netdev - RE: [PATCH net-next 06/12] qlcnic: Convert nested if-else to switch-case

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Date:	Fri, 24 May 2013 09:36:49 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Shahed Shaikh" <shahed.shaikh@...gic.com>, <davem@...emloft.net>
Cc:	<netdev@...r.kernel.org>, <Dept_NX_Linux_NIC_Driver@...gic.com>,
	"Jitendra Kalsaria" <jitendra.kalsaria@...gic.com>
Subject: RE: [PATCH net-next 06/12] qlcnic: Convert nested if-else to switch-case

> -
> -	if (priv_level == QLCNIC_NON_PRIV_FUNC) {
> +	switch (priv_level) {
> +	case QLCNIC_NON_PRIV_FUNC:
>  		ahw->op_mode = QLCNIC_NON_PRIV_FUNC;
>  		ahw->idc.state_entry = qlcnic_83xx_idc_ready_state_entry;
>  		nic_ops->init_driver = qlcnic_83xx_init_non_privileged_vnic;
> -	} else if (priv_level == QLCNIC_PRIV_FUNC) {
> +		break;
> +	case QLCNIC_PRIV_FUNC:
>  		ahw->op_mode = QLCNIC_PRIV_FUNC;
>  		ahw->idc.state_entry = qlcnic_83xx_idc_vnic_pf_entry;
>  		nic_ops->init_driver = qlcnic_83xx_init_privileged_vnic;
> -	} else if (priv_level == QLCNIC_MGMT_FUNC) {
> +		break;
> +	case QLCNIC_MGMT_FUNC:
>  		ahw->op_mode = QLCNIC_MGMT_FUNC;
>  		ahw->idc.state_entry = qlcnic_83xx_idc_ready_state_entry;
>  		nic_ops->init_driver = qlcnic_83xx_init_mgmt_vnic;
> -	} else {
> +		break;
> +	default:
> +		dev_err(&adapter->pdev->dev, "Invalid Virtual NIC opmode\n");
>  		return -EIO;
>  	}

If the NON_PRIV case happens most of the time, then the old code would
have a single conditional branch that could easily be predicted correctly.
The switch statement version will either be a compare and an indirect jump
(a pipeline stall on a lot of cpus) or a compare-branch chain.

So this is necessarily an improvent!

	David



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