package main

import (
	"fmt"
	"math/big"
)

//go:noinline
func f(x uint32) *big.Int {
	return big.NewInt(int64(x))
}
func main() {
	b := f(0xffffffff)
	c := big.NewInt(0xffffffff)
	if b.Cmp(c) != 0 {
		panic(fmt.Sprintf("b:%x c:%x", b, c))
	}
}

The problem is with CL 522975 @jake-ciolek .

I think the issue is that when we change TESTQ to TESTL, a different bit gets copied into the sign bit of the flags register. We previously copied bit 63 (which is zero in the code above) but now we copy bit 31 (which is 1 in the code above).

So the TEST rule in that CL isn't correct. It would work if we just used the result for == or !=, but when we use the results for ordering comparisons it doesn't work.