linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
* [PATCH] drm/i915: Disable -Wtautological-constant-out-of-range-compare
@ 2020-02-11  5:08 Nathan Chancellor
  2020-02-11  6:13 ` [PATCH v2] " Nathan Chancellor
  0 siblings, 1 reply; 12+ messages in thread
From: Nathan Chancellor @ 2020-02-11  5:08 UTC (permalink / raw)
  To: Jani Nikula, Joonas Lahtinen, Rodrigo Vivi
  Cc: intel-gfx, dri-devel, linux-kernel, clang-built-linux, Nathan Chancellor

A recent commit in clang added -Wtautological-compare to -Wall, which is
enabled for i915 so we see the following warning:

../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
result of comparison of constant 576460752303423487 with expression of
type 'unsigned int' is always false
[-Wtautological-constant-out-of-range-compare]
        if (unlikely(remain > N_RELOC(ULONG_MAX)))
            ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~

This warning only happens on x86_64 but that check is relevant for
32-bit x86 so we cannot remove it. -Wtautological-compare on a whole has
good warnings but this one is not really relevant for the kernel because
of all of the different configurations that are used to build the
kernel. When -Wtautological-compare is enabled for the kernel, this
option will remain disabled so do that for i915 now.

Link: https://github.com/ClangBuiltLinux/linux/issues/778
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
---
 drivers/gpu/drm/i915/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
index 38df01c23176..55dbcca179c7 100644
--- a/drivers/gpu/drm/i915/Makefile
+++ b/drivers/gpu/drm/i915/Makefile
@@ -21,6 +21,7 @@ subdir-ccflags-y += $(call cc-disable-warning, unused-but-set-variable)
 subdir-ccflags-y += $(call cc-disable-warning, sign-compare)
 subdir-ccflags-y += $(call cc-disable-warning, sometimes-uninitialized)
 subdir-ccflags-y += $(call cc-disable-warning, initializer-overrides)
+subdir-ccflags-y += $(call cc-disable-warning, tautological-constant-out-of-range-compare)
 subdir-ccflags-$(CONFIG_DRM_I915_WERROR) += -Werror
 
 # Fine grained warnings disable
-- 
2.25.0


^ permalink raw reply	[flat|nested] 12+ messages in thread

* [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
  2020-02-11  5:08 [PATCH] drm/i915: Disable -Wtautological-constant-out-of-range-compare Nathan Chancellor
@ 2020-02-11  6:13 ` Nathan Chancellor
  2020-02-11  9:41   ` Michel Dänzer
  0 siblings, 1 reply; 12+ messages in thread
From: Nathan Chancellor @ 2020-02-11  6:13 UTC (permalink / raw)
  To: Jani Nikula, Joonas Lahtinen, Rodrigo Vivi
  Cc: intel-gfx, dri-devel, linux-kernel, clang-built-linux, Nathan Chancellor

A recent commit in clang added -Wtautological-compare to -Wall, which is
enabled for i915 so we see the following warning:

../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
result of comparison of constant 576460752303423487 with expression of
type 'unsigned int' is always false
[-Wtautological-constant-out-of-range-compare]
        if (unlikely(remain > N_RELOC(ULONG_MAX)))
            ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~

This warning only happens on x86_64 but that check is relevant for
32-bit x86 so we cannot remove it. -Wtautological-compare on a whole has
good warnings but this one is not really relevant for the kernel because
of all of the different configurations that are used to build the
kernel. When -Wtautological-compare is enabled for the kernel, this
option will remain disabled so do that for i915 now.

Link: https://github.com/ClangBuiltLinux/linux/issues/778
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
---

v1 -> v2: https://lore.kernel.org/lkml/20200211050808.29463-1-natechancellor@gmail.com/

* Fix patch application due to basing on a local tree that had
  -Wuninitialized turned on. Can confirm that patch applies on
  latest -next now.

 drivers/gpu/drm/i915/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
index b8c5f8934dbd..159355eb43a9 100644
--- a/drivers/gpu/drm/i915/Makefile
+++ b/drivers/gpu/drm/i915/Makefile
@@ -22,6 +22,7 @@ subdir-ccflags-y += $(call cc-disable-warning, sign-compare)
 subdir-ccflags-y += $(call cc-disable-warning, sometimes-uninitialized)
 subdir-ccflags-y += $(call cc-disable-warning, initializer-overrides)
 subdir-ccflags-y += $(call cc-disable-warning, uninitialized)
+subdir-ccflags-y += $(call cc-disable-warning, tautological-constant-out-of-range-compare)
 subdir-ccflags-$(CONFIG_DRM_I915_WERROR) += -Werror
 
 # Fine grained warnings disable
-- 
2.25.0


^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
  2020-02-11  6:13 ` [PATCH v2] " Nathan Chancellor
@ 2020-02-11  9:41   ` Michel Dänzer
  2020-02-11 20:39     ` Nathan Chancellor
  0 siblings, 1 reply; 12+ messages in thread
From: Michel Dänzer @ 2020-02-11  9:41 UTC (permalink / raw)
  To: Nathan Chancellor, Jani Nikula, Joonas Lahtinen, Rodrigo Vivi
  Cc: clang-built-linux, intel-gfx, linux-kernel, dri-devel

On 2020-02-11 7:13 a.m., Nathan Chancellor wrote:
> A recent commit in clang added -Wtautological-compare to -Wall, which is
> enabled for i915 so we see the following warning:
> 
> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
> result of comparison of constant 576460752303423487 with expression of
> type 'unsigned int' is always false
> [-Wtautological-constant-out-of-range-compare]
>         if (unlikely(remain > N_RELOC(ULONG_MAX)))
>             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
> 
> This warning only happens on x86_64 but that check is relevant for
> 32-bit x86 so we cannot remove it.

That's suprising. AFAICT N_RELOC(ULONG_MAX) works out to the same value
in both cases, and remain is a 32-bit value in both cases. How can it be
larger than N_RELOC(ULONG_MAX) on 32-bit (but not on 64-bit)?


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
  2020-02-11  9:41   ` Michel Dänzer
@ 2020-02-11 20:39     ` Nathan Chancellor
  2020-02-12  8:52       ` Michel Dänzer
  0 siblings, 1 reply; 12+ messages in thread
From: Nathan Chancellor @ 2020-02-11 20:39 UTC (permalink / raw)
  To: Michel Dänzer
  Cc: Jani Nikula, Joonas Lahtinen, Rodrigo Vivi, clang-built-linux,
	intel-gfx, linux-kernel, dri-devel

On Tue, Feb 11, 2020 at 10:41:48AM +0100, Michel Dänzer wrote:
> On 2020-02-11 7:13 a.m., Nathan Chancellor wrote:
> > A recent commit in clang added -Wtautological-compare to -Wall, which is
> > enabled for i915 so we see the following warning:
> > 
> > ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
> > result of comparison of constant 576460752303423487 with expression of
> > type 'unsigned int' is always false
> > [-Wtautological-constant-out-of-range-compare]
> >         if (unlikely(remain > N_RELOC(ULONG_MAX)))
> >             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
> > 
> > This warning only happens on x86_64 but that check is relevant for
> > 32-bit x86 so we cannot remove it.
> 
> That's suprising. AFAICT N_RELOC(ULONG_MAX) works out to the same value
> in both cases, and remain is a 32-bit value in both cases. How can it be
> larger than N_RELOC(ULONG_MAX) on 32-bit (but not on 64-bit)?
> 

Hi Michel,

Can't this condition be true when UINT_MAX == ULONG_MAX? clang does not
warn on a 32-bit x86 build from what I remember. Honestly, my
understanding of overflow is pretty shoddy, this is mostly based on what
I have heard from others.

I sent a patch trying to remove that check but had it rejected:

https://lore.kernel.org/lkml/20191123195321.41305-1-natechancellor@gmail.com/

Cheers,
Nathan

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
  2020-02-11 20:39     ` Nathan Chancellor
@ 2020-02-12  8:52       ` Michel Dänzer
  2020-02-12 17:07         ` Nathan Chancellor
  0 siblings, 1 reply; 12+ messages in thread
From: Michel Dänzer @ 2020-02-12  8:52 UTC (permalink / raw)
  To: Nathan Chancellor
  Cc: intel-gfx, linux-kernel, clang-built-linux, dri-devel, Rodrigo Vivi

On 2020-02-11 9:39 p.m., Nathan Chancellor wrote:
> On Tue, Feb 11, 2020 at 10:41:48AM +0100, Michel Dänzer wrote:
>> On 2020-02-11 7:13 a.m., Nathan Chancellor wrote:
>>> A recent commit in clang added -Wtautological-compare to -Wall, which is
>>> enabled for i915 so we see the following warning:
>>>
>>> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
>>> result of comparison of constant 576460752303423487 with expression of
>>> type 'unsigned int' is always false
>>> [-Wtautological-constant-out-of-range-compare]
>>>         if (unlikely(remain > N_RELOC(ULONG_MAX)))
>>>             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
>>>
>>> This warning only happens on x86_64 but that check is relevant for
>>> 32-bit x86 so we cannot remove it.
>>
>> That's suprising. AFAICT N_RELOC(ULONG_MAX) works out to the same value
>> in both cases, and remain is a 32-bit value in both cases. How can it be
>> larger than N_RELOC(ULONG_MAX) on 32-bit (but not on 64-bit)?
>>
> 
> Hi Michel,
> 
> Can't this condition be true when UINT_MAX == ULONG_MAX?

Oh, right, I think I was wrongly thinking long had 64 bits even on 32-bit.


Anyway, this suggests a possible better solution:

#if UINT_MAX == ULONG_MAX
	if (unlikely(remain > N_RELOC(ULONG_MAX)))
		return -EINVAL;
#endif


Or if that can't be used for some reason, something like

	if (unlikely((unsigned long)remain > N_RELOC(ULONG_MAX)))
		return -EINVAL;

should silence the warning.


Either of these should be better than completely disabling the warning
for the whole file.


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
  2020-02-12  8:52       ` Michel Dänzer
@ 2020-02-12 17:07         ` Nathan Chancellor
  2020-02-12 17:17           ` Michel Dänzer
  0 siblings, 1 reply; 12+ messages in thread
From: Nathan Chancellor @ 2020-02-12 17:07 UTC (permalink / raw)
  To: Michel Dänzer
  Cc: intel-gfx, linux-kernel, clang-built-linux, dri-devel, Rodrigo Vivi

On Wed, Feb 12, 2020 at 09:52:52AM +0100, Michel Dänzer wrote:
> On 2020-02-11 9:39 p.m., Nathan Chancellor wrote:
> > On Tue, Feb 11, 2020 at 10:41:48AM +0100, Michel Dänzer wrote:
> >> On 2020-02-11 7:13 a.m., Nathan Chancellor wrote:
> >>> A recent commit in clang added -Wtautological-compare to -Wall, which is
> >>> enabled for i915 so we see the following warning:
> >>>
> >>> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
> >>> result of comparison of constant 576460752303423487 with expression of
> >>> type 'unsigned int' is always false
> >>> [-Wtautological-constant-out-of-range-compare]
> >>>         if (unlikely(remain > N_RELOC(ULONG_MAX)))
> >>>             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
> >>>
> >>> This warning only happens on x86_64 but that check is relevant for
> >>> 32-bit x86 so we cannot remove it.
> >>
> >> That's suprising. AFAICT N_RELOC(ULONG_MAX) works out to the same value
> >> in both cases, and remain is a 32-bit value in both cases. How can it be
> >> larger than N_RELOC(ULONG_MAX) on 32-bit (but not on 64-bit)?
> >>
> > 
> > Hi Michel,
> > 
> > Can't this condition be true when UINT_MAX == ULONG_MAX?
> 
> Oh, right, I think I was wrongly thinking long had 64 bits even on 32-bit.
> 
> 
> Anyway, this suggests a possible better solution:
> 
> #if UINT_MAX == ULONG_MAX
> 	if (unlikely(remain > N_RELOC(ULONG_MAX)))
> 		return -EINVAL;
> #endif
> 
> 
> Or if that can't be used for some reason, something like
> 
> 	if (unlikely((unsigned long)remain > N_RELOC(ULONG_MAX)))
> 		return -EINVAL;
> 
> should silence the warning.

I do like this one better than the former.

> 
> 
> Either of these should be better than completely disabling the warning
> for the whole file.

Normally, I would agree but I am currently planning to leave
-Wtautological-constant-out-of-range-compare disabled when I turn on
-Wtautological-compare for the whole kernel because there are plenty of
locations in the kernel where these kind of checks depend on various
kernel configuration options and the general attitude of kernel
developers is that this particular warning is not really helpful
for that reason.

I'll see if there is a general consensus before moving further since I
know i915 turns on a bunch of extra warnings from the rest of the kernel
(hence why we are in this situation).

Cheers,
Nathan

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
  2020-02-12 17:07         ` Nathan Chancellor
@ 2020-02-12 17:17           ` Michel Dänzer
  2020-02-13 14:37             ` [Intel-gfx] " Jani Nikula
  2020-02-13 22:43             ` Nick Desaulniers
  0 siblings, 2 replies; 12+ messages in thread
From: Michel Dänzer @ 2020-02-12 17:17 UTC (permalink / raw)
  To: Nathan Chancellor
  Cc: intel-gfx, linux-kernel, clang-built-linux, dri-devel, Rodrigo Vivi

On 2020-02-12 6:07 p.m., Nathan Chancellor wrote:
> On Wed, Feb 12, 2020 at 09:52:52AM +0100, Michel Dänzer wrote:
>> On 2020-02-11 9:39 p.m., Nathan Chancellor wrote:
>>> On Tue, Feb 11, 2020 at 10:41:48AM +0100, Michel Dänzer wrote:
>>>> On 2020-02-11 7:13 a.m., Nathan Chancellor wrote:
>>>>> A recent commit in clang added -Wtautological-compare to -Wall, which is
>>>>> enabled for i915 so we see the following warning:
>>>>>
>>>>> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
>>>>> result of comparison of constant 576460752303423487 with expression of
>>>>> type 'unsigned int' is always false
>>>>> [-Wtautological-constant-out-of-range-compare]
>>>>>         if (unlikely(remain > N_RELOC(ULONG_MAX)))
>>>>>             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
>>>>>
>>>>> This warning only happens on x86_64 but that check is relevant for
>>>>> 32-bit x86 so we cannot remove it.
>>>>
>>>> That's suprising. AFAICT N_RELOC(ULONG_MAX) works out to the same value
>>>> in both cases, and remain is a 32-bit value in both cases. How can it be
>>>> larger than N_RELOC(ULONG_MAX) on 32-bit (but not on 64-bit)?
>>>>
>>>
>>> Hi Michel,
>>>
>>> Can't this condition be true when UINT_MAX == ULONG_MAX?
>>
>> Oh, right, I think I was wrongly thinking long had 64 bits even on 32-bit.
>>
>>
>> Anyway, this suggests a possible better solution:
>>
>> #if UINT_MAX == ULONG_MAX
>> 	if (unlikely(remain > N_RELOC(ULONG_MAX)))
>> 		return -EINVAL;
>> #endif
>>
>>
>> Or if that can't be used for some reason, something like
>>
>> 	if (unlikely((unsigned long)remain > N_RELOC(ULONG_MAX)))
>> 		return -EINVAL;
>>
>> should silence the warning.
> 
> I do like this one better than the former.

FWIW, one downside of this one compared to all alternatives (presumably)
is that it might end up generating actual code even on 64-bit, which
always ends up skipping the return.


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
  2020-02-12 17:17           ` Michel Dänzer
@ 2020-02-13 14:37             ` Jani Nikula
  2020-02-13 21:48               ` Nathan Chancellor
  2020-02-13 22:43             ` Nick Desaulniers
  1 sibling, 1 reply; 12+ messages in thread
From: Jani Nikula @ 2020-02-13 14:37 UTC (permalink / raw)
  To: Michel Dänzer, Nathan Chancellor
  Cc: clang-built-linux, intel-gfx, linux-kernel, dri-devel

On Wed, 12 Feb 2020, Michel Dänzer <michel@daenzer.net> wrote:
> On 2020-02-12 6:07 p.m., Nathan Chancellor wrote:
>> On Wed, Feb 12, 2020 at 09:52:52AM +0100, Michel Dänzer wrote:
>>> On 2020-02-11 9:39 p.m., Nathan Chancellor wrote:
>>>> On Tue, Feb 11, 2020 at 10:41:48AM +0100, Michel Dänzer wrote:
>>>>> On 2020-02-11 7:13 a.m., Nathan Chancellor wrote:
>>>>>> A recent commit in clang added -Wtautological-compare to -Wall, which is
>>>>>> enabled for i915 so we see the following warning:
>>>>>>
>>>>>> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
>>>>>> result of comparison of constant 576460752303423487 with expression of
>>>>>> type 'unsigned int' is always false
>>>>>> [-Wtautological-constant-out-of-range-compare]
>>>>>>         if (unlikely(remain > N_RELOC(ULONG_MAX)))
>>>>>>             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
>>>>>>
>>>>>> This warning only happens on x86_64 but that check is relevant for
>>>>>> 32-bit x86 so we cannot remove it.
>>>>>
>>>>> That's suprising. AFAICT N_RELOC(ULONG_MAX) works out to the same value
>>>>> in both cases, and remain is a 32-bit value in both cases. How can it be
>>>>> larger than N_RELOC(ULONG_MAX) on 32-bit (but not on 64-bit)?
>>>>>
>>>>
>>>> Hi Michel,
>>>>
>>>> Can't this condition be true when UINT_MAX == ULONG_MAX?
>>>
>>> Oh, right, I think I was wrongly thinking long had 64 bits even on 32-bit.
>>>
>>>
>>> Anyway, this suggests a possible better solution:
>>>
>>> #if UINT_MAX == ULONG_MAX
>>> 	if (unlikely(remain > N_RELOC(ULONG_MAX)))
>>> 		return -EINVAL;
>>> #endif
>>>
>>>
>>> Or if that can't be used for some reason, something like
>>>
>>> 	if (unlikely((unsigned long)remain > N_RELOC(ULONG_MAX)))
>>> 		return -EINVAL;
>>>
>>> should silence the warning.
>> 
>> I do like this one better than the former.
>
> FWIW, one downside of this one compared to all alternatives (presumably)
> is that it might end up generating actual code even on 64-bit, which
> always ends up skipping the return.

I like this better than the UINT_MAX == ULONG_MAX comparison because
that creates a dependency on the type of remain.

Then again, a sufficiently clever compiler could see through the cast,
and flag the warning anyway...

BR,
Jani.

-- 
Jani Nikula, Intel Open Source Graphics Center

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
  2020-02-13 14:37             ` [Intel-gfx] " Jani Nikula
@ 2020-02-13 21:48               ` Nathan Chancellor
  2020-02-13 22:05                 ` Jani Nikula
  0 siblings, 1 reply; 12+ messages in thread
From: Nathan Chancellor @ 2020-02-13 21:48 UTC (permalink / raw)
  To: Jani Nikula
  Cc: Michel Dänzer, clang-built-linux, intel-gfx, linux-kernel,
	dri-devel

On Thu, Feb 13, 2020 at 04:37:15PM +0200, Jani Nikula wrote:
> On Wed, 12 Feb 2020, Michel Dänzer <michel@daenzer.net> wrote:
> > On 2020-02-12 6:07 p.m., Nathan Chancellor wrote:
> >> On Wed, Feb 12, 2020 at 09:52:52AM +0100, Michel Dänzer wrote:
> >>> On 2020-02-11 9:39 p.m., Nathan Chancellor wrote:
> >>>> On Tue, Feb 11, 2020 at 10:41:48AM +0100, Michel Dänzer wrote:
> >>>>> On 2020-02-11 7:13 a.m., Nathan Chancellor wrote:
> >>>>>> A recent commit in clang added -Wtautological-compare to -Wall, which is
> >>>>>> enabled for i915 so we see the following warning:
> >>>>>>
> >>>>>> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
> >>>>>> result of comparison of constant 576460752303423487 with expression of
> >>>>>> type 'unsigned int' is always false
> >>>>>> [-Wtautological-constant-out-of-range-compare]
> >>>>>>         if (unlikely(remain > N_RELOC(ULONG_MAX)))
> >>>>>>             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
> >>>>>>
> >>>>>> This warning only happens on x86_64 but that check is relevant for
> >>>>>> 32-bit x86 so we cannot remove it.
> >>>>>
> >>>>> That's suprising. AFAICT N_RELOC(ULONG_MAX) works out to the same value
> >>>>> in both cases, and remain is a 32-bit value in both cases. How can it be
> >>>>> larger than N_RELOC(ULONG_MAX) on 32-bit (but not on 64-bit)?
> >>>>>
> >>>>
> >>>> Hi Michel,
> >>>>
> >>>> Can't this condition be true when UINT_MAX == ULONG_MAX?
> >>>
> >>> Oh, right, I think I was wrongly thinking long had 64 bits even on 32-bit.
> >>>
> >>>
> >>> Anyway, this suggests a possible better solution:
> >>>
> >>> #if UINT_MAX == ULONG_MAX
> >>> 	if (unlikely(remain > N_RELOC(ULONG_MAX)))
> >>> 		return -EINVAL;
> >>> #endif
> >>>
> >>>
> >>> Or if that can't be used for some reason, something like
> >>>
> >>> 	if (unlikely((unsigned long)remain > N_RELOC(ULONG_MAX)))
> >>> 		return -EINVAL;
> >>>
> >>> should silence the warning.
> >> 
> >> I do like this one better than the former.
> >
> > FWIW, one downside of this one compared to all alternatives (presumably)
> > is that it might end up generating actual code even on 64-bit, which
> > always ends up skipping the return.
> 
> I like this better than the UINT_MAX == ULONG_MAX comparison because
> that creates a dependency on the type of remain.
> 
> Then again, a sufficiently clever compiler could see through the cast,
> and flag the warning anyway...

Would you prefer a patch that adds that cast rather than silencing the
warning outright? It does appear to work for clang.

Cheers,
Nathan

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
  2020-02-13 21:48               ` Nathan Chancellor
@ 2020-02-13 22:05                 ` Jani Nikula
  0 siblings, 0 replies; 12+ messages in thread
From: Jani Nikula @ 2020-02-13 22:05 UTC (permalink / raw)
  To: Nathan Chancellor
  Cc: Michel Dänzer, clang-built-linux, intel-gfx, linux-kernel,
	dri-devel

On Thu, 13 Feb 2020, Nathan Chancellor <natechancellor@gmail.com> wrote:
> On Thu, Feb 13, 2020 at 04:37:15PM +0200, Jani Nikula wrote:
>> On Wed, 12 Feb 2020, Michel Dänzer <michel@daenzer.net> wrote:
>> > On 2020-02-12 6:07 p.m., Nathan Chancellor wrote:
>> >> On Wed, Feb 12, 2020 at 09:52:52AM +0100, Michel Dänzer wrote:
>> >>> On 2020-02-11 9:39 p.m., Nathan Chancellor wrote:
>> >>>> On Tue, Feb 11, 2020 at 10:41:48AM +0100, Michel Dänzer wrote:
>> >>>>> On 2020-02-11 7:13 a.m., Nathan Chancellor wrote:
>> >>>>>> A recent commit in clang added -Wtautological-compare to -Wall, which is
>> >>>>>> enabled for i915 so we see the following warning:
>> >>>>>>
>> >>>>>> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
>> >>>>>> result of comparison of constant 576460752303423487 with expression of
>> >>>>>> type 'unsigned int' is always false
>> >>>>>> [-Wtautological-constant-out-of-range-compare]
>> >>>>>>         if (unlikely(remain > N_RELOC(ULONG_MAX)))
>> >>>>>>             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
>> >>>>>>
>> >>>>>> This warning only happens on x86_64 but that check is relevant for
>> >>>>>> 32-bit x86 so we cannot remove it.
>> >>>>>
>> >>>>> That's suprising. AFAICT N_RELOC(ULONG_MAX) works out to the same value
>> >>>>> in both cases, and remain is a 32-bit value in both cases. How can it be
>> >>>>> larger than N_RELOC(ULONG_MAX) on 32-bit (but not on 64-bit)?
>> >>>>>
>> >>>>
>> >>>> Hi Michel,
>> >>>>
>> >>>> Can't this condition be true when UINT_MAX == ULONG_MAX?
>> >>>
>> >>> Oh, right, I think I was wrongly thinking long had 64 bits even on 32-bit.
>> >>>
>> >>>
>> >>> Anyway, this suggests a possible better solution:
>> >>>
>> >>> #if UINT_MAX == ULONG_MAX
>> >>> 	if (unlikely(remain > N_RELOC(ULONG_MAX)))
>> >>> 		return -EINVAL;
>> >>> #endif
>> >>>
>> >>>
>> >>> Or if that can't be used for some reason, something like
>> >>>
>> >>> 	if (unlikely((unsigned long)remain > N_RELOC(ULONG_MAX)))
>> >>> 		return -EINVAL;
>> >>>
>> >>> should silence the warning.
>> >> 
>> >> I do like this one better than the former.
>> >
>> > FWIW, one downside of this one compared to all alternatives (presumably)
>> > is that it might end up generating actual code even on 64-bit, which
>> > always ends up skipping the return.
>> 
>> I like this better than the UINT_MAX == ULONG_MAX comparison because
>> that creates a dependency on the type of remain.
>> 
>> Then again, a sufficiently clever compiler could see through the cast,
>> and flag the warning anyway...
>
> Would you prefer a patch that adds that cast rather than silencing the
> warning outright? It does appear to work for clang.

I'd take the cast.

If that fails for whatever reason, per-file

CFLAGS_gem/i915_gem_execbuffer.o = $(call cc-disable-warning, tautological-constant-out-of-range-compare)

over subdir-ccflags-y would be preferrable I think.

BR,
Jani.



>
> Cheers,
> Nathan

-- 
Jani Nikula, Intel Open Source Graphics Center

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
  2020-02-12 17:17           ` Michel Dänzer
  2020-02-13 14:37             ` [Intel-gfx] " Jani Nikula
@ 2020-02-13 22:43             ` Nick Desaulniers
  2020-02-13 23:27               ` Nathan Chancellor
  1 sibling, 1 reply; 12+ messages in thread
From: Nick Desaulniers @ 2020-02-13 22:43 UTC (permalink / raw)
  To: Michel Dänzer
  Cc: Nathan Chancellor, intel-gfx, LKML, clang-built-linux, dri-devel,
	Rodrigo Vivi

On Wed, Feb 12, 2020 at 9:17 AM Michel Dänzer <michel@daenzer.net> wrote:
>
> On 2020-02-12 6:07 p.m., Nathan Chancellor wrote:
> > On Wed, Feb 12, 2020 at 09:52:52AM +0100, Michel Dänzer wrote:
> >> On 2020-02-11 9:39 p.m., Nathan Chancellor wrote:
> >>> On Tue, Feb 11, 2020 at 10:41:48AM +0100, Michel Dänzer wrote:
> >>>> On 2020-02-11 7:13 a.m., Nathan Chancellor wrote:
> >>>>> A recent commit in clang added -Wtautological-compare to -Wall, which is
> >>>>> enabled for i915 so we see the following warning:
> >>>>>
> >>>>> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
> >>>>> result of comparison of constant 576460752303423487 with expression of
> >>>>> type 'unsigned int' is always false
> >>>>> [-Wtautological-constant-out-of-range-compare]
> >>>>>         if (unlikely(remain > N_RELOC(ULONG_MAX)))
> >>>>>             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
> >>>>>
> >>>>> This warning only happens on x86_64 but that check is relevant for
> >>>>> 32-bit x86 so we cannot remove it.
> >>>>
> >>>> That's suprising. AFAICT N_RELOC(ULONG_MAX) works out to the same value
> >>>> in both cases, and remain is a 32-bit value in both cases. How can it be
> >>>> larger than N_RELOC(ULONG_MAX) on 32-bit (but not on 64-bit)?
> >>>>
> >>>
> >>> Hi Michel,
> >>>
> >>> Can't this condition be true when UINT_MAX == ULONG_MAX?
> >>
> >> Oh, right, I think I was wrongly thinking long had 64 bits even on 32-bit.
> >>
> >>
> >> Anyway, this suggests a possible better solution:
> >>
> >> #if UINT_MAX == ULONG_MAX
> >>      if (unlikely(remain > N_RELOC(ULONG_MAX)))
> >>              return -EINVAL;
> >> #endif
> >>
> >>
> >> Or if that can't be used for some reason, something like
> >>
> >>      if (unlikely((unsigned long)remain > N_RELOC(ULONG_MAX)))
> >>              return -EINVAL;
> >>
> >> should silence the warning.
> >
> > I do like this one better than the former.
>
> FWIW, one downside of this one compared to all alternatives (presumably)
> is that it might end up generating actual code even on 64-bit, which
> always ends up skipping the return.

The warning is pointing out that the conditional is always false,
which is correct on 64b.  The check is only active for 32b.
https://godbolt.org/z/oQrgT_
The cast silences the warning for 64b.  (Note that GCC and Clang also
generate precisely the same instruction sequences in my example, just
GCC doesn't warn on such tautologies).
-- 
Thanks,
~Nick Desaulniers

^ permalink raw reply	[flat|nested] 12+ messages in thread

* Re: [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare
  2020-02-13 22:43             ` Nick Desaulniers
@ 2020-02-13 23:27               ` Nathan Chancellor
  0 siblings, 0 replies; 12+ messages in thread
From: Nathan Chancellor @ 2020-02-13 23:27 UTC (permalink / raw)
  To: Nick Desaulniers
  Cc: Michel Dänzer, intel-gfx, LKML, clang-built-linux,
	dri-devel, Rodrigo Vivi

On Thu, Feb 13, 2020 at 02:43:21PM -0800, Nick Desaulniers wrote:
> On Wed, Feb 12, 2020 at 9:17 AM Michel Dänzer <michel@daenzer.net> wrote:
> >
> > On 2020-02-12 6:07 p.m., Nathan Chancellor wrote:
> > > On Wed, Feb 12, 2020 at 09:52:52AM +0100, Michel Dänzer wrote:
> > >> On 2020-02-11 9:39 p.m., Nathan Chancellor wrote:
> > >>> On Tue, Feb 11, 2020 at 10:41:48AM +0100, Michel Dänzer wrote:
> > >>>> On 2020-02-11 7:13 a.m., Nathan Chancellor wrote:
> > >>>>> A recent commit in clang added -Wtautological-compare to -Wall, which is
> > >>>>> enabled for i915 so we see the following warning:
> > >>>>>
> > >>>>> ../drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c:1485:22: warning:
> > >>>>> result of comparison of constant 576460752303423487 with expression of
> > >>>>> type 'unsigned int' is always false
> > >>>>> [-Wtautological-constant-out-of-range-compare]
> > >>>>>         if (unlikely(remain > N_RELOC(ULONG_MAX)))
> > >>>>>             ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
> > >>>>>
> > >>>>> This warning only happens on x86_64 but that check is relevant for
> > >>>>> 32-bit x86 so we cannot remove it.
> > >>>>
> > >>>> That's suprising. AFAICT N_RELOC(ULONG_MAX) works out to the same value
> > >>>> in both cases, and remain is a 32-bit value in both cases. How can it be
> > >>>> larger than N_RELOC(ULONG_MAX) on 32-bit (but not on 64-bit)?
> > >>>>
> > >>>
> > >>> Hi Michel,
> > >>>
> > >>> Can't this condition be true when UINT_MAX == ULONG_MAX?
> > >>
> > >> Oh, right, I think I was wrongly thinking long had 64 bits even on 32-bit.
> > >>
> > >>
> > >> Anyway, this suggests a possible better solution:
> > >>
> > >> #if UINT_MAX == ULONG_MAX
> > >>      if (unlikely(remain > N_RELOC(ULONG_MAX)))
> > >>              return -EINVAL;
> > >> #endif
> > >>
> > >>
> > >> Or if that can't be used for some reason, something like
> > >>
> > >>      if (unlikely((unsigned long)remain > N_RELOC(ULONG_MAX)))
> > >>              return -EINVAL;
> > >>
> > >> should silence the warning.
> > >
> > > I do like this one better than the former.
> >
> > FWIW, one downside of this one compared to all alternatives (presumably)
> > is that it might end up generating actual code even on 64-bit, which
> > always ends up skipping the return.
> 
> The warning is pointing out that the conditional is always false,
> which is correct on 64b.  The check is only active for 32b.
> https://godbolt.org/z/oQrgT_
> The cast silences the warning for 64b.  (Note that GCC and Clang also
> generate precisely the same instruction sequences in my example, just
> GCC doesn't warn on such tautologies).

Thanks for confirming! I'll send a patch to add the cast later tonight.

Cheers,
Nathan

^ permalink raw reply	[flat|nested] 12+ messages in thread

end of thread, other threads:[~2020-02-13 23:27 UTC | newest]

Thread overview: 12+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-02-11  5:08 [PATCH] drm/i915: Disable -Wtautological-constant-out-of-range-compare Nathan Chancellor
2020-02-11  6:13 ` [PATCH v2] " Nathan Chancellor
2020-02-11  9:41   ` Michel Dänzer
2020-02-11 20:39     ` Nathan Chancellor
2020-02-12  8:52       ` Michel Dänzer
2020-02-12 17:07         ` Nathan Chancellor
2020-02-12 17:17           ` Michel Dänzer
2020-02-13 14:37             ` [Intel-gfx] " Jani Nikula
2020-02-13 21:48               ` Nathan Chancellor
2020-02-13 22:05                 ` Jani Nikula
2020-02-13 22:43             ` Nick Desaulniers
2020-02-13 23:27               ` Nathan Chancellor

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).