[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 <natecha...@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

[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 <natecha...@gmail.com>
---

v1 -> v2: https://lore.kernel.org/lkml/20200211050808.294...@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)

[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)?


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

[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/20191123195321.413...@gmail.com/

Cheers,
Nathan

[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.

[Nathan Chancellor]

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

[Michel Dänzer]

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.

[Jani Nikula]

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

[Nathan Chancellor]

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

Cheers,
Nathan

[Jani Nikula]

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

[Nick Desaulniers]

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

[Nathan Chancellor]

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

Cheers,
Nathan