* L1/L2 cache characteristics and -mtune=znver2 versus run-time detection
@ 2020-12-05 17:46 Sven C. Dack
2020-12-16 16:30 ` Sven C. Dack
0 siblings, 1 reply; 4+ messages in thread
From: Sven C. Dack @ 2020-12-05 17:46 UTC (permalink / raw)
To: gcc-help
Hello,
I've come across an unexpected behaviour of gcc and wanted to ask for
help. I've noticed that when I specify '-march=znver2' does gcc use the
incorrect values for the L1/L2 cache. Only when '-march=native' was used
and gcc detected the CPU at run-time does it use the correct values. For
example on a Ryzen 7 3800X (a Zen 2 CPU):
$ gcc -march=znver2 --help=params -Q|fgrep cache
--param=l1-cache-line-size= 32
--param=l1-cache-size= 64
--param=l2-cache-size= 512
versus:
$ gcc -march=native --help=params -Q|fgrep cache
--param=l1-cache-line-size= 64
--param=l1-cache-size= 32
--param=l2-cache-size= 512
GCC does detect the CPU as 'znver2':
$ gcc -march=native --help=target -Q|fgrep march
-march= znver2
I've also verified this behaviour with '-mtune=znver2', and again does
GCC only use the correct values when it uses auto-detection.
Why does GCC not use the correct L1/L2 cache values when '-march=znver2'
or '-mtune=znver2' is specified?
Sven
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: L1/L2 cache characteristics and -mtune=znver2 versus run-time detection
2020-12-05 17:46 L1/L2 cache characteristics and -mtune=znver2 versus run-time detection Sven C. Dack
@ 2020-12-16 16:30 ` Sven C. Dack
2020-12-16 17:33 ` Segher Boessenkool
0 siblings, 1 reply; 4+ messages in thread
From: Sven C. Dack @ 2020-12-16 16:30 UTC (permalink / raw)
To: gcc-help
Could some of the GCC experts please share their knowledge on this issue?
On 05/12/2020 17:46, Sven C. Dack wrote:
> Hello,
>
> I've come across an unexpected behaviour of gcc and wanted to ask for
> help. I've noticed that when I specify '-march=znver2' does gcc use the
> incorrect values for the L1/L2 cache. Only when '-march=native' was used
> and gcc detected the CPU at run-time does it use the correct values. For
> example on a Ryzen 7 3800X (a Zen 2 CPU):
>
> $ gcc -march=znver2 --help=params -Q|fgrep cache
> --param=l1-cache-line-size= 32
> --param=l1-cache-size= 64
> --param=l2-cache-size= 512
>
> versus:
>
> $ gcc -march=native --help=params -Q|fgrep cache
> --param=l1-cache-line-size= 64
> --param=l1-cache-size= 32
> --param=l2-cache-size= 512
>
> GCC does detect the CPU as 'znver2':
>
> $ gcc -march=native --help=target -Q|fgrep march
> -march= znver2
>
> I've also verified this behaviour with '-mtune=znver2', and again does
> GCC only use the correct values when it uses auto-detection.
>
> Why does GCC not use the correct L1/L2 cache values when '-march=znver2'
> or '-mtune=znver2' is specified?
>
> Sven
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: L1/L2 cache characteristics and -mtune=znver2 versus run-time detection
2020-12-16 16:30 ` Sven C. Dack
@ 2020-12-16 17:33 ` Segher Boessenkool
[not found] ` <3f4c127e-3cdd-5a4e-ef56-d5d62a88e30a@gmx.com>
0 siblings, 1 reply; 4+ messages in thread
From: Segher Boessenkool @ 2020-12-16 17:33 UTC (permalink / raw)
To: Sven C. Dack; +Cc: gcc-help
On Wed, Dec 16, 2020 at 04:30:00PM +0000, Sven C. Dack via Gcc-help wrote:
> Could some of the GCC experts please share their knowledge on this issue?
> >I've come across an unexpected behaviour of gcc and wanted to ask for
> >help. I've noticed that when I specify '-march=znver2' does gcc use the
> >incorrect values for the L1/L2 cache. Only when '-march=native' was used
> >and gcc detected the CPU at run-time does it use the correct values. For
> >example on a Ryzen 7 3800X (a Zen 2 CPU):
-march=native uses cpuid to get the cache info (or in general, looks at
the system it is running on). -march=znver2 does not. I do not know if
-march=znver2 should use better defaults.
> >Why does GCC not use the correct L1/L2 cache values when '-march=znver2'
> >or '-mtune=znver2' is specified?
Please open a PR? https://gcc.gnu.org/bugs.html . Thanks!
Segher
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: L1/L2 cache characteristics and -mtune=znver2 versus run-time detection
[not found] ` <20201218101912.GR2672@gate.crashing.org>
@ 2020-12-18 13:32 ` Sven C. Dack
0 siblings, 0 replies; 4+ messages in thread
From: Sven C. Dack @ 2020-12-18 13:32 UTC (permalink / raw)
To: Segher Boessenkool; +Cc: gcc-help
Hello,
I've seen the PR got resolved now and I still need to compile it. It
appears it got fixed 4 days after my first email to gcc-help regarding
this issue. It only never received a reply. Anyhow, I'm glad it got
fixed now.
Merry Christmas!
On 18/12/2020 10:19, Segher Boessenkool wrote:
> On Fri, Dec 18, 2020 at 04:15:12AM -0600, Segher Boessenkool wrote:
>> On Thu, Dec 17, 2020 at 10:24:17PM +0000, Sven C. Dack wrote:
>>> Thank you. I've created a PR. You can find it here:
>>>
>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98361
>>> <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98361>
>> Thanks you! You might went to not exclude the mailing list though? :-)
> Oh, I see Martin says it is already fixed (by Ke Wen). Could you check
> if you see the same?
>
>
> Segher
^ permalink raw reply [flat|nested] 4+ messages in thread
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2020-12-05 17:46 L1/L2 cache characteristics and -mtune=znver2 versus run-time detection Sven C. Dack
2020-12-16 16:30 ` Sven C. Dack
2020-12-16 17:33 ` Segher Boessenkool
[not found] ` <3f4c127e-3cdd-5a4e-ef56-d5d62a88e30a@gmx.com>
[not found] ` <20201218101512.GQ2672@gate.crashing.org>
[not found] ` <20201218101912.GR2672@gate.crashing.org>
2020-12-18 13:32 ` Sven C. Dack
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