Compilers

Start Lecture #13

Chapter 7: Run Time Environments

Homework: Read Chapter 7.

7.1: Storage Organization

We are discussing storage organization from the point of view of the compiler, which must allocate space for programs to be run. In particular, we are concerned with only virtual addresses and treat them uniformly.

This should be compared with an operating systems treatment, where we worry about how to effectively map this configuration to real memory. For example see these two diagrams in my OS class notes, which illustrate an OS difficulty with our allocation method, which uses a very large virtual address range. Perhaps the most straightforward solution uses multilevel page tables .

Some system require various alignment constraints. For example 4-byte integers might need to begin at a byte address that is a multiple of four. Unaligned data might be illegal or might lower performance. To achieve proper alignment padding is often used.

Areas (Segments) of Memory

As mentioned above, there are various OS issues we are ignoring, for example the mapping from virtual to physical addresses, and consequences of demand paging. In this class we simply allocate memory segments in virtual memory let the operating system worry about managing real memory. In particular, we consider the following four areas of virtual memory.

1. The code (often called text in OS-speak) is fixed size and unchanging (self-modifying code is long out of fashion). If there is OS support, the text could be marked execute only (or perhaps read and execute, but not write). All other areas would be marked non-executable (except for systems like lisp that execute their data).
   
   
2. There is likely data of fixed size whose need can be determined by the compiler by examining the program's structure (and not by determining the program's execution pattern). One example is global data. Storage for this data would be allocated in the next area right after the code. A key point is that since the code and this area are of fixed size that does not change during execution, they, unlike the next two areas, have no need for an expansion region.
   
   
3. The stack is used for memory whose lifetime is stack-like. It is organized into activation records that are created as a procedure is called and destroyed when the procedure exits. It abuts the area of unused memory so can grow easily. Typically the stack is stored at the highest virtual addresses and grows downward (toward small addresses). However, it is sometimes easier in describing the activation records and their uses to pretend that the addresses are increasing (so that increments are positive).
   
   
4. The heap is used for data whose lifetime is not as easily described. This data is allocated by the program itself, typically either with a language construct, such as new, or via a library function call, such as malloc(). It is deallocated either by another executable statement, such as a call to free(), or automatically by the system.

7.1.1: Static Versus Dynamic Storage Allocation

Much (often most) data cannot be statically allocated. Either its size is not known at compile time or its lifetime is only a subset of the program's execution.

Early versions of Fortran used only statically allocated data. This required that each array had a constant size specified in the program. Another consequence of supporting only static allocation was that recursion was forbidden (otherwise the compiler could not tell how many versions of a variable would be needed).

Modern languages, including newer versions of Fortran, support both static and dynamic allocation of memory.

The advantage supporting dynamic storage allocation is the increased flexibility and storage efficiency possible (instead of declaring an array to have a size adequate for the largest data set; just allocate what is needed). The advantage of static storage allocation is that it avoids the runtime costs for allocation/deallocation and may permit faster code sequences for referencing the data.

An (unfortunately, all too common) error is a so-called memory leak where a long running program repeated allocates memory that it fails to delete, even after it can no longer be referenced. To avoid memory leaks and ease programming, several programming language systems employ automatic garbage collection. That means the runtime system itself determines when data can no longer be referenced and automatically deallocates it.

7.2: Stack Allocation of Space

The scheme to be presented achieves the following objectives.

1. Memory is shared by procedure calls that have disjoint durations. Note that we are not able to determine disjointness by just examining the program itself (due to data dependent branches among other issues).
2. The relative address of each (visible) nonlocal variable is constant throughout each execution of a procedure. Note that during this execution the procedure can call other procedures.

7.2.1: Activation Trees

Recall the fibonacci sequence 1,1,2,3,5,8, ... defined by f(1)=f(2)=1 and, for n>2, f(n)=f(n-1)+f(n-2). Consider the function calls that result from a main program calling f(5). Surrounding the more-general pseudocode that calculates (very inefficiently) the first 10 fibonacci numbers, we show the calls and returns that result from main calling f(5). On the left they are shown in a linear fashion and, on the right, we show them in tree form. The latter is sometimes called the activation tree or call tree.

  System starts main               int a[10];
      enter f(5)                   int main(){
          enter f(4)                   int i;
              enter f(3)               for (i=0; i<10; i++){
                  enter f(2)             a[i] = f(i);
                  exit f(2)            }
                  enter f(1)       }
                  exit f(1)        int f (int n) {
              exit f(3)                if (n<3)  return 1;
              enter f(2)               return f(n-1)+f(n-2);
              exit f(2)            }
          exit f(4)
          enter f(3)
              enter f(2)
              exit f(2)
              enter f(1)
              exit f(1)
          exit f(3)
      exit f(5)
  main ends

We can make the following observations about these procedure calls.

1. If an activation of p calls q, then that activation of p terminates no earlier than the activation of q.
   
   
2. The order of activations (procedure calls) corresponds to a preorder traversal of the call tree.
   
   
3. The order of de-activations (procedure returns) corresponds to postorder traversal of the call tree.
   
   
4. If execution is currently in an activation corresponding to a node N of the activation tree, then the activations that are currently live are those corresponding to N and its ancestors in the tree.
   
   
5. These live activations were called in the order given by the root-to-N path in the tree, and the returns will occur in the reverse order.

7.2.2: Activation Records (ARs)

The information needed for each invocation of a procedure is kept in a runtime data structure called an activation record (AR) or frame. The frames are kept in a stack called the control stack.

Note that this is memory used by the compiled program, not by the compiler. The compiler's job is to generate code that obtains the needed memory and to correctly reference the variables stored in the ARs.

At any point in time the number of frames on the stack is the current depth of procedure calls. For example, in the fibonacci execution shown above when f(4) is active there are three activation records on the control stack.

ARs vary with the language and compiler implementation. Typical components are described below and pictured to the right. In the diagrams the stack grows down the page.

1. The arguments (sometimes called the actual parameters). The first few arguments are often placed in registers.
2. The returned value. This is often placed in a register (if it is a scalar).
3. The control link connects the ARs by pointing to the AR of the caller.
4. The access link (described below).
5. Saved status from the caller, which typically includes the return address and the machine registers. The register values are restored when control returns to the caller.
6. Data local to the procedure being activated.
7. Temporaries. For example, recall the temporaries generated during expression evaluation. Often these can be held in machine registers. When that is not possible (e.g., there are more temporaries than registers), the temporary area is used.

The diagram on the right shows (part of) the control stack for the fibonacci example at three points during the execution. The solid lines separate ARs; the dashed lines separate components within an AR.

In the upper left we have the initial state, We show the global variable a, although it is not in an activation record and actually is allocated before the program begins execution (it is statically allocated; recall that the stack and heap are each dynamically allocated). Also shown is the activation record for main, which contains storage for the local variable i. Recall that local variables are near the end of the AR.

Below the initial state we see the next state when main has called f(1) and there are two activation records, one for main and one for f. The activation record for f contains space for the argument n and and also for the returned value. Recall that arguments and the return value are allocated near the beginning of the AR. There are no local variables in f.

At the far right is a later state in the execution when f(4) has been called by main and has in turn called f(2). There are three activation records, one for main and two for f. It is these multiple activations for f that permits the recursive execution. There are two locations for n and two for the returned value.

7.2.3: Calling Sequences

The calling sequence, executed when one procedure (the caller) calls another (the callee), allocates an activation record (AR) on the stack and fills in the fields. Part of this work is done by the caller; the remainder by the callee. Although the work is shared, the AR is called the callee's AR.

Since the procedure being called is defined in one place, but normally called from many places, we would expect to find more instances of the caller activation code than of the callee activation code. Thus it is wise, all else being equal, to assign as much of the work to the callee as possible.

Although details vary among implementations, the following principles are often followed.

1. Values computed by the caller are placed before any items of size unknown by the caller. This way they can be referenced by the caller using fixed offsets. One possibility is to place values computed by the caller at the beginning of the activation record, i.e., near the AR of the caller. The number of arguments may not be the same for different calls of the same function (so called varargs, e.g. printf() in C). However the (compiler of the) caller knows how many arguments there are so, where pink calls blue, the compilers knows how far the return values is from the beginning of the blue AR. Since this beginning of the blue AR is the end of the pink AR (or is one location further depending on how you count), the caller knows (but only at run time) the offset of the return value location from its own stack pointer (sp, see below).
   
   
2. Fixed length items are placed next. Their sizes are known to the caller and callee at compile time. Examples of fixed length items include the links and the saved status.
   
   
3. Finally come items allocated by the callee whose size is known only at run-time, e.g., arrays whose size depends on the parameters.
   
   
4. The stack pointer sp is between the last two. One consequence of this location is that the temporaries and local data are actually above the stack. This would seem more surprising if I used the book's terminology, which is top_sp. Fixed length data can be referenced by fixed offsets (known to the intermediate code generator) from the sp.

The picture above illustrates the situation where a pink procedure (the caller) calls a blue procedure (the callee). Also shown is Blue's AR. Note that responsibility for this single AR is shared by both procedures. The picture is just an approximation: For example, the returned value is actually Blue's responsibility, although the space might well be allocated by Pink. Also some of the saved status, e.g., the old sp, is saved by Pink.

The picture to the right shows what happens when Blue, the callee, itself calls a green procedure and thus Blue is also a caller. You can see that Blue's responsibility includes part of its AR as well as part of Green's.

Calling Sequence

The following actions occur during a call.

1. The caller begins the process of creating the callee's AR by evaluating the arguments and placing them in the AR of the callee. (I use arguments for the caller, parameters for the callee.)
   
   
2. The caller stores the return address and the (soon-to-be-updated) sp in the callee's AR.
   
   
3. The caller increments sp so that instead of pointing into its AR, it points to the corresponding point in the callee's AR.
   
   
4. The callee saves the registers and other (system dependent) information.
   
   
5. The callee allocates and initializes its local data.
   
   
6. The callee begins execution.

Return Sequence

When the procedure returns, the following actions are performed by the callee, essentially undoing the effects of the calling sequence.

1. The callee stores the return value. Note that this address can be determined by the caller using the old (soon-to-be-restored) sp.
   
   
2. The callee restores sp and the registers.
   
   
3. The callee jumps to the return address.

Note that varagrs are supported.

Also note that the values written during the calling sequence are not erased and the space is not explicitly reclaimed. Instead, the sp is restored and, if and when the caller makes another call, the space will be reused.

7.2.4: Variable-Length Data on the Stack

There are two flavors of variable-length data.

• Data obtained by malloc/new have hard to determine lifetimes and are stored in the heap instead of the stack.
• Data, such as arrays with bounds determined by the parameters are still stack like in their lifetimes (if A calls B, these variables of A are allocated before and released after the corresponding variables of B).

It is the second flavor that we wish to allocate on the stack. The goal is for the callee to be able to access these arrays using addresses determined at compile time even though the size of the arrays is not known until the program is called, and indeed often differs from one call to the next (even when the two calls correspond to the same source statement).

The solution is to leave room for pointers to the arrays in the AR. These pointers are fixed size and can thus be accessed using static offsets. When the procedure is invoked and the sizes are known, the pointers are filled in and the space allocated.

A difficulty caused by storing these variable size items on the stack is that it no longer is obvious where the real top of the stack is located relative to sp. Consequently another pointer (call it real-top-of-stack) is also kept. This is used on a call to tell where the new allocation record should begin.

7.3: Access to Nonlocal Data on the Stack

As we shall see the ability of procedure P to access data declared outside of P (either declared globally outside of all procedures or declared inside another procedure Q) offers interesting challenges.

7.3.1: Data Access Without Nested Procedures

In languages like standard C without nested procedures, visible names are either local to the procedure in question or are declared globally.

1. For global names the address is known statically at compile time, providing there is only one source file. If there are multiple source files, the linker knows. In either case no reference to the activation record is needed; the addresses are known prior to execution.
2. For names local to the current procedure, the address needed is in the AR at a known-at-compile-time constant offset from the sp. In the case of variable size arrays, the constant offset refers to a pointer to the actual storage.

7.3.2: Issues With Nested Procedures

With nested procedures a complication arises. Say g is nested inside f. So g can refer to names declared in f. These names refer to objects in the AR for f; the difficulty is finding that AR when g is executing. We can't tell at compile time where the (most recent) AR for f will be relative to the current AR for g since a dynamically-determined (i.e., statically unknown) number of routines could have been called in the middle.

There is an example in the next section. in which g refers to x, which is declared in the immediately outer scope (main) but the AR is 2 away because f was invoked in between. (In that example you can tell at compile time what was called in what order, but with a more complicated program having data-dependent branches, it is not possible.)

7.3.3: A language with Nested Procedure Declarations

The book asserts (correctly) that C doesn't have nested procedures so introduces ML, which does (and is quite slick). However, which many of you don't know ML and I haven't used it. Fortunately, a common extension to C is to permit nested procedures. In particular, gcc supports nested procedures. To check my memory I compiled and ran the following program.

  #include <stdio.h>

  int main (int argc, char *argv[]) {
      int x = 10;

      int g(int y) {
	  int z = x+y;
	  return z;
      }

      int f (int y) {
	  return g(2*y);
      }

  (void) printf("The answer is %d\n", f(x));
  return 0;
  }

The program compiles without errors and the correct answer of 30 is printed.

So we can use C (really the GCC extension of C).

Remark: Many consider this gcc extension to be evil.

7.3.4: Nesting Depth

Outermost procedures have nesting depth 1. Other procedures have nesting depth 1 more than the nesting depth of the immediately outer procedure. In the example above main has nesting depth 1; both f and g have nesting depth 2.

7.3.5: Access Links

The AR for a nested procedure contains an access link that points to the AR of the most recent activation of the immediately outer procedure).

So in the example above the access link for f and the access link for g would each point to the AR of the activation of main. Then when g references x, defined in main, the activation record for main can be found by following the access link in the AR for f. Since f is nested in main, they are compiled together so, once the AR is determined, the same techniques can be used as for variables local to f.

This example was too easy.

1. Everything can be determined at compile time since there are no data dependent branches.
2. This is only one AR for main during all of execution since main is not (directly or indirectly) recursive and there is only one AR for each of f and g.

However the technique is quite general. For a procedure P to access a name defined in the 3-outer scope (i.e., the unique outer scope whose nesting depth is 3 less than that of P; make sure you understand why an outer scope is unique), you follow the access links three times.

The question is how are the access links maintained.

7.3.6: Manipulating Access Links

Let's assume there are no procedure parameters. We are also assuming that the entire program is compiled at once.

Without procedure parameters, the compiler knows the name of the called procedure and the nesting depth.

Let the caller be procedure R (the last letter in caller) and let the called procedure be D. Let N(f) be the nesting depth of f.

We distinguish two cases.

1. N(D)>N(R). The only way D can be visible in R is for D to be declared immediately inside R, i.e., N(D)=N(R)+1. Then when compiling the call from R to D it is easy to include code to have the access link of D point to the AR of R. In this case the access link is the same as the control link.
   
   
2. N(D)≤N(R). This includes the case D=R, i.e., a direct recursive call. For D to be visible in R, there must be another procedure P enclosing both D and R, with D immediately inside P, i.e., N(D)=N(P)+1 and N(R)=N(P)+k+1, with k≥0.

   	P() {
             D() {...}
             P1() {
               P2() {
                 ...
                 Pk() {
                   R(){... D(); ...}
                 }
                ...
               }
             }
           }
         

   Our goal while creating the AR for D at the call from R is to set the access link to point to the AR for P. Note that the entire structure in the skeleton code shown is visible to the compiler. The current (at the time of the call) AR is the one for R and, if we follow the access links k+1 times we get a pointer to the AR for P, which we can then place in the access link for the being-created AR for D.

The above works fine when R is nested (possibly deeply) inside D. It is the picture above but P1 is D.

When k=0 we get the gcc code I showed before and also the case of direct recursion where D=R. I do not know why the book separates out the case k=0, especially since the previous edition didn't.

7.4.5: Manual Deallocation Requests

Stack data is automatically deallocated when the defining procedure returns. What should we do with heap data explicated allocated with new/malloc?

The manual method is to require that the programmer explicitly deallocate these data. Two problems arise.

1. Memory leaks. The programmer forgets to deallocate.

           loop
               allocate X
               use X
               forget to deallocate X
   	end loop
         

   As this program continues to run it will require more and more storage even though is actual usage is not increasing significantly.
   
   
2. Dangling References. The programmer forgets that they did a deallocate.

           allocate X
           use X
           deallocate X
           100,000 lines of code not using X
           use X
         

Both can be disastrous and motivate the next topic, which is covered in programming languages courses.

7.5: Introduction to Garbage Collection

The system detects data that cannot be accessed (no direct or indirect references exist) and deallocates the data automatically.

Covered in programming languages.

Chapter 8: Code Generation

Homework: Read Chapter 8.

Goal: Transform the intermediate code and tables produced by the front end into final machine (or assembly) code. Code generation plus optimization constitutes the back end of the compiler.

8.1: Issues in the Design of a Code Generator

8.1.1: Input to the Code Generator

As expected the input to the code generator is the output of the intermediate code generator. We assume that all syntactic and semantic error checks have been done by the front end. Also, all needed type conversions are already done and any type errors have been detected.

We are using three address instructions for our intermediate language. These instructions have several representations, quads, triples, indirect triples, etc. In this chapter I will tend to use the term quad (for brevity) when I should really say three-address instruction, since the representation doesn't matter.

8.1.2: the Target Program

A RISC (Reduced Instruction Set Computer), e.g. PowerPC, Sparc, MIPS (popular for embedded systems), is characterized by

• Many registers.
• Three address instructions.
• Simple addressing modes.
• Relatively simple ISA (instruction set architecture).
• Only loads and stores touch memory.
• Homogeneous registers.
• Very few instruction lengths.

A CISC (Complex Instruct Set Computer), e.g. x86, x86-64/amd64 is characterized by

• Few registers.
• Two address instructions.
• Variety of addressing modes (some complex).
• Complex ISA.
• Register classes.
• Multiple instruction lengths.

A stack-based computer is characterized by

1. No registers.
2. Zero address instructions (operands and results are implicitly on the runtime stack).
3. The top portion of the stack are kept in hidden registers.

A Little History

IBM 701/704/709/7090/7094 (Moon shot, MIT CTSS) were accumulator based.

Stack based machines were believed to be good compiler targets. They became very unpopular when it was believed that register architecture would perform better. Better compilation (code generation) techniques appeared that could take advantage of the multiple registers.

Pascal P-code and Java byte-code are the machine instructions for a hypothetical stack-based machines, the JVM (Java Virtual Machine) in the case of Java. This code can be interpreted or compiled to native code.

RISC became all the rage in the 1980s.

CISC made a gigantic comeback in the 90s with the intel pentium pro. A key idea of the pentium pro is that the hardware would dynamically translate a complex x86 instruction into a series of simpler RISC-like instructions called ROPs (RISC ops). The actual execution engine dealt with ROPs. The jargon would be that, while the architecture (the ISA) remained the x86, the micro-architecture was quite different and more like the micro-architecture seen in previous RISC processors.

Assemblers and Linkers

For maximum compilation speed of modest size programs, the compiler accepts the entire program at once and produces code that can be loaded and executed (the compilation system can include a simple loader and can start the compiled program). This was popular for student jobs when computer time was expensive. The alternative, where each procedure can be compiled separately, requires a linkage editor.

It eases the compiler's task to produce assembly code instead of machine code and we will do so. This decision increased the total compilation time since it requires an extra assembler pass (or two).

8.1.3: Instruction Selection

A big question is the level of code quality we seek to attain. For example we can simply translate one quadruple at a time. The quad
        x = y + z
can always (assuming the addresses x, y, and z are each a compile time constant off the off a given register, e.g., the sp) be compiled into 4 RISC-like instructions (fewer CISC instructions would suffice) using only 2 registers R0 and R1.

    LD  R0, y
    LD  R1, z
    ADD R0, R0, R1
    ST  x, R0
  

    LD  R0, b
    LD  R1, c
    ADD R0, R0, R1
    ST  a, R0
    LD  R0, a
    LD  R1, e
    ADD R0, R0, R1
    ST  d, R0