306 lines
11 KiB
Plaintext
306 lines
11 KiB
Plaintext
.NH 2
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Implementation.
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.PP
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In this section we will discuss the implementation of the CS phase.
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We will first describe the basic actions that are undertaken
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by the algorithm, than the algorithm itself.
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.NH 3
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Partioning the EM instructions
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.PP
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There are over 100 EM instructions.
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For our purpose we partition this huge set into groups of
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instructions which can be more or less conveniently handled together.
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.PP
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There are groups for all sorts of load instructions:
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simple loads, expensive loads, loads of an array element.
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A load is considered \fIexpensive\fP when more than one EM instructions
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are involved in loading it.
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The load of a lexical entity is also considered expensive.
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For instance: LOF is expensive, LAL is not.
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LAR forms a group on its own,
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because it is not only an expensive load,
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but also implicitly includes the ternary operator AAR,
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which computes the address of the array element.
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.PP
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There are groups for all sorts of operators:
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unary, binary, and ternary.
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The groups of operators are further partitioned according to the size
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of their operand(s) and result.
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\" .PP
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\" The distinction between operators and expensive loads is not always clear.
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\" The ADP instruction for example,
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\" might seem a unary operator because it pops one item
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\" (a pointer) from the stack.
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\" However, two ADP-instructions which pop an item with the same value number
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\" need not have the same result,
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\" because the attributes (an offset, to be added to the pointer)
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\" can be different.
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\" Is it then a binary operator?
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\" That would give rise to the strange, and undesirable,
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\" situation that some binary operators pop two operands
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\" and others pop one.
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\" The conclusion is inevitable:
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\" we have been fooled by the name (ADd Pointer).
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\" The ADP-instruction is an expensive load.
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\" In this context LAF, meaning Load Address of oFfsetted,
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\" would have been a better name,
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\" corresponding to LOF, like LAL,
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\" Load Address of Local, corresponds to LOL.
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.PP
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There are groups for all sorts of stores:
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direct, indirect, array element.
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The SAR forms a group on its own for the same reason
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as appeared with LAR.
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.PP
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The effect of the remaining instructions is less clear.
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They do not help very much in parsing expressions or
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in constructing our pseudo symboltable.
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They are partitioned according to the following criteria:
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.RS
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.IP "-"
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They change the value of an entity without using the stack
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(e.g. ZRL, DEE).
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.IP "-"
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They are subroutine calls (CAI, CAL).
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.IP "-"
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They change the stack in some irreproduceable way (e.g. ASP, LFR, DUP).
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.IP "-"
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They have no effect whatever on the stack or on the entities.
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This does not mean they can be deleted,
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but they can be ignored for the moment
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(e.g. MES, LIN, NOP).
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.IP "-"
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Their effect is too complicate too compute,
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so we just assume worst case behaviour.
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Hopefully, they do not occur very often.
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(e.g. MON, STR, BLM).
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.IP "-"
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They signal the end of the basic block (e.g. BLT, RET, TRP).
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.RE
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.NH 3
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Parsing expressions
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.PP
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To recognize expressions,
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we simulate the behaviour of the EM machine,
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by means of a fake-stack.
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When we scan the instructions in sequential order,
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we first encounter the instructions that load
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the operands on the stack,
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and then the instruction that indicates the operator,
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because EM expressions are postfix.
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When we find an instruction to load an operand,
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we load on the fake-stack a struct with the following information:
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.DS
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(1) the value number of the operand
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(2) the size of the operand
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(3) a pointer to the first line of EM-code
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that constitutes the operand
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.DE
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In most cases, (3) will point to the line
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that loaded the operand (e.g. LOL, LOC),
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i.e. there is only one line that refers to this operand,
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but sometimes some information must be popped
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to load the operand (e.g. LOI, LAR).
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This information must have been pushed before,
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so we also pop a pointer to the first line that pushed
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the information.
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This line is now the first line that defines the operand.
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.PP
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When we find the operator instruction,
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we pop its operand(s) from the fake-stack.
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The first line that defines the first operand is
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now the first line of the expression.
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We now have all information to determine
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whether the just parsed expression has occurred before.
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We also know the first and last line of the expression;
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we need this when we decide to eliminate it.
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Associated with each available expression is a set of
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which the elements contains the first and last line of
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a recurrence of this expression.
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.PP
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Not only will the operand(s) be popped from the fake-stack,
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but the following will be pushed:
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.DS
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(1) the value number of the result
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(2) the size of the result
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(3) a pointer to the first line of the expression
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.DE
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In this way an item on the fake-stack always contains
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the necessary information.
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As you see, EM expressions are parsed bottum up.
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.NH 3
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Updating entities
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.PP
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As said before,
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we build our private "symboltable",
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while scanning the EM-instructions.
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The behaviour of the EM-machine is not only reflected
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in the fake-stack,
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but also in the entities.
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When an entity is created,
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we do not yet know its value,
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so we assign a brand new value number to it.
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Each time a store-instruction is encountered,
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we change the value number of the target entity of this store
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to the value number of the token that was popped
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from the fake-stack.
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Because entities may overlap,
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we must also "forget" the value numbers of entities
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that might be affected by this store.
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Each such entity will be \fIkilled\fP,
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i.e. assigned a brand new valuenumber.
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.PP
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Because we lose information when we forget
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the value number of an entity,
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we try to save as much entities as possible.
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When we store into an external,
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we don't have to kill locals and vice versa.
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Furthermore, we can see whether two locals or
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two externals overlap,
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because we know the offset from the local base,
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resp. the offset within the data block,
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and the size.
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The situation becomes more complicated when we have
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to consider indirection.
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The worst case is that we store through an unknown pointer.
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In that case we kill all entities except those locals
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for which a so-called \fIregister message\fP has been generated;
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this register message indicates that this local can never be
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accessed indirectly.
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If we know this pointer we can be more careful.
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If it points to a local then the entity that is accessed through
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this pointer can never overlap with an external.
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If it points to an external this entity can never overlap with a local.
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Furthermore, in the latter case,
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we can find the data block this entity belongs to.
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Since pointer arithmetic is only defined within a data block,
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this entity can never overlap with entities that are known to
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belong to another data block.
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.PP
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Not only after a store-instruction but also after a
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subroutine-call it may be necessary to kill entities;
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the subroutine may affect global variables or store
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through a pointer.
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If a subroutine is called that is not available as EM-text,
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we assume worst case behaviour,
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i.e. we kill all entities without register message.
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.NH 3
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Additions and replacements.
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.PP
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When a new expression comes available,
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we check whether the result is saved in a local
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that may go in a register.
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The last line of the expression must be followed
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by a STL or SDL instruction,
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depending on the size of the result
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(resp. WS and 2*WS),
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and a register message must be present for
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this local.
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If we have found such a local,
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we store a pointer to it with the available expression.
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Each time a new occurrence of this expression
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is found,
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we compare the value number of the local against
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the value number of the result.
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When they are different we remove the pointer to it,
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because we cannot use it.
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.PP
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The available expressions are singly linked in a list.
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When a new expression comes available,
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we link it at the head of the list.
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In this way expressions that are contained within other
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expressions appear later in the list,
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because EM-expressions are postfix.
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When we are going to eliminate expressions,
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we walk through the list,
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starting at the head, to find the largest expressions first.
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When we decide to eliminate an expression,
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we look at the expressions in the tail of the list,
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starting from where we are now,
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to delete expressions that are contained within
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the chosen one because
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we cannot eliminate an expression more than once.
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.PP
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When we are going to eliminate expressions,
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and we do not have a local that holds the result,
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we emit a STL or SDL after the line where the expression
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was first found.
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The other occurrences are simply removed,
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unless they contain instructions that not only have
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effect on the stack; e.g. messages, stores, calls.
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Before each instruction that needs the result on the stack,
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we emit a LOL or LDL.
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When the expression was an AAR,
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but the instruction was a LAR or a SAR,
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we append a LOI resp. a STI of the number of bytes
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in an array-element after each LOL/LDL.
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.NH 3
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Desirability analysis
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.PP
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Although the global optimizer works on EM code,
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the goal is to improve the quality of the object code.
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Therefore we need some machine dependent information
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to decide whether it is desirable to
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eliminate a given expression.
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Because it is impossible for the CS phase to know
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exactly what code will be generated,
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we use some heuristics.
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In most cases it will save time when we eliminate an
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operator, so we just do it.
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We only look for some special cases.
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.PP
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Some operators can in some cases be translated
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into an addressing mode for the machine at hand.
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We only eliminate such an operator,
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when its operand is itself "expensive",
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i.e. not just a simple load.
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The user of the CS phase has to supply
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a set of such operators.
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.PP
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Eliminating the loading of the Local Base or
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the Argument Base by the LXL resp. LXA instruction
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is only beneficial when the number of lexical levels
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we have to go back exceeds a certain threshold.
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This threshold will be different when registers
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are saved by the back end.
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The user must supply this threshold.
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.PP
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Replacing a SAR or a LAR by an AAR followed by a LOI
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may possibly increase the size of the object code.
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We assume that this is only possible when the
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size of the array element is greater than some
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(user-supplied) limit.
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.PP
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There are back ends that can very efficiently translate
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the index computing instruction sequence LOC SLI ADS.
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If this is the case,
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we do not eliminate the SLI instruction between a LOC
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and an ADS.
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.PP
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To handle unforeseen cases, the user may also supply
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a set of operators that should never be eliminated.
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.NH 3
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The algorithm
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.PP
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After these preparatory explanations,
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we can be short about the algorithm itself.
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For each instruction within our window,
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the following steps are performed in the order given:
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.IP 1.
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We check if this instructin defines an entity.
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If this is the case the set of entities is updated accordingly.
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.IP 2.
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We kill all entities that might be affected by this instruction.
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.IP 3.
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The instruction is simulated on the fake-stack.
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Copy propagation is done.
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If this instruction is an operator,
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we update the list of available expressions accordingly.
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.PP
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When we have processed all instructions this way,
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we have built a list of available expressions plus the information we
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need to eliminate them.
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Those expressions of which desirability analysis tells us so,
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we eliminate.
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The we shift our window and continue.
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