cmd/compile: optimize cmp to cmn under conditions < and >= on arm64
Under the right conditions we can optimize cmp comparisons to cmn
comparisons, such as:
func foo(a, b int) int {
var c int
if a + b < 0 {
c = 1
}
return c
}
Previously it's compiled as:
ADD R1, R0, R1
CMP $0, R1
CSET LT, R0
With this CL it's compiled as:
CMN R1, R0
CSET MI, R0
Here we need to pay attention to the overflow situation of a+b, the MI
flag means N==1, which doesn't honor the overflow flag V, its value
depends only on the sign of the result. So it has the same semantic of
the Go code, so it's correct.
Similarly, this CL also optimizes the case of >= comparison
using the PL conditional flag.
Change-Id: I47179faba5b30cca84ea69bafa2ad5241bf6dfba
Reviewed-on: https://go-review.googlesource.com/c/go/+/476116
Run-TryBot: Eric Fang <eric.fang@arm.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>