compile

Overview ¶

Compile, typically invoked as “go tool compile,” compiles a single Go package comprising the files named on the command line. It then writes a single object file named for the basename of the first source file with a .o suffix. The object file can then be combined with other objects into a package archive or passed directly to the linker (“go tool link”). If invoked with -pack, the compiler writes an archive directly, bypassing the intermediate object file.

The generated files contain type information about the symbols exported by the package and about types used by symbols imported by the package from other packages. It is therefore not necessary when compiling client C of package P to read the files of P's dependencies, only the compiled output of P.

Command Line ¶

Usage:

go tool compile [flags] file...

The specified files must be Go source files and all part of the same package. The same compiler is used for all target operating systems and architectures. The GOOS and GOARCH environment variables set the desired target.

Flags:

-D path
	Set relative path for local imports.
-I dir1 -I dir2
	Search for imported packages in dir1, dir2, etc,
	after consulting $GOROOT/pkg/$GOOS_$GOARCH.
-L
	Show complete file path in error messages.
-N
	Disable optimizations.
-S
	Print assembly listing to standard output (code only).
-S -S
	Print assembly listing to standard output (code and data).
-V
	Print compiler version and exit.
-asmhdr file
	Write assembly header to file.
-asan
	Insert calls to C/C++ address sanitizer.
-buildid id
	Record id as the build id in the export metadata.
-blockprofile file
	Write block profile for the compilation to file.
-c int
	Concurrency during compilation. Set 1 for no concurrency (default is 1).
-complete
	Assume package has no non-Go components.
-cpuprofile file
	Write a CPU profile for the compilation to file.
-dynlink
	Allow references to Go symbols in shared libraries (experimental).
-e
	Remove the limit on the number of errors reported (default limit is 10).
-goversion string
	Specify required go tool version of the runtime.
	Exits when the runtime go version does not match goversion.
-h
	Halt with a stack trace at the first error detected.
-importcfg file
	Read import configuration from file.
	In the file, set importmap, packagefile to specify import resolution.
-installsuffix suffix
	Look for packages in $GOROOT/pkg/$GOOS_$GOARCH_suffix
	instead of $GOROOT/pkg/$GOOS_$GOARCH.
-l
	Disable inlining.
-lang version
	Set language version to compile, as in -lang=go1.12.
	Default is current version.
-linkobj file
	Write linker-specific object to file and compiler-specific
	object to usual output file (as specified by -o).
	Without this flag, the -o output is a combination of both
	linker and compiler input.
-m
	Print optimization decisions. Higher values or repetition
	produce more detail.
-memprofile file
	Write memory profile for the compilation to file.
-memprofilerate rate
	Set runtime.MemProfileRate for the compilation to rate.
-msan
	Insert calls to C/C++ memory sanitizer.
-mutexprofile file
	Write mutex profile for the compilation to file.
-nolocalimports
	Disallow local (relative) imports.
-o file
	Write object to file (default file.o or, with -pack, file.a).
-p path
	Set expected package import path for the code being compiled,
	and diagnose imports that would cause a circular dependency.
-pack
	Write a package (archive) file rather than an object file
-race
	Compile with race detector enabled.
-s
	Warn about composite literals that can be simplified.
-shared
	Generate code that can be linked into a shared library.
-spectre list
	Enable spectre mitigations in list (all, index, ret).
-traceprofile file
	Write an execution trace to file.
-trimpath prefix
	Remove prefix from recorded source file paths.

Flags related to debugging information:

-dwarf
	Generate DWARF symbols.
-dwarflocationlists
	Add location lists to DWARF in optimized mode.
-gendwarfinl int
	Generate DWARF inline info records (default 2).

Flags to debug the compiler itself:

-E
	Debug symbol export.
-K
	Debug missing line numbers.
-d list
	Print debug information about items in list. Try -d help for further information.
-live
	Debug liveness analysis.
-v
	Increase debug verbosity.
-%
	Debug non-static initializers.
-W
	Debug parse tree after type checking.
-f
	Debug stack frames.
-i
	Debug line number stack.
-j
	Debug runtime-initialized variables.
-r
	Debug generated wrappers.
-w
	Debug type checking.

Compiler Directives ¶

The compiler accepts directives in the form of comments. To distinguish them from non-directive comments, directives require no space between the comment opening and the name of the directive. However, since they are comments, tools unaware of the directive convention or of a particular directive can skip over a directive like any other comment.

Line directives come in several forms:

//line :line
//line :line:col
//line filename:line
//line filename:line:col
/*line :line*/
/*line :line:col*/
/*line filename:line*/
/*line filename:line:col*/

In order to be recognized as a line directive, the comment must start with //line or /*line followed by a space, and must contain at least one colon. The //line form must start at the beginning of a line. A line directive specifies the source position for the character immediately following the comment as having come from the specified file, line and column: For a //line comment, this is the first character of the next line, and for a /*line comment this is the character position immediately following the closing */. If no filename is given, the recorded filename is empty if there is also no column number; otherwise it is the most recently recorded filename (actual filename or filename specified by previous line directive). If a line directive doesn't specify a column number, the column is "unknown" until the next directive and the compiler does not report column numbers for that range. The line directive text is interpreted from the back: First the trailing :ddd is peeled off from the directive text if ddd is a valid number > 0. Then the second :ddd is peeled off the same way if it is valid. Anything before that is considered the filename (possibly including blanks and colons). Invalid line or column values are reported as errors.

Examples:

//line foo.go:10      the filename is foo.go, and the line number is 10 for the next line
//line C:foo.go:10    colons are permitted in filenames, here the filename is C:foo.go, and the line is 10
//line  a:100 :10     blanks are permitted in filenames, here the filename is " a:100 " (excluding quotes)
/*line :10:20*/x      the position of x is in the current file with line number 10 and column number 20
/*line foo: 10 */     this comment is recognized as invalid line directive (extra blanks around line number)

Line directives typically appear in machine-generated code, so that compilers and debuggers will report positions in the original input to the generator.

The line directive is a historical special case; all other directives are of the form //go:name, indicating that they are defined by the Go toolchain. Each directive must be placed its own line, with only leading spaces and tabs allowed before the comment. Each directive applies to the Go code that immediately follows it, which typically must be a declaration.

//go:noescape

The //go:noescape directive must be followed by a function declaration without a body (meaning that the function has an implementation not written in Go). It specifies that the function does not allow any of the pointers passed as arguments to escape into the heap or into the values returned from the function. This information can be used during the compiler's escape analysis of Go code calling the function.

//go:uintptrescapes

The //go:uintptrescapes directive must be followed by a function declaration. It specifies that the function's uintptr arguments may be pointer values that have been converted to uintptr and must be on the heap and kept alive for the duration of the call, even though from the types alone it would appear that the object is no longer needed during the call. The conversion from pointer to uintptr must appear in the argument list of any call to this function. This directive is necessary for some low-level system call implementations and should be avoided otherwise.

//go:noinline

The //go:noinline directive must be followed by a function declaration. It specifies that calls to the function should not be inlined, overriding the compiler's usual optimization rules. This is typically only needed for special runtime functions or when debugging the compiler.

//go:norace

The //go:norace directive must be followed by a function declaration. It specifies that the function's memory accesses must be ignored by the race detector. This is most commonly used in low-level code invoked at times when it is unsafe to call into the race detector runtime.

//go:nosplit

The //go:nosplit directive must be followed by a function declaration. It specifies that the function must omit its usual stack overflow check. This is most commonly used by low-level runtime code invoked at times when it is unsafe for the calling goroutine to be preempted.

//go:linkname localname [importpath.name]

The //go:linkname directive conventionally precedes the var or func declaration named by “localname“, though its position does not change its effect. This directive determines the object-file symbol used for a Go var or func declaration, allowing two Go symbols to alias the same object-file symbol, thereby enabling one package to access a symbol in another package even when this would violate the usual encapsulation of unexported declarations, or even type safety. For that reason, it is only enabled in files that have imported "unsafe".

It may be used in two scenarios. Let's assume that package upper imports package lower, perhaps indirectly. In the first scenario, package lower defines a symbol whose object file name belongs to package upper. Both packages contain a linkname directive: package lower uses the two-argument form and package upper uses the one-argument form. In the example below, lower.f is an alias for the function upper.g:

package upper
import _ "unsafe"
//go:linkname g
func g()

package lower
import _ "unsafe"
//go:linkname f upper.g
func f() { ... }

The linkname directive in package upper suppresses the usual error for a function that lacks a body. (That check may alternatively be suppressed by including a .s file, even an empty one, in the package.)

In the second scenario, package upper unilaterally creates an alias for a symbol in package lower. In the example below, upper.g is an alias for the function lower.f.

package upper
import _ "unsafe"
//go:linkname g lower.f
func g()

package lower
func f() { ... }

The declaration of lower.f may also have a linkname directive with a single argument, f. This is optional, but helps alert the reader that the function is accessed from outside the package.

//go:wasmimport importmodule importname

The //go:wasmimport directive is wasm-only and must be followed by a function declaration. It specifies that the function is provided by a wasm module identified by “importmodule“ and “importname“.

//go:wasmimport a_module f
func g()

The types of parameters and return values to the Go function are translated to Wasm according to the following table:

Go types        Wasm types
int32, uint32   i32
int64, uint64   i64
float32         f32
float64         f64
unsafe.Pointer  i32

Any other parameter types are disallowed by the compiler.