Re: [Xen-devel] [PATCH] x86/string: Use compiler builtins whenever possible

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.08.16 at 16:27, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 09/08/16 15:01, Jan Beulich wrote:
>>>>> On 09.08.16 at 14:41, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> The use of -fno-builtin inhibits this automatic transformation.  Manually
>>> tranform the callsites.  This causes constructs such as strlen("literal") to
>>> be evaluated at compile time, and certain simple operations to be replaced
>>> with repeated string operations.
>> Iirc this was specifically avoided in at least older Linux because of
>> compiler issues. Are you reasonably certain that those issues
>> predate gcc 4.1.x (and don't exist at all in clang)? Other than that
>> I'm of course fine with this patch.
> 
> Do you have any reference to these issues?  I cant find any reference.

This was when I barley started working with Linux, i.e. in late 2.4.x
or early 2.6.x times. Querying Google seems to indicate that
someone reported successful use in 2.6.3.

> I notice that Linux doesn't use these builtins, even today, but does
> have arch specific implementations which are more efficient than our
> common ones.  Still, it doesn't appear to be able to do any compile-time
> evaluation.

Isn't it surprising that no-one has submitted Linux patches to
change that? Anyway, please consider the patch ack-ed. We
can always revert it in case we run into problems (and once
my hardware setup will be fully functional again, I will test with
a reasonably old gcc anyway).

Jan


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