internal/chacha20: use x/sys/cpu for s390x feature detection

Use the recently added CPU feature detection API rather than custom
assembly. This will need to be updated to use 'internal/cpu' when
the package is revendored into std.

Change-Id: Icf0f28236ca0c351c88035f36e5b9c29d4bc2ba7
Reviewed-on: https://go-review.googlesource.com/c/164383Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

internal/chacha20/chacha_s390x.go

@@ -6,14 +6,13 @@
 
 package chacha20
 
-var haveAsm = hasVectorFacility()
+import (
+	"golang.org/x/sys/cpu"
+)
 
-const bufSize = 256
+var haveAsm = cpu.S390X.HasVX
 
-// hasVectorFacility reports whether the machine supports the vector
-// facility (vx).
-// Implementation in asm_s390x.s.
-func hasVectorFacility() bool
+const bufSize = 256
 
 // xorKeyStreamVX is an assembly implementation of XORKeyStream. It must only
 // be called when the vector facility is available.

internal/chacha20/chacha_s390x.s

@@ -258,26 +258,3 @@ tail:
 	MOVD R8, R3
 	MOVD $0, R4
 	JMP  continue
-
-// func hasVectorFacility() bool
-TEXT ·hasVectorFacility(SB), NOSPLIT, $24-1
-	MOVD  $x-24(SP), R1
-	XC    $24, 0(R1), 0(R1) // clear the storage
-	MOVD  $2, R0            // R0 is the number of double words stored -1
-	WORD  $0xB2B01000       // STFLE 0(R1)
-	XOR   R0, R0            // reset the value of R0
-	MOVBZ z-8(SP), R1
-	AND   $0x40, R1
-	BEQ   novector
-
-vectorinstalled:
-	// check if the vector instruction has been enabled
-	VLEIB  $0, $0xF, V16
-	VLGVB  $0, V16, R1
-	CMPBNE R1, $0xF, novector
-	MOVB   $1, ret+0(FP)      // have vx
-	RET
-
-novector:
-	MOVB $0, ret+0(FP) // no vx
-	RET