cmd/compile: fix wrong complement for arm64 floating-point comparisons

cmd/compile: fix wrong complement for arm64 floating-point comparisons

Consider the following example,

  func test(a, b float64, x uint64) uint64 {
    if a < b {
      x = 0
    }
    return x
  }

  func main() {
    fmt.Println(test(1, math.NaN(), 123))
  }

The output is 0, but the expectation is 123.

This is because the rewrite rule

  (CSEL [cc] (MOVDconst [0]) y flag) => (CSEL0 [arm64Negate(cc)] y flag)

converts

  FCMP NaN, 1
  CSEL MI, 0, 123, R0 // if 1 < NaN then R0 = 0 else R0 = 123

to

  FCMP NaN, 1
  CSEL GE, 123, 0, R0 // if 1 >= NaN then R0 = 123 else R0 = 0

But both 1 < NaN and 1 >= NaN are false. So the output is 0, not 123.

The root cause is arm64Negate not handle negation of floating comparison
correctly. According to the ARM manual, the meaning of MI, GE, and PL
are

  MI: Less than
  GE: Greater than or equal to
  PL: Greater than, equal to, or unordered

Because NaN cannot be compared with other numbers, the result of such
comparison is unordered. So when NaN is involved, unlike integer, the
result of !(a < b) is not a >= b, it is a >= b || a is NaN || b is NaN.
This is exactly what PL means. We add NotLessThanF to represent PL. Then
the negation of LessThanF is NotLessThanF rather than GreaterEqualF. The
same reason for the other floating comparison operations.

Fixes #43619

Change-Id: Ia511b0027ad067436bace9fbfd261dbeaae01bcd
Reviewed-on: https://go-review.googlesource.com/c/go/+/283572
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Keith Randall <khr@golang.org>