[llvm-dev] [MemSSA] Getting more precise information regarding MemoryPhi (original) (raw)

Alina Sbirlea via llvm-dev llvm-dev at lists.llvm.org
Tue Mar 9 12:37:09 PST 2021

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 dsolocal 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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