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Update overview; move 'choosing approach' section after it

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Peter Dimov
2020-01-21 04:26:38 +02:00
parent 7eaa25ec2a
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2
doc/endian.adoc
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@ -18,10 +18,10 @@ Beman Dawes
include::endian/overview.adoc[]
include::endian/changelog.adoc[]
include::endian/choosing_approach.adoc[]
include::endian/conversion.adoc[]
include::endian/buffers.adoc[]
include::endian/arithmetic.adoc[]
include::endian/choosing_approach.adoc[]
include::endian/history.adoc[]
:leveloffset: -1

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doc/endian/choosing_approach.adoc
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@ -6,19 +6,15 @@ Distributed under the Boost Software License, Version 1.0.
////
[#choosing]
# Choosing Approach
# Choosing between Conversion Functions, Buffer Types, and Arithmetic Types
:idprefix: choosing_
## Introduction
Deciding which is the best endianness approach (conversion functions, buffer
NOTE: Deciding which is the best endianness approach (conversion functions, buffer
types, or arithmetic types) for a particular application involves complex
engineering trade-offs. It is hard to assess those trade-offs without some
understanding of the different interfaces, so you might want to read the
<<conversion,conversion functions>>, <<buffers,buffer types>>, and
<<arithmetic,arithmetic types>> pages before diving into this page.
## Choosing between conversion functions, buffer types, and arithmetic types
<<arithmetic,arithmetic types>> pages before proceeding.
The best approach to endianness for a particular application depends on the
interaction between the application's needs and the characteristics of each of
@ -30,7 +26,7 @@ invest the time to study engineering trade-offs, use
maintain. Use the _<<choosing_anticipating_need,anticipating need>>_ design
pattern locally around performance hot spots like lengthy loops, if needed.
### Background
## Background
A dealing with endianness usually implies a program portability or a data
portability requirement, and often both. That means real programs dealing with
@ -39,13 +35,13 @@ written as multiple functions spread across multiple translation units. They
would involve interfaces that can not be altered as they are supplied by
third-parties or the standard library.
### Characteristics
## Characteristics
The characteristics that differentiate the three approaches to endianness are
the endianness invariants, conversion explicitness, arithmetic operations, sizes
available, and alignment requirements.
#### Endianness invariants
### Endianness invariants
*Endian conversion functions* use objects of the ordinary {cpp} arithmetic types
like `int` or `unsigned short` to hold values. That breaks the implicit
@ -121,7 +117,7 @@ write(data);
A later maintainer can add `third_party::func(data.v3)` and it will just-work.
#### Conversion explicitness
### Conversion explicitness
*Endian conversion functions* and *buffer types* never perform implicit
conversions. This gives users explicit control of when conversion occurs, and
@ -131,7 +127,7 @@ may help avoid unnecessary conversions.
very easy to use, but can result in unnecessary conversions. Failure to hoist
conversions out of inner loops can bring a performance penalty.
#### Arithmetic operations
### Arithmetic operations
*Endian conversion functions* do not supply arithmetic operations, but this is
not a concern since this approach uses ordinary {cpp} arithmetic types to hold
@ -153,7 +149,7 @@ That's sufficient for many applications.
integers. For an application where memory use or I/O speed is the limiting
factor, using sizes tailored to application needs can be useful.
#### Alignments
### Alignments
*Endianness conversion functions* only support aligned integer and
floating-point types. That's sufficient for most applications.
@ -180,7 +176,7 @@ struct S {
};
```
### Design patterns
## Design patterns
Applications often traffic in endian data as records or packets containing
multiple endian data elements. For simplicity, we will just call them records.
@ -189,7 +185,7 @@ If desired endianness differs from native endianness, a conversion has to be
performed. When should that conversion occur? Three design patterns have
evolved.
#### Convert only as needed (i.e. lazy)
### Convert only as needed (i.e. lazy)
This pattern defers conversion to the point in the code where the data
element is actually used.
@ -198,7 +194,7 @@ This pattern is appropriate when which endian element is actually used varies
greatly according to record content or other circumstances
[#choosing_anticipating_need]
#### Convert in anticipation of need
### Convert in anticipation of need
This pattern performs conversion to native endianness in anticipation of use,
such as immediately after reading records. If needed, conversion to the output
@ -213,7 +209,7 @@ from native to the desired output endianness.
This pattern is appropriate when all endian elements in a record are typically
used regardless of record content or other circumstances.
#### Convert only as needed, except locally in anticipation of need
### Convert only as needed, except locally in anticipation of need
This pattern in general defers conversion but for specific local needs does
anticipatory conversion. Although particularly appropriate when coupled with the
@ -264,9 +260,9 @@ cost might be significant if the loop is repeated enough times. On the other
hand, the program may be so dominated by I/O time that even a lengthy loop will
be immaterial.
### Use case examples
## Use case examples
#### Porting endian unaware codebase
### Porting endian unaware codebase
An existing codebase runs on big endian systems. It does not currently deal
with endianness. The codebase needs to be modified so it can run on little
@ -279,7 +275,7 @@ needs. A relatively small number of header files dealing with binary I/O layouts
need to change types. For example, `short` or `int16_t` would change to
`big_int16_t`. No changes are required for `.cpp` files.
#### Porting endian aware codebase
### Porting endian aware codebase
An existing codebase runs on little-endian Linux systems. It already deals with
endianness via
@ -293,7 +289,7 @@ just mechanically changes the calls to `htobe32`, etc. to
`boost::endian::native_to_big`, etc. and replaces `<endian.h>` with
`<boost/endian/conversion.hpp>`.
#### Reliability and arithmetic-speed
### Reliability and arithmetic-speed
A new, complex, multi-threaded application is to be developed that must run
on little endian machines, but do big endian network I/O. The developers believe
@ -304,7 +300,7 @@ slow conversions if full-blown endian arithmetic types are used.
The <<buffers,endian buffers>> approach is made-to-order for this use case.
#### Reliability and ease-of-use
### Reliability and ease-of-use
A new, complex, multi-threaded application is to be developed that must run on
little endian machines, but do big endian network I/O. The developers believe

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doc/endian/overview.adoc
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@ -83,7 +83,8 @@ Boost.Endian provides three different approaches to dealing with endianness. All
three approaches support integers and user-define types (UDTs).
Each approach has a long history of successful use, and each approach has use
cases where it is preferred to the other approaches.
cases where it is preferred to the other approaches. See
<<choosing,Choosing between Conversion Functions, Buffer Types, and Arithmetic Types>>.
<<conversion,Endian conversion functions>>::
The application uses the built-in integer types to hold values, and calls the
@ -113,10 +114,6 @@ Boost Endian is a header-only library. {cpp}11 features affecting interfaces,
such as `noexcept`, are used only if available. See
<<overview_cpp03_support,{cpp}03 support for {cpp}11 features>> for details.
## Choosing between conversion functions, buffer types, and arithmetic types
This section has been moved to its own <<choosing,Choosing the Approach>> page.
[#overview_intrinsics]
## Built-in support for Intrinsics
@ -260,6 +257,28 @@ Iterations: 10'000'000'000, Intrinsics: `<cstdlib>` `_byteswap_ushort`, etc.
|64-bit aligned little endian |3.35 s |2.73 s
|===
[#overview_cpp03_support]
## {cpp}03 support for {cpp}11 features
[%header,cols=2*]
|===
|{cpp}11 Feature
|Action with {cpp}03 Compilers
|Scoped enums
|Uses header
http://www.boost.org/libs/core/doc/html/core/scoped_enum.html[boost/core/scoped_enum.hpp]
to emulate {cpp}11 scoped enums.
|`noexcept`
|Uses `BOOST_NOEXCEPT` macro, which is defined as null for compilers not
supporting this {cpp}11 feature.
|{cpp}11 PODs
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2342.htm[N2342])
|Takes advantage of {cpp}03 compilers that relax {cpp}03 POD rules, but see
Limitations <<buffers_limitations,here>> and <<arithmetic_limitations,here>>.
Also see macros for explicit POD control <<buffers_compilation,here>> and
<<arithmetic_compilation,here>>
|===
[#overview_faq]
## Overall FAQ
@ -326,11 +345,11 @@ the same code being generated for either types.
What are the limitations of integer support?::
Tests have only been performed on machines that use two's complement
arithmetic. The Endian conversion functions only support 16, 32, and 64-bit
arithmetic. The Endian conversion functions only support 8, 16, 32, and 64-bit
aligned integers. The endian types only support 8, 16, 24, 32, 40, 48, 56, and
64-bit unaligned integers, and 8, 16, 32, and 64-bit aligned integers.
Why is there no floating point support?::
Is there floating point support?::
An attempt was made to support four-byte ``float``s and eight-byte
``double``s, limited to
http://en.wikipedia.org/wiki/IEEE_floating_point[IEEE 754] (also known as
@ -338,29 +357,9 @@ ISO/IEC/IEEE 60559) floating point and further limited to systems where floating
point endianness does not differ from integer endianness. Even with those
limitations, support for floating point types was not reliable and was removed.
For example, simply reversing the endianness of a floating point number can
result in a signaling-NAN. For all practical purposes, binary serialization and
endianness for integers are one and the same problem. That is not true for
floating point numbers, so binary serialization interfaces and formats for
floating point does not fit well in an endian-based library.
[#overview_cpp03_support]
## {cpp}03 support for {cpp}11 features
[%header,cols=2*]
|===
|{cpp}11 Feature
|Action with {cpp}03 Compilers
|Scoped enums
|Uses header
http://www.boost.org/libs/core/doc/html/core/scoped_enum.html[boost/core/scoped_enum.hpp]
to emulate {cpp}11 scoped enums.
|`noexcept`
|Uses `BOOST_NOEXCEPT` macro, which is defined as null for compilers not
supporting this {cpp}11 feature.
|{cpp}11 PODs
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2342.htm[N2342])
|Takes advantage of {cpp}03 compilers that relax {cpp}03 POD rules, but see
Limitations <<buffers_limitations,here>> and <<arithmetic_limitations,here>>.
Also see macros for explicit POD control <<buffers_compilation,here>> and
<<arithmetic_compilation,here>>
|===
result in a signaling-NAN.
+
Support for `float` and `double` has since been reinstated for `endian_buffer`
and `endian_arithmetic`. The conversion functions still do not support floating
point due to the above issues; reversing the bytes of a floating point number
does not necessarily produce another valid floating point number.