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Soundness bugfix for barrier<thread_scope_block> on sm_70 #300
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@daniellustig could maybe review? |
griwes
reviewed
Aug 10, 2022
| unsigned int __active = __activeA & __activeB; | ||
| int __inc = __popc(__active) * __update; | ||
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| unsigned __laneid; | ||
| asm volatile ("mov.u32 %0, %laneid;" : "=r"(__laneid)); | ||
| asm ("mov.u32 %0, %laneid;" : "=r"(__laneid)); |
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Why are you removing the volatile here?
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Because it is not necessary. Two "calls" to this assembly instruction from this same thread always "return" the same value, so eliding one is a valid transformation for the compiler to do.
This assembly statement does not modify any memory, so it does not need a "memory" clobber either.
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The cumulativity fix seems reasonable to me |
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@wmaxey ? |
davedsth
approved these changes
Sep 14, 2022
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Thanks for review, David. Merging. |
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For sm_70, barrier arrive has an optimization to "coalesce" all arrives with the same update to the same barrier into a single update performed by a "leader" thread.
This optimization is missing a release fence to establish cummulativity between all coalesced threads and the leader, before the leader performs the update.