[llvm-dev] [MemSSA] Getting more precise information regarding MemoryPhi

[Hao Jin via llvm-dev]

Hi Alina,

Thank you for your reply. Your explanation is really helpful.

Best,
Hao

> On Mar 9, 2021, at 15:37, Alina Sbirlea <alina.sbirlea at gmail.com> wrote:
> 
> Hi,
> 
> Adding another MemoryPhi, or altering the current one will make MemorySSA invalid. So simply creating the instance `MemoryPhi({if.then,1},{if.else,3})` is not a valid option in the current implementation, regardless of where it is done.
> I would suggest doing the querying you need in the analysis that uses MemorySSA, and then figure out if it makes sense to have the same mechanics in a custom walker.
> 
> Best,
> Alina
> 
> 
> On Tue, Mar 9, 2021 at 8:07 AM Hao Jin via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Hello all,
>  
> I am trying to use MemorySSA in my analysis and recently I got a problem about querying the clobbering memory access.
>  
> For the example below, both the loads for %c and %d have the clobbering memory access 5 = MemoryPhi({if.then,2},{if.else,4}).
>  
> I believe it can be further optimized. The stores that actually define %c are; 1 = MemoryDef(liveOnEntry) and ; 3 = MemoryDef(liveOnEntry) so the clobbering memory access for the load of %c could be another MemoryPhi({if.then,1},{if.else,3}).
>  
> I understand that it is the design of memorySSA to have a single MemoryPhi for each Basicblock but I am wondering if I want to have more precise information like this, should this be done as a part of MemSSA/Walker or should the analysis that uses MemorySSA handle this?
>  
> Thank you for your time and I am looking forward to your reply.
> Best,
> Hao
>  
> int foo(int a, int b, int m){
>   int c;
>   int d;
>   if(m){
>     c = a;
>     d = b;
>   }else{
>     c = a;
>     d = b;
>   }
>   return c+d;
> }
>  
> Corresponding IR and MemSSA dump:
> define dso_local i32 @foo(i32 %a, i32 %b, i32 %m) {
> entry:
>   %c = alloca i32, align 4
>   %d = alloca i32, align 4
>   %tobool = icmp ne i32 %m, 0
>   br i1 %tobool, label %if.then, label %if.else
>  
> if.then:                                          ; preds = %entry
> ; 1 = MemoryDef(liveOnEntry)
>   store i32 %a, i32* %c, align 4
> ; 2 = MemoryDef(1)
>   store i32 %b, i32* %d, align 4
>   br label %if.end
>  
> if.else:                                          ; preds = %entry
> ; 3 = MemoryDef(liveOnEntry)
>   store i32 %a, i32* %c, align 4
> ; 4 = MemoryDef(3)
>   store i32 %b, i32* %d, align 4
>   br label %if.end
>  
> if.end:                                           ; preds = %if.else, %if.then
> ; 5 = MemoryPhi({if.then,2},{if.else,4})
> ; MemoryUse(5) MayAlias
>   %0 = load i32, i32* %c, align 4
> ; MemoryUse(5) MayAlias
>   %1 = load i32, i32* %d, align 4
>   %add = add nsw i32 %0, %1
>   ret i32 %add
> }
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