# 94

Compiler Engineer interview questions shared by candidates

## Top Interview Questions

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Junior Compiler Software Engineer was asked...29 March 2018

### Assume your assembly language includes ONLY the following instructions: 'inc REG': increments a given register by one. 'dec REG': decrement a given register by one. 'jnz LABEL': jumps to a given LABEL if the previous instruction's result was not zero. 'HALT': stops running. Task: A and B registers hold non-negative values. The program should calculate the value of |A-B| and locate the result in C. In addition, the language holds registers C,D,...,Z, which you can assume are initialized at program start to zero.

Not well

sub_A: DEC A JNZ sub_B INC D JNZ A_reached_zero sub_B: DEC B JNZ sub_A A_grater: INC C DEC A JNZ A_greater HALT A_reached_zero: DEC B JNZ B_greater HALT B_greater: INC C DEC B JNZ B_greater HALT Less

### Optimize this code (like what compiler would reconstruct an equivalent source code as): int fn (int a, int b) { int sum = 0; for(int i=4*a;i&gt;0;i--) { sum+=b*i*i; } return sum; }

One of them is replace 4*b with a&lt;&lt;2. But there are more ways I didn't find. Less

the loop can be replaced by one line (basic series formula) : sum = b* 4*a *(4*a +1) *(8*a+1)/6 http://www.tug.org/texshowcase/cheat.pdf Less

### LLVM and projects that I did.

I just explained my projects. I also showed my open-source contribution through screen sharing. Less

int isPowoftwo(int n) { while(n&gt;0) { if(n&amp;1) return (n==1) n= n&gt;&gt;1; } return 0; } Less

### What's the difference between C++ and C?

Object oriented programming in C++ is the biggest difference, obviously. They will ask you to expand on this, so make sure you know what you're talking about before answering it. Less

### The performance of a program is slower than the same program before the backend of the compiler has changed. What direction would you look into this issue and solve it?

I will utilize tools to find out hotspots in the target program. Then disassemble some particular hotspots, to see whether it's regard to loop, and CPU pipeline. Then try to unroll the loop, or rearrange the policy of the CG backend. Less

### How would you narrow down bugs, if you came in one day and found 10 new failures with your run?

Check the environment first, so see if anything has changed.

Very good