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doc: update Go memory model

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Following discussion on #47141, make the following changes:

- Document Go's overall approach.
- Document that multiword races can cause crashes.
- Document happens-before for runtime.SetFinalizer.
- Document (or link to) happens-before for more sync types.
- Document happens-before for sync/atomic.
- Document disallowed compiler optimizations.

See also https://research.swtch.com/gomm for background.

Fixes #50859.

Change-Id: I17d837756a77f4d8569f263489c2c45de20a8778
Reviewed-on: https://go-review.googlesource.com/c/go/+/381315
Reviewed-by: Ian Lance Taylor <iant@google.com>
This commit is contained in:
Russ Cox 2022-01-26 16:42:05 -05:00
parent fc66cae490
commit 865911424d
1 changed files with 492 additions and 95 deletions
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@ -8,12 +8,9 @@
p.rule { p.rule {
font-style: italic; font-style: italic;
} }
span.event {
font-style: italic;
}
</style> </style>
<h2>Introduction</h2> <h2 id="introduction">Introduction</h2>
<p> <p>
The Go memory model specifies the conditions under which The Go memory model specifies the conditions under which
@ -22,7 +19,7 @@ observe values produced by writes to the same variable in a different goroutine.
</p> </p>
<h2>Advice</h2> <h3 id="advice">Advice</h3>
<p> <p>
Programs that modify data being simultaneously accessed by multiple goroutines Programs that modify data being simultaneously accessed by multiple goroutines
@ -44,90 +41,237 @@ you are being too clever.
Don't be clever. Don't be clever.
</p> </p>
<h2>Happens Before</h2> <h3 id="overview">Informal Overview</h3>
<p> <p>
Within a single goroutine, reads and writes must behave Go approaches its memory model in much the same way as the rest of the language,
as if they executed in the order specified by the program. aiming to keep the semantics simple, understandable, and useful.
That is, compilers and processors may reorder the reads and writes This section gives a general overview of the approach and should suffice for most programmers.
executed within a single goroutine only when the reordering The memory model is specified more formally in the next section.
does not change the behavior within that goroutine
as defined by the language specification.
Because of this reordering, the execution order observed
by one goroutine may differ from the order perceived
by another. For example, if one goroutine
executes <code>a = 1; b = 2;</code>, another might observe
the updated value of <code>b</code> before the updated value of <code>a</code>.
</p> </p>
<p> <p>
To specify the requirements of reads and writes, we define A <em>data race</em> is defined as
<i>happens before</i>, a partial order on the execution a write to a memory location happening concurrently with another read or write to that same location,
of memory operations in a Go program. If event <span class="event">e<sub>1</sub></span> happens unless all the accesses involved are atomic data accesses as provided by the <code>sync/atomic</code> package.
before event <span class="event">e<sub>2</sub></span>, then we say that <span class="event">e<sub>2</sub></span> happens after <span class="event">e<sub>1</sub></span>. As noted already, programmers are strongly encouraged to use appropriate synchronization
Also, if <span class="event">e<sub>1</sub></span> does not happen before <span class="event">e<sub>2</sub></span> and does not happen to avoid data races.
after <span class="event">e<sub>2</sub></span>, then we say that <span class="event">e<sub>1</sub></span> and <span class="event">e<sub>2</sub></span> happen concurrently. In the absence of data races, Go programs behave as if all the goroutines
</p> were multiplexed onto a single processor.
This property is sometimes referred to as DRF-SC: data-race-free programs
<p class="rule"> execute in a sequentially consistent manner.
Within a single goroutine, the happens-before order is the
order expressed by the program.
</p> </p>
<p> <p>
A read <span class="event">r</span> of a variable <code>v</code> is <i>allowed</i> to observe a write <span class="event">w</span> to <code>v</code> While programmers should write Go programs without data races,
if both of the following hold: there are limitations to what a Go implementation can do in response to a data race.
An implementation may always react to a data race by reporting the race and terminating the program.
Otherwise, each read of a single-word-sized or sub-word-sized memory location
must observe a value actually written to that location (perhaps by a concurrent executing goroutine)
and not yet overwritten.
These implementation constraints make Go more like Java or JavaScript,
in that most races have a limited number of outcomes,
and less like C and C++, where the meaning of any program with a race
is entirely undefined, and the compiler may do anything at all.
Go's approach aims to make errant programs more reliable and easier to debug,
while still insisting that races are errors and that tools can diagnose and report them.
</p> </p>
<h2 id="model">Memory Model</h2>
<p>
The following formal definition of Go's memory model closely follows
the approach presented by Hans-J. Boehm and Sarita V. Adve in
“<a href="https://www.hpl.hp.com/techreports/2008/HPL-2008-56.pdf">Foundations of the C++ Concurrency Memory Model</a>”,
published in PLDI 2008.
The definition of data-race-free programs and the guarantee of sequential consistency
for race-free progams are equivalent to the ones in that work.
</p>
<p>
The memory model describes the requirements on program executions,
which are made up of goroutine executions,
which in turn are made up of memory operations.
</p>
<p>
A <i>memory operation</i> is modeled by four details:
</p>
<ul>
<li>its kind, indicating whether it is an ordinary data read, an ordinary data write,
or a <i>synchronizing operation</i> such as an atomic data access,
a mutex operation, or a channel operation,
<li>its location in the program,
<li>the memory location or variable being accessed, and
<li>the values read or written by the operation.
</ul>
<p>
Some memory operations are <i>read-like</i>, including read, atomic read, mutex lock, and channel receive.
Other memory operations are <i>write-like</i>, including write, atomic write, mutex unlock, channel send, and channel close.
Some, such as atomic compare-and-swap, are both read-like and write-like.
</p>
<p>
A <i>goroutine execution</i> is modeled as a set of memory operations executed by a single goroutine.
</p>
<p>
<b>Requirement 1</b>:
The memory operations in each goroutine must correspond to a correct sequential execution of that goroutine,
given the values read from and written to memory.
That execution must be consistent with the <i>sequenced before</i> relation,
defined as the partial order requirements set out by the <a href="/ref/spec">Go language specification</a>
for Go's control flow constructs as well as the <a href="/ref/spec#Order_of_evaluation">order of evaluation for expressions</a>.
</p>
<p>
A Go <i>program execution</i> is modeled as a set of goroutine executions,
together with a mapping <i>W</i> that specifies the write-like operation that each read-like operation reads from.
(Multiple executions of the same program can have different program executions.)
</p>
<p>
<b>Requirement 2</b>:
For a given program execution, the mapping <i>W</i>, when limited to synchronizing operations,
must be explainable by some implicit total order of the synchronizing operations
that is consistent with sequencing and the values read and written by those operations.
</p>
<p>
The <i>synchronized before</i> relation is a partial order on synchronizing memory operations,
derived from <i>W</i>.
If a synchronizing read-like memory operation <i>r</i>
observes a synchronizing write-like memory operation <i>w</i>
(that is, if <i>W</i>(<i>r</i>) = <i>w</i>),
then <i>w</i> is synchronized before <i>r</i>.
Informally, the synchronized before relation is a subset of the implied total order
mentioned in the previous paragraph,
limited to the information that <i>W</i> directly observes.
</p>
<p>
The <i>happens before</i> relation is defined as the transitive closure of the
union of the sequenced before and synchronized before relations.
</p>
<p>
<b>Requirement 3</b>:
For an ordinary (non-synchronizing) data read <i>r</i> on a memory location <i>x</i>,
<i>W</i>(<i>r</i>) must be a write <i>w</i> that is <i>visible</i> to <i>r</i>,
where visible means that both of the following hold:
<ol> <ol>
<li><span class="event">r</span> does not happen before <span class="event">w</span>.</li> <li><i>w</i> happens before <i>r</i>.
<li>There is no other write <span class="event">w'</span> to <code>v</code> that happens <li><i>w</i> does not happen before any other write <i>w'</i> (to <i>x</i>) that happens before <i>r</i>.
after <span class="event">w</span> but before <span class="event">r</span>.</li>
</ol> </ol>
<p> <p>
To guarantee that a read <span class="event">r</span> of a variable <code>v</code> observes a A <i>read-write data race</i> on memory location <i>x</i>
particular write <span class="event">w</span> to <code>v</code>, ensure that <span class="event">w</span> is the only consists of a read-like memory operation <i>r</i> on <i>x</i>
write <span class="event">r</span> is allowed to observe. and a write-like memory operation <i>w</i> on <i>x</i>,
That is, <span class="event">r</span> is <i>guaranteed</i> to observe <span class="event">w</span> if both of the following hold: at least one of which is non-synchronizing,
</p> which are unordered by happens before
(that is, neither <i>r</i> happens before <i>w</i>
<ol> nor <i>w</i> happens before <i>r</i>).
<li><span class="event">w</span> happens before <span class="event">r</span>.</li>
<li>Any other write to the shared variable <code>v</code>
either happens before <span class="event">w</span> or after <span class="event">r</span>.</li>
</ol>
<p>
This pair of conditions is stronger than the first pair;
it requires that there are no other writes happening
concurrently with <span class="event">w</span> or <span class="event">r</span>.
</p> </p>
<p> <p>
Within a single goroutine, A <i>write-write data race</i> on memory location <i>x</i>
there is no concurrency, so the two definitions are equivalent: consists of two write-like memory operations <i>w</i> and <i>w'</i> on <i>x</i>,
a read <span class="event">r</span> observes the value written by the most recent write <span class="event">w</span> to <code>v</code>. at least one of which is non-synchronizing,
When multiple goroutines access a shared variable <code>v</code>, which are unordered by happens before.
they must use synchronization events to establish
happens-before conditions that ensure reads observe the
desired writes.
</p> </p>
<p> <p>
The initialization of variable <code>v</code> with the zero value Note that if there are no read-write or write-write data races on memory location <i>x</i>,
for <code>v</code>'s type behaves as a write in the memory model. then any read <i>r</i> on <i>x</i> has only one possible <i>W</i>(<i>r</i>):
the single <i>w</i> that immediately precedes it in the happens before order.
</p> </p>
<p> <p>
Reads and writes of values larger than a single machine word More generally, it can be shown that any Go program that is data-race-free,
behave as multiple machine-word-sized operations in an meaning it has no program executions with read-write or write-write data races,
unspecified order. can only have outcomes explained by some sequentially consistent interleaving
of the goroutine executions.
(The proof is the same as Section 7 of Boehm and Adve's paper cited above.)
This property is called DRF-SC.
</p> </p>
<h2>Synchronization</h2> <p>
The intent of the formal definition is to match
the DRF-SC guarantee provided to race-free programs
by other languages, including C, C++, Java, JavaScript, Rust, and Swift.
</p>
<h3>Initialization</h3> <p>
Certain Go language operations such as goroutine creation and memory allocation
act as synchronization opeartions.
The effect of these operations on the synchronized-before partial order
is documented in the “Synchronization” section below.
Individual packages are responsible for providing similar documentation
for their own operations.
</p>
<h2 id="restrictions">Implementation Restrictions for Programs Containing Data Races</h2>
<p>
The preceding section gave a formal definition of data-race-free program execution.
This section informally describes the semantics that implementations must provide
for programs that do contain races.
</p>
<p>
First, any implementation can, upon detecting a data race,
report the race and halt execution of the program.
Implementations using ThreadSanitizer
(accessed with “<code>go</code> <code>build</code> <code>-race</code>”)
do exactly this.
</p>
<p>
Otherwise, a read <i>r</i> of a memory location <i>x</i>
that is not larger than a machine word must observe
some write <i>w</i> such that <i>r</i> does not happen before <i>w</i>
and there is no write <i>w'</i> such that <i>w</i> happens before <i>w'</i>
and <i>w'</i> happens before <i>r</i>.
That is, each read must observe a value written by a preceding or concurrent write.
</p>
<p>
Additionally, observation of acausal and “out of thin air” writes is disallowed.
</p>
<p>
Reads of memory locations larger than a single machine word
are encouraged but not required to meet the same semantics
as word-sized memory locations,
observing a single allowed write <i>w</i>.
For performance reasons,
implementations may instead treat larger operations
as a set of individual machine-word-sized operations
in an unspecified order.
This means that races on multiword data structures
can lead to inconsistent values not corresponding to a single write.
When the values depend on the consistency
of internal (pointer, length) or (pointer, type) pairs,
as can be the case for interface values, maps,
slices, and strings in most Go implementations,
such races can in turn lead to arbitrary memory corruption.
</p>
<p>
Examples of incorrect synchronization are given in the
“Incorrect synchronization” section below.
</p>
<p>
Examples of the limitations on implementations are given in the
“Incorrect compilation” section below.
</p>
<h2 id="synchronization">Synchronization</h2>
<h3 id="init">Initialization</h3>
<p> <p>
Program initialization runs in a single goroutine, Program initialization runs in a single goroutine,
@ -141,15 +285,15 @@ If a package <code>p</code> imports package <code>q</code>, the completion of
</p> </p>
<p class="rule"> <p class="rule">
The start of the function <code>main.main</code> happens after The completion of all <code>init</code> functions is synchronized before
all <code>init</code> functions have finished. the start of the function <code>main.main</code>.
</p> </p>
<h3>Goroutine creation</h3> <h3 id="go">Goroutine creation</h3>
<p class="rule"> <p class="rule">
The <code>go</code> statement that starts a new goroutine The <code>go</code> statement that starts a new goroutine
happens before the goroutine's execution begins. is synchronized before the start of the goroutine's execution.
</p> </p>
<p> <p>
@ -174,11 +318,12 @@ calling <code>hello</code> will print <code>"hello, world"</code>
at some point in the future (perhaps after <code>hello</code> has returned). at some point in the future (perhaps after <code>hello</code> has returned).
</p> </p>
<h3>Goroutine destruction</h3> <h3 id="goexit">Goroutine destruction</h3>
<p> <p>
The exit of a goroutine is not guaranteed to happen before The exit of a goroutine is not guaranteed to be synchronized before
any event in the program. For example, in this program: any event in the program.
For example, in this program:
</p> </p>
<pre> <pre>
@ -203,7 +348,7 @@ use a synchronization mechanism such as a lock or channel
communication to establish a relative ordering. communication to establish a relative ordering.
</p> </p>
<h3>Channel communication</h3> <h3 id="chan">Channel communication</h3>
<p> <p>
Channel communication is the main method of synchronization Channel communication is the main method of synchronization
@ -213,8 +358,8 @@ usually in a different goroutine.
</p> </p>
<p class="rule"> <p class="rule">
A send on a channel happens before the corresponding A send on a channel is synchronized before the completion of the
receive from that channel completes. corresponding receive from that channel.
</p> </p>
<p> <p>
@ -239,13 +384,13 @@ func main() {
<p> <p>
is guaranteed to print <code>"hello, world"</code>. The write to <code>a</code> is guaranteed to print <code>"hello, world"</code>. The write to <code>a</code>
happens before the send on <code>c</code>, which happens before is sequenced before the send on <code>c</code>, which is synchronized before
the corresponding receive on <code>c</code> completes, which happens before the corresponding receive on <code>c</code> completes, which is sequenced before
the <code>print</code>. the <code>print</code>.
</p> </p>
<p class="rule"> <p class="rule">
The closing of a channel happens before a receive that returns a zero value The closing of a channel is synchronized before a receive that returns a zero value
because the channel is closed. because the channel is closed.
</p> </p>
@ -256,8 +401,8 @@ yields a program with the same guaranteed behavior.
</p> </p>
<p class="rule"> <p class="rule">
A receive from an unbuffered channel happens before A receive from an unbuffered channel is synchronized before the completion of
the send on that channel completes. the corresponding send on that channel.
</p> </p>
<p> <p>
@ -283,8 +428,8 @@ func main() {
<p> <p>
is also guaranteed to print <code>"hello, world"</code>. The write to <code>a</code> is also guaranteed to print <code>"hello, world"</code>. The write to <code>a</code>
happens before the receive on <code>c</code>, which happens before is sequenced before the receive on <code>c</code>, which is synchronized before
the corresponding send on <code>c</code> completes, which happens the corresponding send on <code>c</code> completes, which is sequenced
before the <code>print</code>. before the <code>print</code>.
</p> </p>
@ -296,7 +441,7 @@ crash, or do something else.)
</p> </p>
<p class="rule"> <p class="rule">
The <i>k</i>th receive on a channel with capacity <i>C</i> happens before the <i>k</i>+<i>C</i>th send from that channel completes. The <i>k</i>th receive on a channel with capacity <i>C</i> is synchronized before the completion of the <i>k</i>+<i>C</i>th send from that channel completes.
</p> </p>
<p> <p>
@ -330,7 +475,7 @@ func main() {
} }
</pre> </pre>
<h3>Locks</h3> <h3 id="locks">Locks</h3>
<p> <p>
The <code>sync</code> package implements two lock data types, The <code>sync</code> package implements two lock data types,
@ -339,7 +484,7 @@ The <code>sync</code> package implements two lock data types,
<p class="rule"> <p class="rule">
For any <code>sync.Mutex</code> or <code>sync.RWMutex</code> variable <code>l</code> and <i>n</i> &lt; <i>m</i>, For any <code>sync.Mutex</code> or <code>sync.RWMutex</code> variable <code>l</code> and <i>n</i> &lt; <i>m</i>,
call <i>n</i> of <code>l.Unlock()</code> happens before call <i>m</i> of <code>l.Lock()</code> returns. call <i>n</i> of <code>l.Unlock()</code> is synchronized before call <i>m</i> of <code>l.Lock()</code> returns.
</p> </p>
<p> <p>
@ -365,19 +510,29 @@ func main() {
<p> <p>
is guaranteed to print <code>"hello, world"</code>. is guaranteed to print <code>"hello, world"</code>.
The first call to <code>l.Unlock()</code> (in <code>f</code>) happens The first call to <code>l.Unlock()</code> (in <code>f</code>) is synchronized
before the second call to <code>l.Lock()</code> (in <code>main</code>) returns, before the second call to <code>l.Lock()</code> (in <code>main</code>) returns,
which happens before the <code>print</code>. which is sequenced before the <code>print</code>.
</p> </p>
<p class="rule"> <p class="rule">
For any call to <code>l.RLock</code> on a <code>sync.RWMutex</code> variable <code>l</code>, For any call to <code>l.RLock</code> on a <code>sync.RWMutex</code> variable <code>l</code>,
there is an <i>n</i> such that the <code>l.RLock</code> happens (returns) after call <i>n</i> to there is an <i>n</i> such that the <i>n</i>th call to <code>l.Unlock</code>
<code>l.Unlock</code> and the matching <code>l.RUnlock</code> happens is synchronized before the return from <code>l.RLock</code>,
before call <i>n</i>+1 to <code>l.Lock</code>. and the matching call to <code>l.RUnlock</code> is synchronized before the return from call <i>n</i>+1 to <code>l.Lock</code>.
</p> </p>
<h3>Once</h3> <p class="rule">
A successful call to <code>l.TryLock</code> (or <code>l.TryRLock</code>)
is equivalent to a call to <code>l.Lock</code> (or <code>l.RLock</code>).
An unsuccessful call has no synchronizing effect at all.
As far as the memory model is concerned,
<code>l.TryLock</code> (or <code>l.TryRLock</code>)
may be considered to be able to return false
even when the mutex <i>l</i> is unlocked.
</p>
<h3 id="once">Once</h3>
<p> <p>
The <code>sync</code> package provides a safe mechanism for The <code>sync</code> package provides a safe mechanism for
@ -389,7 +544,8 @@ until <code>f()</code> has returned.
</p> </p>
<p class="rule"> <p class="rule">
A single call of <code>f()</code> from <code>once.Do(f)</code> happens (returns) before any call of <code>once.Do(f)</code> returns. The completion of a single call of <code>f()</code> from <code>once.Do(f)</code>
is synchronized before the return of any call of <code>once.Do(f)</code>.
</p> </p>
<p> <p>
@ -424,13 +580,60 @@ The result will be that <code>"hello, world"</code> will be printed
twice. twice.
</p> </p>
<h2>Incorrect synchronization</h2> <h3 id="atomic">Atomic Values</h3>
<p> <p>
Note that a read <span class="event">r</span> may observe the value written by a write <span class="event">w</span> The APIs in the <a href="/pkg/sync/atomic/"><code>sync/atomic</code></a>
that happens concurrently with <span class="event">r</span>. package are collectively “atomic operations”
Even if this occurs, it does not imply that reads happening after <span class="event">r</span> that can be used to synchronize the execution of different goroutines.
will observe writes that happened before <span class="event">w</span>. If the effect of an atomic operation <i>A</i> is observed by atomic operation <i>B</i>,
then <i>A</i> is synchronized before <i>B</i>.
All the atomic operations executed in a program behave as though executed
in some sequentially consistent order.
</p>
<p>
The preceding definition has the same semantics as C++’s sequentially consistent atomics
and Java’s <code>volatile</code> variables.
</p>
<h3 id="finalizer">Finalizers</h3>
<p>
The <a href="/pkg/runtime/"><code>runtime</code></a> package provides
a <code>SetFinalizer</code> function that adds a finalizer to be called when
a particular object is no longer reachable by the program.
A call to <code>SetFinalizer(x, f)</code> is synchronized before the finalization call <code>f(x)</code>.
</p>
<h3 id="more">Additional Mechanisms</h3>
<p>
The <code>sync</code> package provides additional synchronization abstractions,
including <a href="/pkg/sync/#Cond">condition variables</a>,
<a href="/pkg/sync/#Map">lock-free maps</a>,
<a href="/pkg/sync/#Pool">allocation pools</a>,
and
<a href="/pkg/sync/#WaitGroup">wait groups</a>.
The documentation for each of these specifies the guarantees it
makes concerning synchronization.
</p>
<p>
Other packages that provide synchronization abstractions
should document the guarantees they make too.
</p>
<h2 id="badsync">Incorrect synchronization</h2>
<p>
Programs with races are incorrect and
can exhibit non-sequentially consistent executions.
In particular, note that a read <i>r</i> may observe the value written by any write <i>w</i>
that executes concurrently with <i>r</i>.
Even if this occurs, it does not imply that reads happening after <i>r</i>
will observe writes that happened before <i>w</i>.
</p> </p>
<p> <p>
@ -566,3 +769,197 @@ value for <code>g.msg</code>.
In all these examples, the solution is the same: In all these examples, the solution is the same:
use explicit synchronization. use explicit synchronization.
</p> </p>
<h2 id="badcompiler">Incorrect compilation</h2>
<p>
The Go memory model restricts compiler optimizations as much as it does Go programs.
Some compiler optimizations that would be valid in single-threaded programs are not valid in all Go programs.
In particular, a compiler must not introduce writes that do not exist in the original program,
it must not allow a single read to observe multiple values,
and it must not allow a single write to write multiple values.
</p>
<p>
All the following examples assume that `*p` and `*q` refer to
memory locations accessible to multiple goroutines.
</p>
<p>
Not introducing data races into race-free programs means not moving
writes out of conditional statements in which they appear.
For example, a compiler must not invert the conditional in this program:
</p>
<pre>
*p = 1
if cond {
*p = 2
}
</pre>
<p>
That is, the compiler must not rewrite the program into this one:
</p>
<pre>
*p = 2
if !cond {
*p = 1
}
</pre>
<p>
If <code>cond</code> is false and another goroutine is reading <code>*p</code>,
then in the original program, the other goroutine can only observe any prior value of <code>*p</code> and <code>1</code>.
In the rewritten program, the other goroutine can observe <code>2</code>, which was previously impossible.
</p>
<p>
Not introducing data races also means not assuming that loops terminate.
For example, a compiler must in general not move the accesses to <code>*p</code> or <code>*q</code>
ahead of the loop in this program:
</p>
<pre>
n := 0
for e := list; e != nil; e = e.next {
n++
}
i := *p
*q = 1
</pre>
<p>
If <code>list</code> pointed to a cyclic list,
then the original program would never access <code>*p</code> or <code>*q</code>,
but the rewritten program would.
(Moving `*p` ahead would be safe if the compiler can prove `*p` will not panic;
moving `*q` ahead would also require the compiler proving that no other
goroutine can access `*q`.)
</p>
<p>
Not introducing data races also means not assuming that called functions
always return or are free of synchronization operations.
For example, a compiler must not move the accesses to <code>*p</code> or <code>*q</code>
ahead of the function call in this program
(at least not without direct knowledge of the precise behavior of <code>f</code>):
</p>
<pre>
f()
i := *p
*q = 1
</pre>
<p>
If the call never returned, then once again the original program
would never access <code>*p</code> or <code>*q</code>, but the rewritten program would.
And if the call contained synchronizing operations, then the original program
could establish happens before edges preceding the accesses
to <code>*p</code> and <code>*q</code>, but the rewritten program would not.
</p>
<p>
Not allowing a single read to observe multiple values means
not reloading local variables from shared memory.
For example, a compiler must not discard <code>i</code> and reload it
a second time from <code>*p</code> in this program:
</p>
<pre>
i := *p
if i &lt; 0 || i &gt;= len(funcs) {
panic("invalid function index")
}
... complex code ...
// compiler must NOT reload i = *p here
funcs[i]()
</pre>
<p>
If the complex code needs many registers, a compiler for single-threaded programs
could discard <code>i</code> without saving a copy and then reload
<code>i = *p</code> just before
<code>funcs[i]()</code>.
A Go compiler must not, because the value of <code>*p</code> may have changed.
(Instead, the compiler could spill <code>i</code> to the stack.)
</p>
<p>
Not allowing a single write to write multiple values also means not using
the memory where a local variable will be written as temporary storage before the write.
For example, a compiler must not use <code>*p</code> as temporary storage in this program:
</p>
<pre>
*p = i + *p/2
</pre>
<p>
That is, it must not rewrite the program into this one:
</p>
<pre>
*p /= 2
*p += i
</pre>
<p>
If <code>i</code> and <code>*p</code> start equal to 2,
the original code does <code>*p = 3</code>,
so a racing thread can read only 2 or 3 from <code>*p</code>.
The rewritten code does <code>*p = 1</code> and then <code>*p = 3</code>,
allowing a racing thread to read 1 as well.
</p>
<p>
Note that all these optimizations are permitted in C/C++ compilers:
a Go compiler sharing a back end with a C/C++ compiler must take care
to disable optimizations that are invalid for Go.
</p>
<p>
Note that the prohibition on introducing data races
does not apply if the compiler can prove that the races
do not affect correct execution on the target platform.
For example, on essentially all CPUs, it is valid to rewrite
</p>
<pre>
n := 0
for i := 0; i < m; i++ {
n += *shared
}
</pre>
into:
<pre>
n := 0
local := *shared
for i := 0; i < m; i++ {
n += local
}
</pre>
<p>
provided it can be proved that <code>*shared</code> will not fault on access,
because the potential added read will not affect any existing concurrent reads or writes.
On the other hand, the rewrite would not be valid in a source-to-source translator.
</p>
<h2 id="conclusion">Conclusion</h2>
<p>
Go programmers writing data-race-free programs can rely on
sequentially consistent execution of those programs,
just as in essentially all other modern programming languages.
</p>
<p>
When it comes to programs with races,
both programmers and compilers should remember the advice:
don't be clever.
</p>