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Re: option -mprfchw
- From: NightStrike <nightstrike at gmail dot com>
- To: gcc-help <gcc-help at gcc dot gnu dot org>
- Date: Sun, 24 Apr 2016 22:36:22 -0400
- Subject: Re: option -mprfchw
- Authentication-results: sourceware.org; auth=none
- References: <CAF1jjLu50nNXw4dOOvhrLN9vQCwrE6aVcTiPVwJ0QiYyDFdFBA at mail dot gmail dot com> <CAF1jjLtqmxW547y15gHmsQXMw7jyuK=1K-Ft1r0Wr5jvvFaL+A at mail dot gmail dot com>
Ping?
On Mon, Apr 18, 2016 at 12:38 AM, NightStrike <nightstrike@gmail.com> wrote:
> Also, FWIW:
>
> 1) The march=native version that uses prefetcht0 is very repeatedly
> faster by about 15% in the particular test case I'm looking at.
>
> 2) The compilers in both instances are not just the same version, they
> are the same compiler installed on an NFS mount and shared to both
> computers.
>
>
> On Mon, Apr 18, 2016 at 12:26 AM, NightStrike <nightstrike@gmail.com> wrote:
>> Not sure if this applies:
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54210
>>
>> If I compile on a k8 Opteron 248 with -march=native, I do not see
>> -mprfchw listed in the options in -fverbose-asm. In the assembly, I
>> see this:
>>
>> prefetcht0 (%rax) # ivtmp.1160
>> prefetcht0 304(%rcx) #
>> prefetcht0 (%rax) # ivtmp.1160
>>
>> If I compile on a bdver2 Opteron 6386 SE with -march=k8 (thus trying
>> to target the older system), I do see it listed in the options in
>> -fverbose-asm. In the assembly, I see this:
>>
>> prefetcht0 (%rax) # ivtmp.1160
>> prefetcht0 304(%rcx) #
>> prefetchw (%rax) # ivtmp.1160
>>
>> (The third line is the only difference)
>>
>> In both cases, I'm using gcc 4.9.3. Which is correct for a k8 Opteron 248?