Inline Expansion
Remarks#
Inline expansion is a common optimization in compiled code that prioritized performance over binary size. It lets the compiler replace a function call with the actual body of the function; effectively copy/pasting code from one place to another at compile time. Since the call site is expanded to just contain the machine instructions that the compiler generated for the function, we don’t have to perform a CALL or PUSH (the x86 equivalant of a GOTO statement or a stack frame push) or their equivalant on other architectures.
The inliner makes decisions about whether or not to inline a function based on a number of heuristics, but in general Go inlines by default. Because the inliner gets rid of function calls, it effectively gets to decide where the scheduler is allowed to preempt a goroutine.
Function calls will not be inlined if any of the following are true (there are many other reasons too, this list is incomplete):
- Functions are variadic (eg. they have
...args) - Functions have a “max hairyness” greater than the budget (they recurse too much or can’t be analyzed for some other reason)
- They contain
panic,recover, ordefer
Disabling inline expansion
Inline expansion can be disabled with the go:noinline pragma. For example, if we build the following simple program:
package main
func printhello() {
println("Hello")
}
func main() {
printhello()
}we get output that looks like this (trimmed for readability):
$ go version
go version go1.6.2 linux/amd64
$ go build main.go
$ ./main
Hello
$ go tool objdump main
TEXT main.main(SB) /home/sam/main.go
main.go:7 0x401000 64488b0c25f8ffffff FS MOVQ FS:0xfffffff8, CX
main.go:7 0x401009 483b6110 CMPQ 0x10(CX), SP
main.go:7 0x40100d 7631 JBE 0x401040
main.go:7 0x40100f 4883ec10 SUBQ $0x10, SP
main.go:8 0x401013 e8281f0200 CALL runtime.printlock(SB)
main.go:8 0x401018 488d1d01130700 LEAQ 0x71301(IP), BX
main.go:8 0x40101f 48891c24 MOVQ BX, 0(SP)
main.go:8 0x401023 48c744240805000000 MOVQ $0x5, 0x8(SP)
main.go:8 0x40102c e81f290200 CALL runtime.printstring(SB)
main.go:8 0x401031 e89a210200 CALL runtime.printnl(SB)
main.go:8 0x401036 e8851f0200 CALL runtime.printunlock(SB)
main.go:9 0x40103b 4883c410 ADDQ $0x10, SP
main.go:9 0x40103f c3 RET
main.go:7 0x401040 e87b9f0400 CALL runtime.morestack_noctxt(SB)
main.go:7 0x401045 ebb9 JMP main.main(SB)
main.go:7 0x401047 cc INT $0x3
main.go:7 0x401048 cc INT $0x3
main.go:7 0x401049 cc INT $0x3
main.go:7 0x40104a cc INT $0x3
main.go:7 0x40104b cc INT $0x3
main.go:7 0x40104c cc INT $0x3
main.go:7 0x40104d cc INT $0x3
main.go:7 0x40104e cc INT $0x3
main.go:7 0x40104f cc INT $0x3
…note that there is no CALL to printhello. However, if we then build the program with the pragma in place:
package main
//go:noinline
func printhello() {
println("Hello")
}
func main() {
printhello()
}The output contains the printhello function and a CALL main.printhello:
$ go version
go version go1.6.2 linux/amd64
$ go build main.go
$ ./main
Hello
$ go tool objdump main
TEXT main.printhello(SB) /home/sam/main.go
main.go:4 0x401000 64488b0c25f8ffffff FS MOVQ FS:0xfffffff8, CX
main.go:4 0x401009 483b6110 CMPQ 0x10(CX), SP
main.go:4 0x40100d 7631 JBE 0x401040
main.go:4 0x40100f 4883ec10 SUBQ $0x10, SP
main.go:5 0x401013 e8481f0200 CALL runtime.printlock(SB)
main.go:5 0x401018 488d1d01130700 LEAQ 0x71301(IP), BX
main.go:5 0x40101f 48891c24 MOVQ BX, 0(SP)
main.go:5 0x401023 48c744240805000000 MOVQ $0x5, 0x8(SP)
main.go:5 0x40102c e83f290200 CALL runtime.printstring(SB)
main.go:5 0x401031 e8ba210200 CALL runtime.printnl(SB)
main.go:5 0x401036 e8a51f0200 CALL runtime.printunlock(SB)
main.go:6 0x40103b 4883c410 ADDQ $0x10, SP
main.go:6 0x40103f c3 RET
main.go:4 0x401040 e89b9f0400 CALL runtime.morestack_noctxt(SB)
main.go:4 0x401045 ebb9 JMP main.printhello(SB)
main.go:4 0x401047 cc INT $0x3
main.go:4 0x401048 cc INT $0x3
main.go:4 0x401049 cc INT $0x3
main.go:4 0x40104a cc INT $0x3
main.go:4 0x40104b cc INT $0x3
main.go:4 0x40104c cc INT $0x3
main.go:4 0x40104d cc INT $0x3
main.go:4 0x40104e cc INT $0x3
main.go:4 0x40104f cc INT $0x3
TEXT main.main(SB) /home/sam/main.go
main.go:8 0x401050 64488b0c25f8ffffff FS MOVQ FS:0xfffffff8, CX
main.go:8 0x401059 483b6110 CMPQ 0x10(CX), SP
main.go:8 0x40105d 7606 JBE 0x401065
main.go:9 0x40105f e89cffffff CALL main.printhello(SB)
main.go:10 0x401064 c3 RET
main.go:8 0x401065 e8769f0400 CALL runtime.morestack_noctxt(SB)
main.go:8 0x40106a ebe4 JMP main.main(SB)
main.go:8 0x40106c cc INT $0x3
main.go:8 0x40106d cc INT $0x3
main.go:8 0x40106e cc INT $0x3
main.go:8 0x40106f cc INT $0x3
…