[Intel-gfx] [PATCH] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[Intel-gfx] [PATCH] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[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

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[Intel-gfx] ✗ Fi.CI.BUILD: failure for drm/i915: Disable -Wtautological-constant-out-of-range-compare

[Patchwork]

== Series Details ==

Series: drm/i915: Disable -Wtautological-constant-out-of-range-compare
URL   : https://patchwork.freedesktop.org/series/73271/
State : failure

== Summary ==

Applying: drm/i915: Disable -Wtautological-constant-out-of-range-compare
error: sha1 information is lacking or useless (drivers/gpu/drm/i915/Makefile).
error: could not build fake ancestor
hint: Use 'git am --show-current-patch' to see the failed patch
Patch failed at 0001 drm/i915: Disable -Wtautological-constant-out-of-range-compare
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

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[Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[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

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Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[Michel Dänzer]

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)?


-- 
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Libre software enthusiast             |             Mesa and X developer
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Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[Nathan Chancellor]

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
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Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[Michel Dänzer]

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.


-- 
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Libre software enthusiast             |             Mesa and X developer
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Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[Nathan Chancellor]

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
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Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[Michel Dänzer]

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.


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Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[Jani Nikula]

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.

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Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[Nathan Chancellor]

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
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Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[Jani Nikula]

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

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Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[Nick Desaulniers]

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
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Re: [Intel-gfx] [PATCH v2] drm/i915: Disable -Wtautological-constant-out-of-range-compare

[Nathan Chancellor]

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
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[Intel-gfx] ✗ Fi.CI.BAT: failure for drm/i915: Disable -Wtautological-constant-out-of-range-compare (rev2)

[Patchwork]

== Series Details ==

Series: drm/i915: Disable -Wtautological-constant-out-of-range-compare (rev2)
URL   : https://patchwork.freedesktop.org/series/73271/
State : failure

== Summary ==

CI Bug Log - changes from CI_DRM_7905 -> Patchwork_16515
====================================================

Summary
-------

  **FAILURE**

  Serious unknown changes coming with Patchwork_16515 absolutely need to be
  verified manually.
  
  If you think the reported changes have nothing to do with the changes
  introduced in Patchwork_16515, please notify your bug team to allow them
  to document this new failure mode, which will reduce false positives in CI.

  External URL: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_16515/index.html

Possible new issues
-------------------

  Here are the unknown changes that may have been introduced in Patchwork_16515:

### IGT changes ###

#### Possible regressions ####

  * igt@i915_selftest@live_blt:
    - fi-hsw-4770r:       [PASS][1] -> [INCOMPLETE][2]
   [1]: https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_7905/fi-hsw-4770r/igt@i915_selftest@live_blt.html
   [2]: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_16515/fi-hsw-4770r/igt@i915_selftest@live_blt.html

  * igt@i915_selftest@live_objects:
    - fi-bwr-2160:        [PASS][3] -> [INCOMPLETE][4]
   [3]: https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_7905/fi-bwr-2160/igt@i915_selftest@live_objects.html
   [4]: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_16515/fi-bwr-2160/igt@i915_selftest@live_objects.html

  
Known issues
------------

  Here are the changes found in Patchwork_16515 that come from known issues:

### IGT changes ###

#### Possible fixes ####

  * igt@gem_close_race@basic-threads:
    - fi-hsw-peppy:       [INCOMPLETE][5] ([i915#694] / [i915#816]) -> [PASS][6]
   [5]: https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_7905/fi-hsw-peppy/igt@gem_close_race@basic-threads.html
   [6]: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_16515/fi-hsw-peppy/igt@gem_close_race@basic-threads.html

  * igt@i915_selftest@live_gem_contexts:
    - fi-cfl-8700k:       [INCOMPLETE][7] ([i915#424]) -> [PASS][8]
   [7]: https://intel-gfx-ci.01.org/tree/drm-tip/CI_DRM_7905/fi-cfl-8700k/igt@i915_selftest@live_gem_contexts.html
   [8]: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_16515/fi-cfl-8700k/igt@i915_selftest@live_gem_contexts.html

  
  [i915#424]: https://gitlab.freedesktop.org/drm/intel/issues/424
  [i915#694]: https://gitlab.freedesktop.org/drm/intel/issues/694
  [i915#816]: https://gitlab.freedesktop.org/drm/intel/issues/816


Participating hosts (46 -> 40)
------------------------------

  Additional (4): fi-skl-lmem fi-glk-dsi fi-snb-2520m fi-kbl-r 
  Missing    (10): fi-ilk-m540 fi-bsw-n3050 fi-byt-j1900 fi-hsw-4200u fi-byt-squawks fi-bsw-cyan fi-gdg-551 fi-blb-e6850 fi-byt-clapper fi-bsw-nick 


Build changes
-------------

  * CI: CI-20190529 -> None
  * Linux: CI_DRM_7905 -> Patchwork_16515

  CI-20190529: 20190529
  CI_DRM_7905: db98da3dd757a19dbaaeaef8640276fe7be2fc4e @ git://anongit.freedesktop.org/gfx-ci/linux
  IGT_5433: 6a96c17f3a1b4e1f90b1a0b0ce42a7219875d1a4 @ git://anongit.freedesktop.org/xorg/app/intel-gpu-tools
  Patchwork_16515: 2b06b9a9a6d6785235ca559832db83a0e69928ff @ git://anongit.freedesktop.org/gfx-ci/linux


== Linux commits ==

2b06b9a9a6d6 drm/i915: Disable -Wtautological-constant-out-of-range-compare

== Logs ==

For more details see: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_16515/index.html
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