[PATCH] Refine memory dependence's notion of volatile semantics

[Philip Reames]

ping x2

On 01/16/2015 11:35 AM, Philip Reames wrote:
> ping
> On 01/09/2015 11:19 AM, Philip Reames wrote:
>> Hi hfinkel, atrick, bob.wilson, void, morisset,
>>
>> (For reviewers who get duplicates, sorry!  Forgot to add llvm-commits 
>> to the first review and had to close it and open another.)
>>
>> According to my reading of the LangRef, volatiles are only ordered 
>> with respect to other volatiles. It is entirely legal and profitable 
>> to forward unrelated loads over the volatile load. This patch 
>> implements this for GVN by refining the transition rules 
>> MemoryDependenceAnalysis uses when encountering a volatile.
>>
>> The added test cases show where the extra flexibility is profitable 
>> for local dependence optimizations. I have a related patch 
>> (http://reviews.llvm.org/D6895) which will extend this to non-local 
>> dependence (i.e. PRE), but that's essentially orthogonal to the 
>> semantic change in this patch. I have tested the two together and can 
>> confirm that PRE works over a volatile load with both changes.
>>
>> One question for reviewers: should a load being volatile change the 
>> default handling for a *may* alias load? For a non-volatile load, 
>> we'll keep scanning past looking for an actual mustalias dependency. 
>> Is this the behaviour we want for volatiles? While I think this 
>> matches the letter of the LangRef, I'm not entirely sure it matches 
>> the expectations of mapping C volatile to LLVM volatile. In 
>> particular, should the following code always return 0? With my 
>> current patch, we can and will forward the first load to third 
>> without changing the second.
>> int example(int* p, int *q) { // q == p at runtime
>>    int x = *p;
>>    *(volatile int*)q
>>    int y = *p;
>>    return x-y;
>> }
>>
>> http://reviews.llvm.org/D6901
>>
>> Files:
>>    lib/Analysis/MemoryDependenceAnalysis.cpp
>>    test/Transforms/GVN/volatile.ll
>>
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>
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