math: optimize dim and remove s390x assembly implementation

By calculating dim directly, rather than calling max, we can simplify
the generated code significantly. The compiler now reports that dim
is easily inlineable, but it can't be inlined because there is still
an assembly stub for Dim.

Since dim is now very simple I no longer think it is worth having
assembly implementations of it. I have therefore removed the s390x
assembly. Removing the other assembly for Dim is #21913.

name  old time/op  new time/op  delta
Dim   4.29ns ± 0%  3.53ns ± 0%  -17.62%  (p=0.000 n=9+8)

Change-Id: Ic38a6b51603cbc661dcdb868ecf2b1947e9f399e
Reviewed-on: https://go-review.googlesource.com/64194
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/dim.go

@@ -13,7 +13,18 @@ package math
 func Dim(x, y float64) float64
 
 func dim(x, y float64) float64 {
-	return max(x-y, 0)
+	// The special cases result in NaN after the subtraction:
+	//      +Inf - +Inf = NaN
+	//      -Inf - -Inf = NaN
+	//       NaN - y    = NaN
+	//         x - NaN  = NaN
+	v := x - y
+	if v <= 0 {
+		// v is negative or 0
+		return 0
+	}
+	// v is positive or NaN
+	return v
 }
 
 // Max returns the larger of x or y.

src/math/dim_s390x.s

@@ -10,42 +10,6 @@
 #define NaN    0x7FF8000000000001
 #define NegInf 0xFFF0000000000000
 
-// func Dim(x, y float64) float64
-TEXT ·Dim(SB),NOSPLIT,$0
-	// (+Inf, +Inf) special case
-	MOVD    x+0(FP), R2
-	MOVD    y+8(FP), R3
-	MOVD    $PosInf, R4
-	CMPUBNE R4, R2, dim2
-	CMPUBEQ R4, R3, bothInf
-dim2:	// (-Inf, -Inf) special case
-	MOVD    $NegInf, R4
-	CMPUBNE R4, R2, dim3
-	CMPUBEQ R4, R3, bothInf
-dim3:	// (NaN, x) or (x, NaN)
-	MOVD    $~(1<<63), R5
-	MOVD    $PosInf, R4
-	AND     R5, R2 // x = |x|
-	CMPUBLT R4, R2, isDimNaN
-	AND     R5, R3 // y = |y|
-	CMPUBLT R4, R3, isDimNaN
-
-	FMOVD   x+0(FP), F1
-	FMOVD   y+8(FP), F2
-	FSUB    F2, F1
-	FMOVD   $(0.0), F2
-	FCMPU   F2, F1
-	BGE     +3(PC)
-	FMOVD   F1, ret+16(FP)
-	RET
-	FMOVD   F2, ret+16(FP)
-	RET
-bothInf: // Dim(-Inf, -Inf) or Dim(+Inf, +Inf)
-isDimNaN:
-	MOVD    $NaN, R4
-	MOVD    R4, ret+16(FP)
-	RET
-
 // func ·Max(x, y float64) float64
 TEXT ·Max(SB),NOSPLIT,$0
 	// +Inf special cases

src/math/stubs_s390x.s

@@ -4,6 +4,9 @@
 
 #include "textflag.h"
 
+TEXT ·Dim(SB),NOSPLIT,$0
+	BR ·dim(SB)
+
 TEXT ·Exp2(SB),NOSPLIT,$0
 	BR ·exp2(SB)