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>

1 files changed, 43 insertions, 0 deletions

diff --git a/test/codegen/comparisons.go b/test/codegen/comparisons.go
index ee732e06d9..99589c4ce8 100644
--- a/test/codegen/comparisons.go
+++ b/test/codegen/comparisons.go
@@ -456,6 +456,7 @@ func CmpToZero_ex5(e, f int32, u uint32) int {
 	}
 	return 0
 }
+
 func UintLtZero(a uint8, b uint16, c uint32, d uint64) int {
 	// amd64: -`(TESTB|TESTW|TESTL|TESTQ|JCC|JCS)`
 	// arm64: -`(CMPW|CMP|BHS|BLO)`
@@ -704,3 +705,45 @@ func cmpToCmn(a, b, c, d int) int {
 	}
 	return c1 + c2 + c3 + c4 + c5 + c6 + c7 + c8 + c9 + c10 + c11
 }
+
+func cmpToCmnLessThan(a, b, c, d int) int {
+	var c1, c2, c3, c4 int
+	// arm64:`CMN`,`CSET\tMI`,-`CMP`
+	if a+1 < 0 {
+		c1 = 1
+	}
+	// arm64:`CMN`,`CSET\tMI`,-`CMP`
+	if a+b < 0 {
+		c2 = 1
+	}
+	// arm64:`CMN`,`CSET\tMI`,-`CMP`
+	if a*b+c < 0 {
+		c3 = 1
+	}
+	// arm64:`CMP`,`CSET\tMI`,-`CMN`
+	if a-b*c < 0 {
+		c4 = 1
+	}
+	return c1 + c2 + c3 + c4
+}
+
+func cmpToCmnGreaterThanEqual(a, b, c, d int) int {
+	var c1, c2, c3, c4 int
+	// arm64:`CMN`,`CSET\tPL`,-`CMP`
+	if a+1 >= 0 {
+		c1 = 1
+	}
+	// arm64:`CMN`,`CSET\tPL`,-`CMP`
+	if a+b >= 0 {
+		c2 = 1
+	}
+	// arm64:`CMN`,`CSET\tPL`,-`CMP`
+	if a*b+c >= 0 {
+		c3 = 1
+	}
+	// arm64:`CMP`,`CSET\tPL`,-`CMN`
+	if a-b*c >= 0 {
+		c4 = 1
+	}
+	return c1 + c2 + c3 + c4
+}