Common Subexpressions Common subexpression Previously computed The values of the variables not changed ·Local:: t6=4*i B5 t6=4*1 B5 x a[t6] x a[t6] t7=4*1 t8=4*j t8=4*j t9=a[t8] t9=a[t8] a[t6]=t9 a[t7]=t9 a[t8]=x t10=4*j goto B2 a[t10]=x goto B2 (a)Before. (b)After. CS308 Compiler Theory 8Common Subexpressions • Common subexpression – Previously compute d – The values of the variables not changed • Local: CS308 Compiler Theory 8