Re: [PATCH] drm/i915: prevent out of range pt in the PDE macros (take 3)

[Michel Thierry <michel.thierry@intel.com>]

On 10/1/2015 5:09 PM, Chris Wilson wrote:
> On Thu, Oct 01, 2015 at 04:59:35PM +0100, Michel Thierry wrote:
>> ---
>>   drivers/gpu/drm/i915/i915_gem_gtt.h | 14 ++++++++++----
>>   1 file changed, 10 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.h b/drivers/gpu/drm/i915/i915_gem_gtt.h
>> index 9fbb07d..94f8344 100644
>> --- a/drivers/gpu/drm/i915/i915_gem_gtt.h
>> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.h
>> @@ -394,7 +394,9 @@ struct i915_hw_ppgtt {
>>    */
>>   #define gen6_for_each_pde(pt, pd, start, length, temp, iter) \
>>   	for (iter = gen6_pde_index(start); \
>> -	     pt = (pd)->page_table[iter], length > 0 && iter < I915_PDES; \
>> +	     pt = (length > 0 && iter < I915_PDES) ? \
>> +			(pd)->page_table[iter] : NULL, \
>> +	     length > 0 && iter < I915_PDES; \
>
> length > 0 && iter < I915_PDES ? (pt = (pd)->page_table[iter]) : 0,
>
> as the compiler wouldn't be able to CSE it otherwise (I think).

Even after that change, the compiler keeps doing an optimization when 
page_table[iter] is null (takes the null assignment as the break condition).

I've been playing with these examples
http://paste.ubuntu.com/12638106/

Only the 1st example (a) iterates over all elements, b & c stop after 
the 1st run.

Thanks,

-Michel
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