MOVQ	_cgo_sys_thread_create(SB), AX
	TESTQ	AX, AX
	JZ	nocgo

	// We're calling back to C.
	// Align stack per ELF ABI requirements.
	MOVQ	SP, BX  // Callee-save in C ABI
	ANDQ	$~15, SP
	MOVQ	$_rt0_amd64_lib_go(SB), DI
	MOVQ	$0, SI
	CALL	AX
	MOVQ	BX, SP
	JMP	restore

nocgo:
	ADJSP	$16
	MOVQ	$0x800000, 0(SP)		// stacksize
	MOVQ	$_rt0_amd64_lib_go(SB), AX
	MOVQ	AX, 8(SP)			// fn

// Lowering shifts
// Unsigned shifts need to return 0 if shift amount is >= width of shifted value.
//   result = (arg << shift) & (shift >= argbits ? 0 : 0xffffffffffffffff)
(Lsh64x(64|32|16|8) <t> x y) && !shiftIsBounded(v) => (ANDQ (SHLQ <t> x y) (SBBQcarrymask <t> (CMP(Q|L|W|B)const y [64])))
(Lsh32x(64|32|16|8) <t> x y) && !shiftIsBounded(v) => (ANDL (SHLL <t> x y) (SBBLcarrymask <t> (CMP(Q|L|W|B)const y [32])))