[PATCH] [SCEV][LoopVectorize] Allow ScalarEvolution to make assumptions about overflows

[Silviu Baranga]

Ping? (http://reviews.llvm.org/D10161)

@Sanjoy: would you be willing to review this? It looks like you might be
the person to do it.

Also CC'ing Adam as he might be interested as well.

Thanks,
Silviu


>> -----Original Message-----
>> From: llvm-commits-bounces at cs.uiuc.edu [mailto:llvm-commits-
>> bounces at cs.uiuc.edu] On Behalf Of silviu.baranga at arm.com
>> Sent: 01 June 2015 17:16
>> To: Silviu Baranga
>> Cc: llvm-commits at cs.uiuc.edu
>> Subject: [PATCH] [SCEV][LoopVectorize] Allow ScalarEvolution to make
>> assumptions about overflows
>> 
>> Add a new pass - AssumingScalarEvolution - that extends SCEV, but can
>> make assumptions and generate code that can check these assumptions.
>> This pass uses the normal ScalarEvolution pass for anything outside the
>> current analyzed loop, but uses its own data structures to handle the SCEVs
>> within the loop. For now, the pass assumes that chrec expressions will not
>> overflow.
>> 
>> The AssumingScalarEvolution pass can add checks for the made
>> assumptions, so that a loop can be versioned.
>> 
>> We use this pass in order to add runtime overflow checks in the Loop
>> Vectorize pass of expressions which can in theory overflow and would
>> prevent the vectorization a loop. Also note that the runtime checks will
>> almost always pass, since having an overflow is usually a sign of a coding
>> error on the part of user.
>> 
>> The main reasons behind inheriting from ScalarEvolution:
>> Since ScalarEvolution maintains its own cache for various results, making any
>> assumption would likely require invalidating the entire cache.
>> 
>> This way we can use the new pass as an almost drop-in replacement for
>> ScalarEvolution. The users that are doing a transformation would have to
>> know about it in order to add the checks.
>> 
>> However, there are probably a lot more ways in which this could be
>> implemented.
>> 
>> Any comments are much appreciated!
>> 
>> Thanks,
>> Silviu
>> 
>> http://reviews.llvm.org/D10161
>> 
>> Files:
>>   include/llvm/Analysis/LoopAccessAnalysis.h
>>   include/llvm/Analysis/ScalarEvolution.h
>>   include/llvm/InitializePasses.h
>>   lib/Analysis/Analysis.cpp
>>   lib/Analysis/LoopAccessAnalysis.cpp
>>   lib/Analysis/ScalarEvolution.cpp
>>   lib/Transforms/Vectorize/LoopVectorize.cpp
>>   test/Transforms/LoopVectorize/safegep.ll
>>   test/Transforms/LoopVectorize/scev-overflow-check.ll
>>   test/Transforms/LoopVectorize/version-mem-access.ll
>> 
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