CompilerOptimizations

Compiler Optimizations

This page lists optimizations done by the compilers. Note that these are not guaranteed by the language specification.

Interface values

Zero-width types in interface values

Putting a zero-width type in an interface value doesn't allocate.

• gc: 1.0+
• gccgo: ?

Word-sized value in an interface value

Putting a word-sized-or-less non-pointer type in an interface value doesn't allocate.

• gc: 1.0-1.3, but not in 1.4+
• gccgo: never

string and []byte

Map lookup by []byte

For a map m of type map[string]T and []byte b, m[string(b)] doesn't allocate. (the temporary string copy of the byte slice isn't made)

• gc: 1.4+
• gccgo: ?

Escape analysis and Inlining

Use -gcflags -m to observe the result of escape analysis and inlining decisions for the gc toolchain.

(TODO: explain the output of -gcflags -m).

Escape analysis

Gc compiler does global escape analysis across function and package boundaries. However, there are lots of cases where it gives up. For example, anything assigned to any indirection (*p = ...; a[i] = ...) is considered escaped. Other things that can inhibit analysis are: function calls, package boundaries, slice literals, subslicing and indexing, etc. Full rules are too complex to describe, so check the -m output.

• gc: 1.0+
• gccgo: not yet.

Function Inlining

Only short and simple functions are inlined. To be inlined a function must contain less than ~40 expressions and does not contain complex things like function calls, loops, labels, closures, panic's, recover's, select's, switch'es, etc.

• gc: 1.0+
• gccgo: -O1 and above.

Idioms

Optimized memclr

For a slice or array s, loops of the form

for i := range s {
	a[i] = <zero value for element of s>
}

are converted into efficient runtime memclr calls. Issue and commit.

• gc: 1.5+
• gccgo: ?