[LLVMdev] llvm cse optimization

[Nadav Rotem]

On Apr 11, 2014, at 10:13 AM, xiaoyong liu <xiaoyongliu at outlook.com> wrote:

> I'm working on the across the subprogram call optimization, and let's say the following llvm IR,
>  
>  @ng0 = internal global [2 x i32] [i32 2, i32 0], align 4
>  
>   %t7 = alloca [16 x i8], align 16
>   %add.ptr = getelementptr inbounds i8* %t1, i64 40
>   %call = call i8* @handle_value(i8* %add.ptr, i32 3) #3
>   %arraydecay = getelementptr inbounds [8 x i8]* %t3, i64 0, i64 0
>   %arraydecay1 = getelementptr inbounds [16 x i8]* %t4, i64 0, i64 0
>  call void @select_value(i8* %arraydecay, i32 1, i32 1, i8* %call, i8* %arraydecay1, i8* null, i32 1, i32 2, i32 1, i8* bitcast ([2 x i32]* @ng0 to i8*), i32 32, i32 1) #3
>   %call3 = call i8* @handle_value(i8* %add.ptr, i32 3) #3
>  
> ; Function Attrs: nounwind readonly
> declare i8* @handle_value(i8* nocapture, i32) #1
> ; Function Attrs: nounwind
> declare void @select_value(i8*, i32, i32, i8* nocapture readonly, i8* nocapture readonly, i8*, i32, i32, i32, i8*, i32, i32) #2 //select value will only write to the first parameter, other parameter is readonly

The function ’select_value’ is not marked as *not* having side effects. They compiler has to assume that it could write to memory or print to the screen so it can’t merge them.  

>  
> For the above case, the second handle_value function call should be replaced with %call with cse or gvn optimization.
>  
> Is anything I can do to optimize this case?
>  
> thanks
> 
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