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1、Compiler ConstrucionS1Three-Address CodeThe most basic instruction of three address codex=y op zThe use of the address x differs from the addresses of y and z.y and z can represent constants and literal values.Compiler ConstrucionS2Form of Intermediate CodeIntermediate code is a linearization of the
2、 syntax tree.Intermediate code Can be very high level,representing operations almost as abstractly as the syntax tree or can closely resemble target code.May use or not used detailed information about the target machine and runtime environment.Compiler ConstrucionS3Example2*a+(b-3)+*-2ab3t1=2*at2=b-
3、3t3=t1+t2t1=b-3t2=2*at3=t2+t1Left-to-right linearization Right-to-left linearization Compiler ConstrucionS4Three-Address Code(cont)It is necessary to vary form of the three-address code to express all constructs(e.g.t2=-t1)No standard form exists.Compiler ConstrucionS5Implementation of Three-Address
4、 CodeEach three-address instruction is implemented as a record structure containing several fields.The entire sequence is an array or a linked list.The most common implementation requires four fields quadruple One for operation and three for addresses.For instructions that need fewer number of addre
5、sses,one or more addresses fields is given null or“empty”values.Compiler ConstrucionS6Factorial Program Sample program in TINY language-computes factorial read x;input an integer if 0 x then dont compute if x=0 fact:=1;repeat fact:=fact*x;x:=x-1 until x=0;write fact output factorial of x endCompiler ConstrucionS7Syntax Tree for Factorial Program