[PATCH] D25172: [MSSA] Initial version of avoiding unnecessary use walks when calling getClobberingMemoryAccess

[George Burgess IV via llvm-commits]

> First, it looks like your attempt to add basicAA in setupAnalyses is not
doing anything

r283158 should fix that -- thanks.

> We have no way to catch all the Use.set's that i can find

Can we keep an `OldDef` member that tracks the ID of the Def/Phi we last
optimized to? It makes isOptimized() a bit more expensive, but I can't
think of a valid transformation that swaps the DefiningAccess of Foo from
Bar to Baz to Bar while making Bar not the clobber for Foo.

On Sun, Oct 2, 2016 at 8:21 PM, Daniel Berlin <dberlin at dberlin.org> wrote:

> Sigh.
>
> Two things need your thoughts on george.
>
> First, it looks like your attempt to add basicAA in setupAnalyses is not
> doing anything:
>  76   AliasResult AAResults::alias(const MemoryLocation &LocA,
>    77                               const MemoryLocation &LocB) {
> -> 78    for (const auto &AA : AAs) {
>    79      auto Result = AA->alias(LocA, LocB);
>    80      if (Result != MayAlias)
>    81        return Result;
> (lldb) p AAs
> (std::__1::vector<std::__1::unique_ptr<llvm::AAResults::Concept,
> std::__1::default_delete<llvm::AAResults::Concept> >,
> std::__1::allocator<std::__1::unique_ptr<llvm::AAResults::Concept,
> std::__1::default_delete<llvm::AAResults::Concept> > > >) $33 = size=0 {}
>
>
>
> Simple way to see this:
>
> In the unit test, the following:
>   Type *Int8 = Type::getInt8Ty(C);
>   Value *AllocaA = B.CreateAlloca(Int8, ConstantInt::get(Int8, 1), "A");
>   Instruction *SIA = B.CreateStore(ConstantInt::get(Int8, 0), AllocaA);
>   Value *AllocaB = B.CreateAlloca(Int8, ConstantInt::get(Int8, 1), "B");
>   Instruction *SIB = B.CreateStore(ConstantInt::get(Int8, 0), AllocaB);
>   Instruction *LIA = B.CreateLoad(AllocaA);
>
> results in:
> define void @F() {
>   %A = alloca i8, i8 1
> ; 1 = MemoryDef(liveOnEntry)
>   store i8 0, i8* %A
>   %B = alloca i8, i8 1
> ; 2 = MemoryDef(1)
>   store i8 0, i8* %B
> ; MemoryUse(2)
>   %1 = load i8, i8* %A
> }
>
> (The load is not being optimized because there is no AA).
>
> Second, we can only reset optimized properly if all operand setting is
> done through setDefinedAccess.
>
> We have no way to catch all the Use.set's that i can find.
>
> If that's correct, i'll drop the optimized flag, and just keep the part to
> reset setDefiningAccess.
>
> This means we will still pay a cost for calling getclobberingmemoryaccess
> on loads.
>
>
> On Sun, Oct 2, 2016 at 7:39 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
>
>> dberlin added a comment.
>>
>> (Unittest that resets the defining access, and calls getClobberingAccess,
>> coming)
>>
>>
>> https://reviews.llvm.org/D25172
>>
>>
>>
>>
>
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