syscall: avoid zeroing unused syscall arguments

Zeroing unused registers is not required. Removing it makes the code
very slightly smaller and very slightly faster.

Change-Id: I1ec17b497db971ca8a3641e3e94c063571419f27
GitHub-Last-Rev: f721bb263637717e8ff9fd2c34148b5b2762e8c4
GitHub-Pull-Request: golang/go#31596
Reviewed-on: https://go-review.googlesource.com/c/go/+/173160Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/asm_darwin_amd64.s

@@ -17,9 +17,6 @@ TEXT	·Syscall(SB),NOSPLIT,$0-56
 	MOVQ	a1+8(FP), DI
 	MOVQ	a2+16(FP), SI
 	MOVQ	a3+24(FP), DX
-	MOVQ	$0, R10
-	MOVQ	$0, R8
-	MOVQ	$0, R9
 	MOVQ	trap+0(FP), AX	// syscall entry
 	ADDQ	$0x2000000, AX
 	SYSCALL
@@ -100,9 +97,6 @@ TEXT ·RawSyscall(SB),NOSPLIT,$0-56
 	MOVQ	a1+8(FP), DI
 	MOVQ	a2+16(FP), SI
 	MOVQ	a3+24(FP), DX
-	MOVQ	$0, R10
-	MOVQ	$0, R8
-	MOVQ	$0, R9
 	MOVQ	trap+0(FP), AX	// syscall entry
 	ADDQ	$0x2000000, AX
 	SYSCALL

src/syscall/asm_linux_amd64.s

@@ -19,9 +19,6 @@ TEXT ·Syscall(SB),NOSPLIT,$0-56
 	MOVQ	a1+8(FP), DI
 	MOVQ	a2+16(FP), SI
 	MOVQ	a3+24(FP), DX
-	MOVQ	$0, R10
-	MOVQ	$0, R8
-	MOVQ	$0, R9
 	MOVQ	trap+0(FP), AX	// syscall entry
 	SYSCALL
 	CMPQ	AX, $0xfffffffffffff001
@@ -70,9 +67,6 @@ TEXT ·RawSyscall(SB),NOSPLIT,$0-56
 	MOVQ	a1+8(FP), DI
 	MOVQ	a2+16(FP), SI
 	MOVQ	a3+24(FP), DX
-	MOVQ	$0, R10
-	MOVQ	$0, R8
-	MOVQ	$0, R9
 	MOVQ	trap+0(FP), AX	// syscall entry
 	SYSCALL
 	CMPQ	AX, $0xfffffffffffff001
@@ -139,9 +133,6 @@ TEXT ·rawSyscallNoError(SB),NOSPLIT,$0-48
 	MOVQ	a1+8(FP), DI
 	MOVQ	a2+16(FP), SI
 	MOVQ	a3+24(FP), DX
-	MOVQ	$0, R10
-	MOVQ	$0, R8
-	MOVQ	$0, R9
 	MOVQ	trap+0(FP), AX	// syscall entry
 	SYSCALL
 	MOVQ	AX, r1+32(FP)

src/syscall/asm_unix_amd64.s

@@ -21,9 +21,6 @@ TEXT	·Syscall(SB),NOSPLIT,$0-56
 	MOVQ	a1+8(FP), DI
 	MOVQ	a2+16(FP), SI
 	MOVQ	a3+24(FP), DX
-	MOVQ	$0, R10
-	MOVQ	$0, R8
-	MOVQ	$0, R9
 	SYSCALL
 	JCC	ok
 	MOVQ	$-1, r1+32(FP)	// r1
@@ -65,9 +62,6 @@ TEXT	·RawSyscall(SB),NOSPLIT,$0-56
 	MOVQ	a1+8(FP), DI
 	MOVQ	a2+16(FP), SI
 	MOVQ	a3+24(FP), DX
-	MOVQ	$0, R10
-	MOVQ	$0, R8
-	MOVQ	$0, R9
 	MOVQ	trap+0(FP), AX	// syscall entry
 	SYSCALL
 	JCC	ok1