I am writing a pass to do constant folding. Like this C code:
int a = 4; int b = a + 5; I want to transform it to:
int b = 4 + 5; But the first segment of code will generate an instruction for int a:
store i32 4, i32* %a, align 4 How can I get the Value "%a" from this pointer "i32* %a" in my Pass? So that I can replace all use of the Value %a to the ConstantInt ?
Use the load instruction:
%1 = load i32* %a, align 4 
%a is unknown unless you run the program. A Pass doesn't run the program. It simply inspects and/or transform your program.%a is a "constant". So is there any other method to do this constant folding ? I know a constant 4 is stored into the address, I want to replace all the use of %a to 4 .store and use it as a replacement in load. This might get complicated though, because %a could be loaded into %1, %2, ... and chains of other variables.For that, first you need to get all the uses of Value %a
Val.user_begin() to val.user_end() via Value::const_user_iterator it and check if there Stored instruction is there and only const is stored
if (const StoreInst* store = dyn_cast<StoreInst>(*it)) then you can replace all instances with that const. Note: this is vary simple algo for inst that you have given
a = 5; b = a +4;
alloca.