doc: update Implementation and Performance sections of the FAQ

Changes are mostly about making more about now than about the past,
changing some verb tenses, and mentioning gollvm (which should
be pronounced "gollum" if you ask me).

Update #26107

Change-Id: I6c14f42b9fc2684259d4ba8bc149d7ec9bb83d15
Reviewed-on: https://go-review.googlesource.com/123917Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go_faq.html

@@ -1851,38 +1851,69 @@ But we encourage most new code to live elsewhere.
 What compiler technology is used to build the compilers?</h3>
 
 <p>
-<code>Gccgo</code> has a front end written in C++, with a recursive descent parser coupled to the
-standard GCC back end. <code>Gc</code> is written in Go with a recursive descent parser
+There are several production compilers for Go, and a number of others
+in development for various platforms.
+</p>
+
+<p>
+The default compiler, <code>gc</code>, is included with the
+Go distribution as part of the support for the <code>go</code>
+command.
+<code>Gc</code> was originally written in C
+because of the difficulties of bootstrapping&mdash;you'd need a Go compiler to
+set up a Go environment.
+But things have advanced and since the Go 1.5 release the compiler has been
+a Go program.
+The compiler was converted from C to Go using automatic translation tools, as
+described in this <a href="/s/go13compiler">design document</a>
+and <a href="https://talks.golang.org/2015/gogo.slide#1">talk</a>.
+Thus the compiler is now "self-hosting", which means we needed to face
+the bootstrapping problem.
+The solution is to have a working Go installation already in place,
+just as one normally has with a working C installation.
+The story of how to bring up a new Go environment from source
+is described <a href="/s/go15bootstrap">here</a> and
+<a href="/doc/install/source">here</a>.
+</p>
+
+<p>
+<code>Gc</code> is written in Go with a recursive descent parser
 and uses a custom loader, also written in Go but
 based on the Plan 9 loader, to generate ELF/Mach-O/PE binaries.
 </p>
 
 <p>
-We considered using LLVM for <code>gc</code> but we felt it was too large and
-slow to meet our performance goals.
+At the beginning of the project we considered using LLVM for
+<code>gc</code> but decided it was too large and slow to meet
+our performance goals.
+More important in retrospect, starting with LLVM would have made it
+harder to introduce some of the ABI and related changes, such as
+stack management, that Go requires but not are not part of the
+standard C setup.
+A new <a href="https://go.googlesource.com/gollvm/">LLVM implementation</a>
+is starting to come together now, however.
 </p>
 
 <p>
-The original <code>gc</code>, the Go compiler, was written in C
-because of the difficulties of bootstrapping&mdash;you'd need a Go compiler to
-set up a Go environment.
-But things have advanced and as of Go 1.5 the compiler is written in Go.
-It was converted from C to Go using automatic translation tools, as
-described in <a href="/s/go13compiler">this design document</a>
-and <a href="https://talks.golang.org/2015/gogo.slide#1">a recent talk</a>.
-Thus the compiler is now "self-hosting", which means we must face
-the bootstrapping problem.
-The solution, naturally, is to have a working Go installation already,
-just as one normally has a working C installation in place.
-The story of how to bring up a new Go installation from source
-is described <a href="/s/go15bootstrap">separately</a>.
+The <code>Gccgo</code> compiler is a front end written in C++
+with a recursive descent parser coupled to the
+standard GCC back end.
+</p>
+
+<p>
+Go turned out to be a fine language in which to implement a Go compiler,
+although that was not its original goal.
+Not being self-hosting from the beginning allowed Go's design to
+concentrate on its original use case, which was networked servers.
+Had we decided Go should compile itself early on, we might have
+ended up with a language targeted more for compiler construction,
+which is a worthy goal but not the one we had initially.
 </p>
 
 <p>
-Go is a fine language in which to implement a Go compiler.
 Although <code>gc</code> does not use them (yet?), a native lexer and
 parser are available in the <a href="/pkg/go/"><code>go</code></a> package
-and there is also a <a href="/pkg/go/types">type checker</a>.
+and there is also a native <a href="/pkg/go/types">type checker</a>.
 </p>
 
 <h3 id="How_is_the_run_time_support_implemented">
@@ -1896,6 +1927,8 @@ tiny bit of assembler) but it has since been translated to Go
 The <code>gccgo</code> compiler implements goroutines using
 a technique called segmented stacks,
 supported by recent modifications to the gold linker.
+<code>Gollvm</code> similarly is built on the corresponding
+LLVM infrastructure. 
 </p>
 
 <h3 id="Why_is_my_trivial_program_such_a_large_binary">
@@ -1905,7 +1938,7 @@ Why is my trivial program such a large binary?</h3>
 The linker in the <code>gc</code> toolchain
 creates statically-linked binaries by default.
 All Go binaries therefore include the Go
-run-time, along with the run-time type information necessary to support dynamic
+runtime, along with the run-time type information necessary to support dynamic
 type checks, reflection, and even panic-time stack traces.
 </p>
 
@@ -1918,6 +1951,14 @@ An equivalent Go program using
 more powerful run-time support and type and debugging information.
 </p>
 
+<p>
+A Go program compiled with <code>gc</code> can be linked with
+the <code>-ldflags=-w</code> flag to disable DWARF generation,
+removing debugging information from the binary but with no
+other loss of functionality.
+This can reduce the binary size substantially.
+</p>
+
 <h3 id="unused_variables_and_imports">
 Can I stop these complaints about my unused variable/import?</h3>
 
@@ -2157,7 +2198,7 @@ brace to live on the next line.  We disagree.  Since Go code is meant
 to be formatted automatically by
 <a href="/cmd/gofmt/"><code>gofmt</code></a>,
 <i>some</i> style must be chosen.  That style may differ from what
-you've used in C or Java, but Go is a new language and
+you've used in C or Java, but Go is a different language and
 <code>gofmt</code>'s style is as good as any other.  More
 important&mdash;much more important&mdash;the advantages of a single,
 programmatically mandated format for all Go programs greatly outweigh
@@ -2170,15 +2211,19 @@ Go can use the standard syntax one line at a time without special rules.
 Why do garbage collection?  Won't it be too expensive?</h3>
 <p>
 One of the biggest sources of bookkeeping in systems programs is
-memory management.  We feel it's critical to eliminate that
+memory management.
+In languages in which it is done manually,
+it can consume a significant amount of programmer time and is
+often the cause of pernicious bugs.
+We felt it was critical to eliminate that
 programmer overhead, and advances in garbage collection
-technology in the last few years give us confidence that we can
-implement it with low enough overhead and no significant
+technology in the last few years gave us confidence that it
+could be implemented with low enough overhead and no significant
 latency.
 </p>
 
 <p>
-Another point is that a large part of the difficulty of concurrent
+Another issue is that a large part of the difficulty of concurrent
 and multi-threaded programming is memory management;
 as objects get passed among threads it becomes cumbersome
 to guarantee they become freed safely.
@@ -2194,12 +2239,15 @@ simpler because they don't need to specify how memory is managed across them.
 </p>
 
 <p>
-The current implementation is a parallel mark-and-sweep collector.
-Recent improvements, documented in
-<a href="/s/go14gc">this design document</a>,
-have introduced bounded pause times and improved the
-parallelism.
-Future versions might attempt new approaches.
+The current implementation is a mark-and-sweep collector that runs
+in parallel with the main program on a separate CPU core if the
+machine is a multiprocessor.
+Major work on the collector in recent years has reduced pause times
+often to the sub-millisecond range, even for large heaps,
+all but eliminating one of the major objections to garbage collection
+in networked servers.
+Work continues to refine the algorithm, reduce overhead and
+latency further, and to explore new approaches.
 </p>
 
 <p>