linux-kernel - RE: Why is (2 < 2) true? Is it a gcc bug?

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Message-ID: <D9FFE20C522965449E182ACE73889AEB1A6A352E@IRSMSX105.ger.corp.intel.com>
Date:	Fri, 17 Jan 2014 13:55:00 +0000
From:	"Dorau, Lukasz" <lukasz.dorau@...el.com>
To:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>
Subject: RE: Why is (2 < 2) true? Is it a gcc bug?

On Friday, January 17, 2014 2:37 PM Dorau, Lukasz <lukasz.dorau@...el.com> wrote:
> 
> Hi
> 
> My story is very simply...
> I applied the following patch:
> 
> diff --git a/drivers/scsi/isci/init.c b/drivers/scsi/isci/init.c
> --- a/drivers/scsi/isci/init.c
> +++ b/drivers/scsi/isci/init.c
> @@ -698,8 +698,11 @@ static int isci_pci_probe(struct pci_dev *pdev, const struct
> pci_device_id *id)
>  	if (err)
>  		goto err_host_alloc;
> 
> -	for_each_isci_host(i, isci_host, pdev)
> +	for_each_isci_host(i, isci_host, pdev) {
> +		pr_err("(%d < %d) == %d\n",\
> +		       i, SCI_MAX_CONTROLLERS, (i < SCI_MAX_CONTROLLERS));
>  		scsi_scan_host(to_shost(isci_host));
> +	}
> 
>  	return 0;
> 
> --
> 1.8.3.1
> 
> Then I issued the command 'modprobe isci' on platform with two SCU controllers
> (Patsburg D or T chipset)
> and received the following, very strange, output:
> 
> (0 < 2) == 1
> (1 < 2) == 1
> (2 < 2) == 1
> 
> Can anyone explain why (2 < 2) is true? Is it a gcc bug?
> 
> (The kernel was compiled using gcc version 4.8.2.)
> 

Some additional information:

The loop 'for' in macro ' for_each_isci_host ' defined as (drivers/scsi/isci/host.h:313):

#define for_each_isci_host(id, ihost, pdev) \
        for (id = 0, ihost = to_pci_info(pdev)->hosts[id]; \
             id < ARRAY_SIZE(to_pci_info(pdev)->hosts) && ihost; \
             ihost = to_pci_info(pdev)->hosts[++id])

should be executed only for i = 0 and 1, because:
ARRAY_SIZE(to_pci_info(pdev)->hosts) = SCI_MAX_CONTROLLERS = 2

but it was executed also for i=2 regardless the above loop's end condition. 

Lukasz

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