1 March 16, March 16, 2016March 16, 2016March 16, 2016 Azusa, CA Sheldon X. Liang Ph. D. Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction CS400 Compiler Construction

2 Code generation: front-end and back end Front-end and back-end are generalized terms that refer to the initial and the end stages of a process. The front-end is responsible for collecting input in various forms from the user and processing it to conform to a specification the back-end can use. The connection of the front-end to the back-end is a kind of interface (IR). Many programs are divided conceptually into front and back-ends, but in most cases, the "back-end" is hidden from the user. However, sometimes programs are written which serve simply as a front-end to another, already existing program, such as a graphical user interface (GUI) which is built on top of a command-line interface. March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

3 March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, Keep in mind following questions Back End Code Generation Hidden from user Directly face machine IR, interface between FE & BE Instruction set and selection IR to assembly Criteria CISC vs RISC Code Generation For expressions Post-order traversal Why post order? CS400 Compiler Construction

4 Code generation Our book's target machine: opcode source1, source2, destination add r1, r2, r3 addI r1, c, r2 loadI c, r2 load r1, r2 loadAI r1, c, r2// r2 := *(r1+c) loadAO r1, r2, r3 i2i r1, r2// r2 := r1 (for integers) cmp_LE r1, r2, r3// if r1<=r2, r3:=true, else r3:=false cbr r1, L1, L2// if (r1) goto L1, else goto L2 jump r1 represents x's offset from the sp March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

5 Code generation Let's start with some examples. Generate code from a tree representing x = a+2 - (c+d-4) Issues: which children should go first? what if we already had a-c in a register? Does it make a difference if a and c are floating point as opposed to integer? Generate code for a switch statement Generate code for w = w*2*x*y*zw = w*2*x*y*z March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

6 Code generation Code generation = CISC vs RISC instruction selection instruction scheduling register allocation March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

7 CISC vs RISC Architecture March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

8 CISC vs RISC Architecture March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

9 Instruction selection IR to assembly Why is it an issue? Example: copy a value from r1 to r2 Let me count the ways... Criteria How hard is it? Use a cost model to choose. How about register usage? March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

10 Instruction selection How hard is it? Can make locally optimal choices Global optimality is NP-complete Criteria speed of generated code size of generated code power consumption Considering registers Assume enough registers are available, let register allocator figure it out. March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

11 Instruction scheduling Reorder instructions to hide latencies. Example: ( ) r1 ( ) addr1, r1, r1 ( ) r2 ( ) multr1, r2, r1 ( ) r2 ( ) mult r1, r2, r1 ( ) loadAI r2 ( ) multr1, r2, r1 ( ) storeAIr1, memory ops : 3 cycles multiply : 2 cycles everything else: 1 cycle March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

12 Instruction scheduling Reorder instructions to hide latencies. Example: (1) r1 (4) addr1, r1, r1 (5) r2 (8) multr1, r2, r1 (9) r2 (12) mult r1, r2, r1 (13) loadAI r2 (16) multr1, r2, r1 (18)storeAIr1, (20) (1) r1 (2) r2 (3) r3 (4) addr1, r1, r1 (5) multr1, r2, r1 (6) loadAI r2 (7) mult r1, r3, r1 (9) multr1, r2, r1 (11) storeAIr1, (13) March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

13 Instruction scheduling Reorder instructions to hide latencies. Example2: (1) r1 (4) multr1, r1, r1 (6) mult r1, r1, r1 (8) multr1, r1, r1 (10) storeAIr1, March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

14 Instruction scheduling Reorder instructions to hide latencies. We need to collect dependence info Scheduling affects register lifetimes ==> different demand for registers Should we do register allocation before or after? How hard is it? more than one instructions may be ready too many variables may be live at the same time NP-complete! March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

15 Register allocation Consists of two parts: register allocation register assignment Goal : minimize spills How hard is it? BB w/ one size of data: polynomial otherwise, NP-complete based on graph coloring. March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

16 Code generation Generating code for simple expressions If the expression is represented by a tree, a post-order walk gives an evaluation sequence  Why post-order? Changing the evaluation sequence may result in code that uses fewer registers. Idea: find out how many registers are needed for each subtree and evaluate the most expensive one first. Consider the following algorithm that labels tree nodes with the number of registers needed (for the ILOC architecture): 1 if n is a left leaf, or right leaf and variable 0 if n is a right leaf and constant max(label lchild, label rchild ) if label lchild  label rchild label lchild +1 if label lchild == label rchild label(n) = March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

17 Code generation Generating code for simple expressions The idea behind our algorithm: Variables need to be loaded in registers, so leaves need one register For rhs constants we can use opI operations, so no extra register needed If we need k registers for each subtree, we'll use k for the left one, keep 1 for the result, and then use another k for the right subtree. That's a total of k+1 registers If we need k registers for one subtree and m<k for the other, then use k for the one subtree, keep the result in 1, use the remaining k-1 (which is at least m) for the other subtree. That's a total of k registers. When generating code, make sure to evaluate the most expensive subtree first. March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

18 Code generation Generating code for expressions What if an expression contains a function call? Should the compiler be allowed to change the evaluation order? Big idea #1 wrt to optimizing or moving code around : Be conservative! Generating code for boolean operators relational operators read the book array references March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction

19 March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, Got it with following questions Back End Code Generation Hidden from user Directly face machine IR, interface between FE & BE Instruction set and selection IR to assembly Criteria CISC vs RISC Code Generation For expressions Post-order traversal Why post order? CS400 Compiler Construction

20 Thank you very much! Questions? March 16, Azusa Pacific University, Azusa, CA 91702, Tel: (800) Department of Computer Science, CS400 Compiler Construction