Liveness of a variable

Question

https://gateoverflow.in/?qa=blob&qa_blobid=6419625116648850773

Comments

They have assumed {b,d} to be live at the end of the program because I scanned the pdf completely, but could not find any sequence of statements which tells that {b,d} should be live at the end.

If anyone has a better explanation, please do post.

Answer 1

Live variable means those which will be used again- not necessarily by same block.

int a = 5;
if(a < b)
{
   b = 7;
}
c = a;

Here the statements inside the if are in a separate basic block and a is live there because a is used again though in a different block. Compiler use live variable analysis to see for which all variables it can free the registers.