do we need register to allocate to all a,b,c initially.....if following is program sequence

Question

Answer 1

If you'll use a good compiler then it'll find that both expressions can be evaluated as compile time since operands are constants. So there is not any need of executing any expression at runtime.

But if constant propagation optimization is restricted, then it can be evaluated with the help of two registers.

d= a+b;

R1 <- R1 + R2  //before execution   R1 : contains value of a, R2 : contains value of b,

c= e+d;

R1 <- R1 + R2 //before execution R1 contains value of variable d and R2 contains variable e, result will be in register R1

Comments

but here in this when program will be executed sequencially then regsters should be allocated to a ,b ,c in first three statements of program???....(only consider no optimizations)

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It depends on the compiler implementation, If compiler has more than two registers then it can keep the constant value in the registers and use it while evaluating the expression or another option would be to keep only two variables in the register and  will load the another variable(which is C in this case) after evaluation of first expression.

 

regarding the initialisation of a, b and c variables, there is some instructions in almost every architecture, which writes the constant directly to memory (not any need of register).

Answer 2

Answer is 1

MOV a R1 // move "a" to R1

ADD b R1 // Add "b" with R1

ADD c R1 // Add "c" with R1

Now value of e is in R1.