runtime: GOMAXPROCS ignores the requested value

[cpunion]

Summary

runtime.GOMAXPROCS(n) in an LLGo-generated program returns the host maximum but does not apply positive n. This also makes go test -cpu=1 ineffective and causes the testing package to report that every benchmark left GOMAXPROCS at the host CPU count.

Tested on xgo-dev/llgo main at 60c30f2ff, Go 1.26.5, LLVM 22.1.8, Darwin/arm64.

Minimal reproduction

package main

import "runtime"

func main() {
    before := runtime.GOMAXPROCS(0)
    previous := runtime.GOMAXPROCS(1)
    after := runtime.GOMAXPROCS(0)
    println(before, previous, after)
    if previous != before || after != 1 {
        panic("runtime.GOMAXPROCS did not apply the requested value")
    }
}

With GOMAXPROCS=4, the host Go toolchain prints:

4 4 1

The current LLGo main build prints and then panics:

16 16 16
panic: runtime.GOMAXPROCS did not apply the requested value

The implementation currently ignores n:

func GOMAXPROCS(n int) int {
    return int(c_maxprocs())
}

Impact

• testing cannot enforce -test.cpu / go test -cpu.
• Benchmarks emit messages such as testing: BenchmarkDirectCall left GOMAXPROCS set to 16.
• Programs that use GOMAXPROCS to bound parallelism observe incorrect standard-library behavior.

A regression test can use the program above and assert that the setter returns the previous value and that a subsequent query returns the requested value.