cmd/compile: should delete loops without side-effects #69889
Description
As of go1.24-cbdb3545ad, none of the following loops get optimized away:
package main
func main() {
for range 10 {
// Just an empty loop
}
var x int
for range 20 {
// x never gets read after the loop
x++
}
var y [30]int
for i := range y {
// The bounds check gets optimized out, turning this into an empty loop
_ = y[i]
}
for range "13 characters" {
// An empty loop that parses UTF-8
}
const debugFoo = false
const debugBar = false
for range 40 {
// Loop body is dead and gets optimized out
if debugFoo {
println()
}
if debugBar {
println()
}
}
}
main.main STEXT size=152 args=0x0 locals=0x20 funcid=0x0 align=0x0
0x0000 00000 (...../foo.go:3) TEXT main.main(SB), ABIInternal, $32-0
0x0000 00000 (...../foo.go:3) CMPQ SP, 16(R14)
0x0004 00004 (...../foo.go:3) PCDATA $0, $-2
0x0004 00004 (...../foo.go:3) JLS 142
0x000a 00010 (...../foo.go:3) PCDATA $0, $-1
0x000a 00010 (...../foo.go:3) PUSHQ BP
0x000b 00011 (...../foo.go:3) MOVQ SP, BP
0x000e 00014 (...../foo.go:3) SUBQ $24, SP
0x0012 00018 (...../foo.go:3) FUNCDATA $0, gclocals·M83szM6+gDKfH9vuf1h0yw==(SB)
0x0012 00018 (...../foo.go:3) FUNCDATA $1, gclocals·M83szM6+gDKfH9vuf1h0yw==(SB)
0x0012 00018 (...../foo.go:3) XORL AX, AX
0x0014 00020 (...../foo.go:4) JMP 25
0x0016 00022 (...../foo.go:4) INCQ AX
0x0019 00025 (...../foo.go:4) CMPQ AX, $10
0x001d 00029 (...../foo.go:4) JLT 22
0x001f 00031 (...../foo.go:4) XORL AX, AX
0x0021 00033 (...../foo.go:4) JMP 38
0x0023 00035 (...../foo.go:9) INCQ AX
0x0026 00038 (...../foo.go:9) CMPQ AX, $20
0x002a 00042 (...../foo.go:9) JLT 35
0x002c 00044 (...../foo.go:9) XORL AX, AX
0x002e 00046 (...../foo.go:15) JMP 51
0x0030 00048 (...../foo.go:15) INCQ AX
0x0033 00051 (...../foo.go:15) CMPQ AX, $30
0x0037 00055 (...../foo.go:15) JLT 48
0x0039 00057 (...../foo.go:15) XORL AX, AX
0x003b 00059 (...../foo.go:15) JMP 64
0x003d 00061 (...../foo.go:20) MOVQ SI, AX
0x0040 00064 (...../foo.go:20) CMPQ AX, $13
0x0044 00068 (...../foo.go:20) JGE 123
0x0046 00070 (...../foo.go:20) LEAQ go:string."13 characters"(SB), DX
0x004d 00077 (...../foo.go:20) MOVBLZX (DX)(AX*1), SI
0x0051 00081 (...../foo.go:20) CMPL SI, $128
0x0057 00087 (...../foo.go:20) JGE 95
0x0059 00089 (...../foo.go:20) LEAQ 1(AX), SI
0x005d 00093 (...../foo.go:20) JMP 61
0x005f 00095 (...../foo.go:20) MOVL $13, BX
0x0064 00100 (...../foo.go:20) MOVQ AX, CX
0x0067 00103 (...../foo.go:20) MOVQ DX, AX
0x006a 00106 (...../foo.go:20) PCDATA $1, $0
0x006a 00106 (...../foo.go:20) CALL runtime.decoderune(SB)
0x006f 00111 (...../foo.go:20) LEAQ go:string."13 characters"(SB), DX
0x0076 00118 (...../foo.go:20) MOVQ BX, SI
0x0079 00121 (...../foo.go:20) JMP 61
0x007b 00123 (...../foo.go:20) XORL AX, AX
0x007d 00125 (...../foo.go:20) JMP 130
0x007f 00127 (...../foo.go:26) INCQ AX
0x0082 00130 (...../foo.go:26) CMPQ AX, $40
0x0086 00134 (...../foo.go:26) JLT 127
0x0088 00136 (...../foo.go:35) ADDQ $24, SP
0x008c 00140 (...../foo.go:35) POPQ BP
0x008d 00141 (...../foo.go:35) RET
0x008e 00142 (...../foo.go:35) NOP
0x008e 00142 (...../foo.go:3) PCDATA $1, $-1
0x008e 00142 (...../foo.go:3) PCDATA $0, $-2
0x008e 00142 (...../foo.go:3) CALL runtime.morestack_noctxt(SB)
0x0093 00147 (...../foo.go:3) PCDATA $0, $-1
0x0093 00147 (...../foo.go:3) JMP 0