cmd/compile: channel operations require but do not use a type dictionary

[mcy]

Consider the following program:

package p

func f[T any](s chan T) {
    s <- <-s
}

var x = f[int]

This produces:

        TEXT    command-line-arguments.f[go.shape.int](SB), DUPOK|ABIInternal, $40-16
        PUSHQ   BP
        MOVQ    SP, BP
        SUBQ    $32, SP
        MOVQ    BX, .s+56(SP)
        MOVQ    $0, ..autotmp_2+24(SP)
        MOVQ    BX, AX // (lmao)
        LEAQ    ..autotmp_2+24(SP), BX
        CALL    runtime.chanrecv1(SB)
        MOVQ    ..autotmp_2+24(SP), CX
        MOVQ    CX, ..autotmp_3+16(SP)
        MOVQ    .s+56(SP), AX
        LEAQ    ..autotmp_3+16(SP), BX
        NOP
        CALL    runtime.chansend1(SB)
        ADDQ    $32, SP
        POPQ    BP
        RET

Notice (lmao), where rax is unceremoniously clobbered. This is the type dictionary, which is not used by either chanrecv or chansend, because those functions only take the channel pointer itself and a pointer to the input or output. Other channel operations have similar ABI.

[gabyhelp]

Related Issues

• cmd/compile: closures in generic functions capture type dictionaries unconditionally #78370
• runtime/internal/maps: `runtime.mapaccess2_fast64` causes unnecessary type dictionary requirement #78378
• runtime: chanrecv funcs don't actually use the *chantype parameter #19591 (closed)
• cmd/compile: optimize away select receive cases with known-zero chans #75608

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[mwriter]

s <- <-s

Wow, and where can this be practically useful?

[mcy]

It's just a silly gadget to generate code that both sends and receives to some channel without me needing to get my hands on a variable of type T.

[dmitshur]

CC @golang/compiler.