[llvm-dev] [RFC] bitfield access shrinking

[Hal Finkel via llvm-dev]

On 03/09/2017 12:28 PM, Krzysztof Parzyszek via llvm-dev wrote:
> We could add intrinsics to extract/insert a bitfield, which would 
> simplify a lot of that bitwise logic.

But then you need to teach a bunch of places about how to simply them, 
fold using bitwise logic and other things that reduce demanded bits into 
them, etc. This seems like a difficult tradeoff.

  -Hal

>
> -Krzysztof
>
>
> On 3/9/2017 12:14 PM, Wei Mi via llvm-dev wrote:
>> In 
>> http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20120827/063200.html,
>> consecutive bitfields are wrapped as a group and represented as a
>> large integer and emits loads stores and bit operations appropriate
>> for extracting bits from within it. It fixes the problem of violating
>> C++11 memory model that original widen load/store of bitfield was
>> facing. It also brings more coalescing opportunities across bitfields.
>>
>> If some bitfields are natural aligned and their num of bits can form a
>> legal integer type that the target supports, it is more efficient to
>> access them directly than doing a large integer load/store plus a
>> series of bit operations. We call such reverse transformation legal
>> type bitfield shrinking. Currently, llvm depends on DAGCombiner to do
>> such shrinking.
>>
>> However, DAGCombiner has the one-basic-block-a-time limitation, so we
>> started to implement a new shrinking optimization in
>> https://reviews.llvm.org/D30416, and initially put it in instcombine,
>> then moved it to CGP because we want to use some TargetLowering
>> information.
>>
>> The initial implementation in D30416 only supports load-and-or-store
>> pattern matching, and it uses a inst-by-inst scan as a safety check to
>> make sure there is no other memory write insn between the load and
>> store (no alias query is done).  After getting the initial
>> implementation, we found more problems: EarlyCSE, LoadPRE and even
>> InstCombine itself can do coalescing before the shrinking (LoadPRE
>> does it the most thoroughly).
>> The coalescing can move the load many BasicBlocks earlier and make
>> simple inst-by-inst scan unscalable and oftentimes fail. It also
>> breaks the load-and-or-store pattern. An example is below:
>>
>> Before coalescing done by earlycse/loadpre:
>> %bf.load = load i64, i64* %0, align 8
>> %bf.clear = and i64 %bf.load, -65536
>> %bf.set = or i64 %bf.value, %bf.clear
>> store i64 %bf.set2, i64* %9, align 8
>> .....
>> %bf.load1 = load i64, i64* %0, align 8
>> %bf.clear1 = and i64 %bf.load1, -4294901761
>> %bf.set1 = or i64 %bf.value1, %bf.clear1
>> store i64 %bf.set2, i64* %9, align 8
>> .....
>> %bf.load2 = load i64, i64* %0, align 8
>> %bf.clear2 = and i64 %bf.load2, -4294901761
>> %bf.set2 = or i64 %bf.value2, %bf.clear2
>> store i64 %bf.set2, i64* %9, align 8
>>
>> After coalescing, it will become:
>> %bf.load = load i64, i64* %0, align 8
>> %bf.clear = and i64 %bf.load, -65536
>> %bf.set = or i64 %bf.value, %bf.clear
>> .....
>> %bf.clear1 = and i64 %bf.set, -4294901761
>> %bf.set1 = or i64 %bf.value1, %bf.clear1
>> .....
>> %bf.clear2 = and i64 %bf.set1, -4294901761
>> %bf.set2 = or i64 %bf.value2, %bf.clear2
>> store i64 %bf.set2, i64* %9, align 8
>>
>> After load-store coalescing, %bf.load now is far away from the store,
>> and the previous load-and-or-store pattern disappears.
>>
>> A simple idea to fix it is to move the shrinking in a very early pass
>> before the first pass of EarlyCSE. However, if we move shrinking
>> earlier, it is possible to block the coalescing of other ilegal type
>> bitfields which can not be shrinked. So for coalescing and shrinking,
>> no matter which one is first, it will block the other one.
>>
>> After some discussions with Eli and Michael, I come up with another
>> idea to let shrinking stay in the late pipeline. It needs two changes
>> to the current patch:
>>
>> 1. extending the pattern match to handle store(or(and(or(and...))
>> pattern above. It needs to analyze the bit mask of every and-or pairs.
>> If the last and-or pair touch different section with the other and-or
>> pairs, we can split the original store into two, and do the shrinking
>> for the second store, like below
>>
>> %bf.load = load i64, i64* %0, align 8
>> %bf.clear = and i64 %bf.load, -65536
>> %bf.set = or i64 %bf.value, %bf.clear
>> .....
>>
>> %bf.clear1 = and i64 %bf.set, -4294901761
>> %bf.set1 = or i64 %bf.value1, %bf.clear1
>> store i64 %bf.set1, i64* %0, align 8                         // the 
>> first store.
>> .....
>>
>> %bf.value2.shrinked = shrink_op %bf.value2
>> store i16 %bf.value2.shrinked, i64* %0, align 8       // shrinked store.
>>
>> 2. use memoryssa + alias query to do the safety check. Because
>> dominator tree is not properly updated in CGP, I have to create a new
>> pass and put it before CGP, in order to use memoryssa.
>>
>> Eli suggested me to ask for more opinions before start writting code.
>> I think it is a good idea and here is the post. Comments are
>> appreciated.
>>
>> Thanks,
>> Wei.
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>>
>

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory