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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