On 08/07/2013 06:13 AM, Eric W. Biederman wrote:
> Andrew Morton <a...@linux-foundation.org> writes:
>
>> On Tue, 06 Aug 2013 15:29:42 +0800 Chen Gang <gang.c...@asianux.com> wrote:
>>
>>> Improve the usage of return value 'result', so not only can make code
>>> clearer to readers, but also can improve the performance.
>>
>> It used to be pervasive kernel style do to
>>
>> ret = -ENOMEM;
>> foo = alloc(...);
>> if (!foo)
>> goto out;
>>
>> whereas nowadays people usually do the more straightforward
>>
>> foo = alloc(...);
>> if (!foo) {
>> ret = -ENOMEM;
>> goto out;
>> }
>>
>> The thinking was that the old style generated better code, but for the
>> life of me I can't remember why :(
>
> Because doing the assignment outside of the if() goto . Allows the
> compiler to emit the if() goto as a single branch.
>
> While a smart compiler may perform the code motion across the branch,
> it is much easier for the compiler to branch to somewhere else perform
> the assignment and then branch out.
>
For my opinion, for assembly code, the old style is clearer than the new
style. And commonly, the old style will be faster than new style.
Thanks.
> Eric
>
>
>> Your patch switches from old-style to new-style. And it appears to
>> have increased the text size. I did this, to switch three sites back
>> to old-style:
>>
>> ---
>> a/kernel/sysctl_binary.c~kernel-sysctl_binaryc-improve-the-usage-of-return-value-result-fix
>> +++ a/kernel/sysctl_binary.c
>> @@ -941,17 +941,15 @@ static ssize_t bin_string(struct file *f
>> copied = result;
>> lastp = oldval + copied - 1;
>>
>> - if (get_user(ch, lastp)) {
>> - result = -EFAULT;
>> + result = -EFAULT;
>> + if (get_user(ch, lastp))
>> goto out;
>> - }
>>
>> /* Trim off the trailing newline */
>> if (ch == '\n') {
>> - if (put_user('\0', lastp)) {
>> - result = -EFAULT;
>> + result = -EFAULT;
>> + if (put_user('\0', lastp))
>> goto out;
>> - }
>> copied -= 1;
>> }
>> }
>> @@ -976,11 +974,10 @@ static ssize_t bin_intvec(struct file *f
>> char *buffer;
>> ssize_t result;
>>
>> + result = -ENOMEM;
>> buffer = kmalloc(BUFSZ, GFP_KERNEL);
>> - if (!buffer) {
>> - result = -ENOMEM;
>> + if (!buffer)
>> goto out;
>> - }
>>
>> if (oldval && oldlen) {
>> unsigned __user *vec = oldval;
>> _
>>
>> and kernel/sysctl_binary.o's .text got six bytes smaller.
>>
>> Now, smaller text doesn't mean faster code. But it probably means
>> larger cache footprint, which can mean slower code.
>>
>> IOW, it isn't obvious that this was an improvement.
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--
Chen Gang
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