cmd/compile: unnecessary LoadReg

[cherrymui]

What version of Go are you using (go version)?

tip (6f4a458)

Compile this code:

func f(a []int) {
        for i, x := range a {
                g(i, x)
        }
}

//go:noinline
func g(i, x int) {}

It generates

  b1:
    v1 = InitMem <mem>
    v29 = Arg <int> {a} [8] : a+8[int]
    v32 = Arg <*int> {a} : a[*int]
    v6 = LoadReg <int> v29 : AX
    v33 = TESTQ <flags> v6 v6
    GT v33 -> b2 b3 (likely)
  b2: <- b1
    v2 = SP <uintptr> : SP
    v28 = LoadReg <*int> v32 : CX
    v11 = MOVQconst <int> [0] : DX
    Plain -> b4
  b4: <- b2 b5
    v12 = Phi <*int> v28 v3 : CX
    v13 = Phi <mem> v1 v21
    v16 = Phi <int> v11 v17 : DX
    v30 = StoreReg <*int> v12 : .autotmp_5[*int]
    v23 = StoreReg <int> v16 : i[int]
    v15 = MOVQload <int> v12 v13 : BX
    v18 = MOVQstore <mem> v2 v16 v13
    v20 = MOVQstore <mem> [8] v2 v15 v18
    v21 = CALLstatic <mem> {"".g} [16] v20
    v26 = LoadReg <*int> v30 : AX
    v24 = ADDQconst <*int> [8] v26 : AX
    v19 = LoadReg <int> v23 : CX
    v27 = ADDQconst <int> [1] v19 : CX
    v5 = LoadReg <int> v29 : DX
    v7 = CMPQ <flags> v27 v5
    LT v7 -> b5 b3 (likely)
  b5: <- b4
    v17 = Copy <int> v27 : DX
    v3 = Copy <*int> v24 : CX
    v4 = LoadReg <int> v29 : AX     // <------ this
    Plain -> b4
  b3: <- b1 b4
    v31 = Phi <mem> v1 v21
    Ret v31

Note v4 loads a value in AX, but it is never used. It would be good not to generate this.

[randall77]

Pretty sure this is a dup of #14761

[gopherbot]

CL https://golang.org/cl/38457 mentions this issue.