Re: [PATCH 2/7] Makefile, x86, LLVM: disable unsupported optimization flags

[Masahiro Yamada]

Hi.


2017-04-06 4:11 GMT+09:00 Michael Davidson :
> It "works" for the cases that I currently care about but I have to say
> that I am uneasy about adding -Werror to the cc-option test in this
> way.
>
> Suppose that one of the *other* flags that is implicitly passed to the
> compiler by cc-option - eg something that was explicitly specified in
> $(KBUILD_CFLAGS) - triggers a warning. In that case all calls to
> cc-option will silently fail because of the -Werror and valid options
> will not be detected correctly.

Theoretically, options explicitly specified in KBUILD_CFLAGS
should be always valid.
Options that may not be supported in some cases
should be wrapped with $(call cc-option ).



> If everyone is OK with that because "it shouldn't normally ever
> happen" then that is fine, but if does result in a subtle change from
> existing behavior (and a trap that I almost immediately fell into
> after applying a similar patch).

There is a rare case where a particular combination fails
(such as the conflict between -pg and -ffunction-sections
as reported in https://patchwork.kernel.org/patch/9624573/).

In a such case, we may end up with swapping the order,
but this should not happen quite often.



> On Wed, Apr 5, 2017 at 12:01 PM, Matthias Kaehlcke  wrote:
>> Hi Masahiro,
>>
>> El Thu, Apr 06, 2017 at 03:08:26AM +0900 Masahiro Yamada ha dit:
>>
>>> 2017-03-17 9:15 GMT+09:00 Michael Davidson :
>>> > Unfortunately, while clang generates a warning about these flags
>>> > being unsupported it still exits with a status of 0 so we have
>>> > to explicitly disable them instead of just using a cc-option check.
>>> >
>>> > Signed-off-by: Michael Davidson 
>>>
>>>
>>> Instead, does the following work for you?
>>> https://patchwork.kernel.org/patch/9657285/
>>
>> Thanks for the pointer, I was about to give this change (or rather its
>> ancestor) a rework myself :)
>>
>>> You need to use
>>> $(call cc-option, ...)
>>> for -falign-jumps=1 and -falign-loops=1
>>
>> I can confirm that this works.
>>
>> Thanks
>>
>> Matthias
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-- 
Best Regards
Masahiro Yamada

Re: [PATCH 2/7] Makefile, x86, LLVM: disable unsupported optimization flags

[Michael Davidson]

It "works" for the cases that I currently care about but I have to say
that I am uneasy about adding -Werror to the cc-option test in this
way.

Suppose that one of the *other* flags that is implicitly passed to the
compiler by cc-option - eg something that was explicitly specified in
$(KBUILD_CFLAGS) - triggers a warning. In that case all calls to
cc-option will silently fail because of the -Werror and valid options
will not be detected correctly.

If everyone is OK with that because "it shouldn't normally ever
happen" then that is fine, but if does result in a subtle change from
existing behavior (and a trap that I almost immediately fell into
after applying a similar patch).

On Wed, Apr 5, 2017 at 12:01 PM, Matthias Kaehlcke  wrote:
> Hi Masahiro,
>
> El Thu, Apr 06, 2017 at 03:08:26AM +0900 Masahiro Yamada ha dit:
>
>> 2017-03-17 9:15 GMT+09:00 Michael Davidson :
>> > Unfortunately, while clang generates a warning about these flags
>> > being unsupported it still exits with a status of 0 so we have
>> > to explicitly disable them instead of just using a cc-option check.
>> >
>> > Signed-off-by: Michael Davidson 
>>
>>
>> Instead, does the following work for you?
>> https://patchwork.kernel.org/patch/9657285/
>
> Thanks for the pointer, I was about to give this change (or rather its
> ancestor) a rework myself :)
>
>> You need to use
>> $(call cc-option, ...)
>> for -falign-jumps=1 and -falign-loops=1
>
> I can confirm that this works.
>
> Thanks
>
> Matthias

Re: [PATCH 2/7] Makefile, x86, LLVM: disable unsupported optimization flags

[Matthias Kaehlcke]

Hi Masahiro,

El Thu, Apr 06, 2017 at 03:08:26AM +0900 Masahiro Yamada ha dit:

> 2017-03-17 9:15 GMT+09:00 Michael Davidson :
> > Unfortunately, while clang generates a warning about these flags
> > being unsupported it still exits with a status of 0 so we have
> > to explicitly disable them instead of just using a cc-option check.
> >
> > Signed-off-by: Michael Davidson 
> 
> 
> Instead, does the following work for you?
> https://patchwork.kernel.org/patch/9657285/

Thanks for the pointer, I was about to give this change (or rather its
ancestor) a rework myself :)

> You need to use
> $(call cc-option, ...)
> for -falign-jumps=1 and -falign-loops=1

I can confirm that this works.

Thanks

Matthias

Re: [PATCH 2/7] Makefile, x86, LLVM: disable unsupported optimization flags

[H. Peter Anvin]

On 03/17/17 14:32, H. Peter Anvin wrote:
> 
> NAK.  Fix your compiler, or use a wrapper script or something.  It is
> absolutely *not* acceptable to disable this since future versions of
> clang *should* support that.
> 
> That being said, it might make sense to look for a key pattern like
> "(un|not )supported" on stderr the try-run macro.  Is there really no
> -Wno- or -Werror= option to turn off this craziness?
> 

Well, guess what... I found it myself.

-W{no-,error=}ignored-optimization-argument

Either variant will make this sane.

-hpa

[PATCH 2/7] Makefile, x86, LLVM: disable unsupported optimization flags

[Michael Davidson]

Unfortunately, while clang generates a warning about these flags
being unsupported it still exits with a status of 0 so we have
to explicitly disable them instead of just using a cc-option check.

Signed-off-by: Michael Davidson 
---
 Makefile  | 2 ++
 arch/x86/Makefile | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/Makefile b/Makefile
index b21fd0ca2946..5e97e5fc1eea 100644
--- a/Makefile
+++ b/Makefile
@@ -629,7 +629,9 @@ ARCH_AFLAGS :=
 ARCH_CFLAGS :=
 include arch/$(SRCARCH)/Makefile
 
+ifneq ($(cc-name),clang)
 KBUILD_CFLAGS  += $(call cc-option,-fno-delete-null-pointer-checks,)
+endif
 KBUILD_CFLAGS  += $(call cc-disable-warning,frame-address,)
 
 ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 2d449337a360..894a8d18bf97 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -87,11 +87,13 @@ else
 KBUILD_AFLAGS += -m64
 KBUILD_CFLAGS += -m64
 
+ifneq ($(cc-name),clang)
 # Align jump targets to 1 byte, not the default 16 bytes:
 KBUILD_CFLAGS += -falign-jumps=1
 
 # Pack loops tightly as well:
 KBUILD_CFLAGS += -falign-loops=1
+endif
 
 # Don't autogenerate traditional x87 instructions
 KBUILD_CFLAGS += $(call cc-option,-mno-80387)
-- 
2.12.0.367.g23dc2f6d3c-goog