space used to store the local slice, so I'm a little worried if it's worth the benefits.

huonw · 2014-08-17T03:00:58Z

as well as stack space used to store the local slice

(This can be addressed by storing a single pointer to a struct/tuple/vtable containing the two function pointers.)

huonw · 2014-08-17T03:10:59Z

src/libcore/fmt/num.rs

+                let log2base = 1 << $log2log2base;
+
+                // Get the number of digits in the target base.
+                let binary_digits = width - (num | log2base).leading_zeros() as uint;


Why this |?

To adjust for the fact that zero has one digit, so that

there are at most width - log2base - 1 leading zeros

binary_digits is at least log2base + 1

binary_digits / log2base is never zero

pnkfelix · 2014-08-17T16:57:54Z

Note also that instead of changing the definition of format!, you could consider just changing the definition of to_string (while leaving the default method in Show itself). Of course, the choice here largely depends on the tradeoff of adding more code to format! (and thus increasing its code size) versus getting the broader benefits of having format! produce a more precisely sized String. I mention this largely as a way to try to semi-address @alexcrichton 's concerns. (Though either way, it would still be best to have concrete numbers presented as well, and logged in the issue or PR.)

pczarn · 2014-08-17T17:36:18Z

@pnkfelix, I agree.

The change to format! increases the size of compiled librustc from 22.054 to 23.429MB, even though format! is practically never called during a successful compilation by default. I guess this bloat is so drastic due to a large number of debug!s and error messages.

I've just made these measurements.

before
wc -c x86_64-unknown-linux-gnu/stage2/lib/*
   67363 x86_64-unknown-linux-gnu/stage2/lib/libarena-4e7c5e5c.so
  160066 x86_64-unknown-linux-gnu/stage2/lib/libdebug-4e7c5e5c.so
  127453 x86_64-unknown-linux-gnu/stage2/lib/libflate-4e7c5e5c.so
   75365 x86_64-unknown-linux-gnu/stage2/lib/libfmt_macros-4e7c5e5c.so
  181903 x86_64-unknown-linux-gnu/stage2/lib/libgetopts-4e7c5e5c.so
   63877 x86_64-unknown-linux-gnu/stage2/lib/libgraphviz-4e7c5e5c.so
   93812 x86_64-unknown-linux-gnu/stage2/lib/liblog-4e7c5e5c.so
  579422 x86_64-unknown-linux-gnu/stage2/lib/libnative-4e7c5e5c.so
  281492 x86_64-unknown-linux-gnu/stage2/lib/librbml-4e7c5e5c.so
22054464 x86_64-unknown-linux-gnu/stage2/lib/librustc-4e7c5e5c.so
  400373 x86_64-unknown-linux-gnu/stage2/lib/librustc_back-4e7c5e5c.so
25064481 x86_64-unknown-linux-gnu/stage2/lib/librustc_llvm-4e7c5e5c.so
 2217782 x86_64-unknown-linux-gnu/stage2/lib/librustrt-4e7c5e5c.so
  942856 x86_64-unknown-linux-gnu/stage2/lib/libserialize-4e7c5e5c.so
 2742769 x86_64-unknown-linux-gnu/stage2/lib/libstd-4e7c5e5c.so
  868949 x86_64-unknown-linux-gnu/stage2/lib/libsync-4e7c5e5c.so
 7834446 x86_64-unknown-linux-gnu/stage2/lib/libsyntax-4e7c5e5c.so
  403999 x86_64-unknown-linux-gnu/stage2/lib/libterm-4e7c5e5c.so
  130546 x86_64-unknown-linux-gnu/stage2/lib/libtime-4e7c5e5c.so

after
wc -c x86_64-unknown-linux-gnu/copy\ of\ stage2/lib/*
   72573 x86_64-unknown-linux-gnu/copy of stage2/lib/libarena-4e7c5e5c.so
  165192 x86_64-unknown-linux-gnu/copy of stage2/lib/libdebug-4e7c5e5c.so
  132902 x86_64-unknown-linux-gnu/copy of stage2/lib/libflate-4e7c5e5c.so
   76314 x86_64-unknown-linux-gnu/copy of stage2/lib/libfmt_macros-4e7c5e5c.so
  182738 x86_64-unknown-linux-gnu/copy of stage2/lib/libgetopts-4e7c5e5c.so
   63342 x86_64-unknown-linux-gnu/copy of stage2/lib/libgraphviz-4e7c5e5c.so
   94202 x86_64-unknown-linux-gnu/copy of stage2/lib/liblog-4e7c5e5c.so
  598900 x86_64-unknown-linux-gnu/copy of stage2/lib/libnative-4e7c5e5c.so
  299335 x86_64-unknown-linux-gnu/copy of stage2/lib/librbml-4e7c5e5c.so
23429064 x86_64-unknown-linux-gnu/copy of stage2/lib/librustc-4e7c5e5c.so
  415548 x86_64-unknown-linux-gnu/copy of stage2/lib/librustc_back-4e7c5e5c.so
25064674 x86_64-unknown-linux-gnu/copy of stage2/lib/librustc_llvm-4e7c5e5c.so
 2271760 x86_64-unknown-linux-gnu/copy of stage2/lib/librustrt-4e7c5e5c.so
  977342 x86_64-unknown-linux-gnu/copy of stage2/lib/libserialize-4e7c5e5c.so
 2824152 x86_64-unknown-linux-gnu/copy of stage2/lib/libstd-4e7c5e5c.so
  914657 x86_64-unknown-linux-gnu/copy of stage2/lib/libsync-4e7c5e5c.so
 8043605 x86_64-unknown-linux-gnu/copy of stage2/lib/libsyntax-4e7c5e5c.so
  413601 x86_64-unknown-linux-gnu/copy of stage2/lib/libterm-4e7c5e5c.so
  146047 x86_64-unknown-linux-gnu/copy of stage2/lib/libtime-4e7c5e5c.so

pczarn · 2014-08-17T18:08:03Z

Here's why size_hint is not useful for cases other than to_string: every instance of format! is easily replaceable with a write! to a buffer if it's inside a tight loop and/or causes higher memory usage.

I forgot about it before.

lilyball · 2014-08-20T19:11:36Z

size_hint() is an awfully generic name to attach to all Show instances. I'd rather see something like

pub trait Show {
    // ...
    fn size_hint(val: &Self) -> Option<uint> { None }
}

thus turning usage into e.g.

let mut output = io::MemWriter::with_capacity(Show::size_hint(self).unwrap_or(128));

Although I am unsure how UFCS affects static trait methods.

lilyball · 2014-08-20T19:12:46Z

If UFCS makes the static method behave the same as a regular method (I know it does for regular impls but not sure how that works for traits) then instead you could have a function fmt::size_hint() that does this.

~~The idea here is that I should be able to implement size_hint() on my own type that also implements Show without causing an ambiguity error when I try and call size_hint().~~

Edit: What am I thinking, third-party types can't implement fmt::size_hint().

pczarn · 2014-08-20T19:33:35Z

This method has to work for many types. I agree that a method used once internally shouldn't have a generic name. Well, at least iterators don't implement Show.

How about changing it to fmt_size_hint()?

eddyb · 2014-09-13T05:29:50Z

@pczarn @kballard keep in mind that Show is not in the prelude, and changing it affects much less code than actually would otherwise.

pczarn · 2014-09-13T20:04:55Z

Renamed to formatter_len_hint.

pczarn · 2014-09-22T09:50:12Z

Updated with a simplified implementation for floats. Added two tests.

pczarn · 2014-09-29T20:08:38Z

This is good to go, right? I have a crude memory chart from stage1.

Before:magenta, after:blue, the peak is smaller by 4MB.

Implements `formatter_len_hint` for several simple types. Length of the formatted string is approximated for floats and integers.

alexcrichton · 2014-10-01T04:56:28Z

Note that there has been talk of separating Show and ToString, which may require tweaks to this patch or a whole different strategy altogether.

Committing to this style of API is taking us pretty hard down the road of using Show for transforming into a stream of bytes, which is a strong commitment to consider when Show is such a commonly used trait.

I'm curious, @aturon, do you have an opinion on this? This is something that can always be added backwards-compatibly later on, and I'd almost rather consider the stabilization of Show before considering optimizations like this (for what seems like only a small win?)

eddyb · 2014-10-01T05:23:24Z

I believe the win is bigger, but most of it is already in (using into_string in the interner).

aturon · 2014-10-01T15:30:25Z

@pczarn, thanks for taking this on!

I agree with @alexcrichton that, while this seems like a reasonable extension, it seems wise to resolve the basic question about the role of Show and its connection to to_string before adding this functionality. I plan to work on an RFC on this topic very soon and can incorporate this idea as part of the proposal -- maybe best to wait on landing this PR until the community has had a chance to evaluate the overall design space. (And, as @alexcrichton says, depending on the design it may be that a completely different strategy for solving this problem is needed.)

I'll try to get a draft together in the next day or two, and ping you for feedback before posting it.

alexcrichton · 2014-10-24T16:28:22Z

Closing to help clear out the queue, @aturon do you have an update on the RFC you were planning to write?

SimonSapin · 2014-12-11T10:11:12Z

Iterator::size_hint has both a lower-bound (which can default to zero when unknown) and an optional upper bound, rather than a single number. Would it make sense to do something similar here? (Vec::from_iter ignore the upper bound and takes the lower bound as the initial capacity.)

…ykril Add completions to show only traits in trait `impl` statement This is prerequisite PR for adding the assist mentioned in rust-lang#12500 P.S: If wanted, I will add the implementation of the assist in this PR as well.

huonw reviewed Aug 17, 2014
View reviewed changes

pczarn changed the title ~~(WIP) Add the size_hint method to Show as a default method~~ (WIP) Add the size_hint method to Show as a default method and use it in to_string Aug 17, 2014

pczarn changed the title ~~(WIP) Add the size_hint method to Show as a default method and use it in to_string~~ Add the size_hint method to Show as a default method and use it in to_string Aug 19, 2014

retep998 mentioned this pull request Sep 13, 2014

Improve memory usage of libsyntax #17223

Merged

pczarn force-pushed the formatting_size_hint branch from 91ccbd5 to fbb2981 Compare September 21, 2014 12:26

pczarn force-pushed the formatting_size_hint branch from 6cfef4e to 0e2ba80 Compare September 29, 2014 20:10

pczarn changed the title ~~Add the size_hint method to Show as a default method and use it in to_string~~ Add the formatter_len_hint method to Show and use it in to_string Sep 30, 2014

Add the formatter_len_hint method to Show and use it in to_string

fbc04e6

Implements `formatter_len_hint` for several simple types. Length of the formatted string is approximated for floats and integers.

pczarn force-pushed the formatting_size_hint branch from 0e2ba80 to fbc04e6 Compare September 30, 2014 20:07

alexcrichton closed this Oct 24, 2014

aturon mentioned this pull request Dec 10, 2014

RFC: Split Show into String and Show rust-lang/rfcs#504

Merged

alexcrichton mentioned this pull request Dec 16, 2014

std: Stabilize the std::str module #19741

Merged

pczarn mentioned this pull request Apr 12, 2015

Add fmt size_hints rust-lang/rfcs#583

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Add the `formatter_len_hint` method to Show and use it in `to_string` #16544

Add the `formatter_len_hint` method to Show and use it in `to_string` #16544

pczarn commented Aug 17, 2014

huonw Aug 17, 2014

alexcrichton commented Aug 17, 2014

huonw commented Aug 17, 2014

huonw Aug 17, 2014

pczarn Aug 17, 2014

pnkfelix commented Aug 17, 2014

pczarn commented Aug 17, 2014

pczarn commented Aug 17, 2014

lilyball commented Aug 20, 2014

lilyball commented Aug 20, 2014

pczarn commented Aug 20, 2014

eddyb commented Sep 13, 2014

pczarn commented Sep 13, 2014

pczarn commented Sep 22, 2014

pczarn commented Sep 29, 2014

alexcrichton commented Oct 1, 2014

eddyb commented Oct 1, 2014

aturon commented Oct 1, 2014

alexcrichton commented Oct 24, 2014

SimonSapin commented Dec 11, 2014

Add the formatter_len_hint method to Show and use it in to_string #16544

Add the formatter_len_hint method to Show and use it in to_string #16544

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pczarn commented Aug 17, 2014

Examples

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alexcrichton commented Oct 1, 2014

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SimonSapin commented Dec 11, 2014

Add the `formatter_len_hint` method to Show and use it in `to_string` #16544

Add the `formatter_len_hint` method to Show and use it in `to_string` #16544