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@c \input texinfo

@c %**start of header

@c @setfilename agentexpr.info

@c @settitle GDB Agent Expressions

@c @setchapternewpage off

@c %**end of header

@c This file is part of the GDB manual.

@c

@c Copyright (C) 2003, 2004, 2005, 2006, 2009, 2010

@c               Free Software Foundation, Inc.

@c

@c See the file gdb.texinfo for copying conditions.

@node Agent Expressions

@appendix The GDB Agent Expression Mechanism

In some applications, it is not feasible for the debugger to interrupt

the program's execution long enough for the developer to learn anything

helpful about its behavior.  If the program's correctness depends on its

real-time behavior, delays introduced by a debugger might cause the

program to fail, even when the code itself is correct.  It is useful to

be able to observe the program's behavior without interrupting it.

Using GDB's @code{trace} and @code{collect} commands, the user can

specify locations in the program, and arbitrary expressions to evaluate

when those locations are reached.  Later, using the @code{tfind}

command, she can examine the values those expressions had when the

program hit the trace points.  The expressions may also denote objects

in memory --- structures or arrays, for example --- whose values GDB

should record; while visiting a particular tracepoint, the user may

inspect those objects as if they were in memory at that moment.

However, because GDB records these values without interacting with the

user, it can do so quickly and unobtrusively, hopefully not disturbing

the program's behavior.

When GDB is debugging a remote target, the GDB @dfn{agent} code running

on the target computes the values of the expressions itself.  To avoid

having a full symbolic expression evaluator on the agent, GDB translates

expressions in the source language into a simpler bytecode language, and

then sends the bytecode to the agent; the agent then executes the

bytecode, and records the values for GDB to retrieve later.

The bytecode language is simple; there are forty-odd opcodes, the bulk

of which are the usual vocabulary of C operands (addition, subtraction,

shifts, and so on) and various sizes of literals and memory reference

operations.  The bytecode interpreter operates strictly on machine-level

values --- various sizes of integers and floating point numbers --- and

requires no information about types or symbols; thus, the interpreter's

internal data structures are simple, and each bytecode requires only a

few native machine instructions to implement it.  The interpreter is

small, and strict limits on the memory and time required to evaluate an

expression are easy to determine, making it suitable for use by the

debugging agent in real-time applications.

@menu

* General Bytecode Design::     Overview of the interpreter.

* Bytecode Descriptions::       What each one does.

* Using Agent Expressions::     How agent expressions fit into the big picture.

* Varying Target Capabilities:: How to discover what the target can do.

* Rationale::                   Why we did it this way.

@end menu

@c @node Rationale

@c @section Rationale

@node General Bytecode Design

@section General Bytecode Design

The agent represents bytecode expressions as an array of bytes.  Each

instruction is one byte long (thus the term @dfn{bytecode}).  Some

instructions are followed by operand bytes; for example, the @code{goto}

instruction is followed by a destination for the jump.

The bytecode interpreter is a stack-based machine; most instructions pop

their operands off the stack, perform some operation, and push the

result back on the stack for the next instruction to consume.  Each

element of the stack may contain either a integer or a floating point

value; these values are as many bits wide as the largest integer that

can be directly manipulated in the source language.  Stack elements

carry no record of their type; bytecode could push a value as an

integer, then pop it as a floating point value.  However, GDB will not

generate code which does this.  In C, one might define the type of a

stack element as follows:

@example

union agent_val @{

  LONGEST l;

  DOUBLEST d;

@};

@end example

@noindent

where @code{LONGEST} and @code{DOUBLEST} are @code{typedef} names for

the largest integer and floating point types on the machine.

By the time the bytecode interpreter reaches the end of the expression,

the value of the expression should be the only value left on the stack.

For tracing applications, @code{trace} bytecodes in the expression will

have recorded the necessary data, and the value on the stack may be

discarded.  For other applications, like conditional breakpoints, the

value may be useful.

Separate from the stack, the interpreter has two registers:

@table @code

@item pc

The address of the next bytecode to execute.

@item start

The address of the start of the bytecode expression, necessary for

interpreting the @code{goto} and @code{if_goto} instructions.

@end table

@noindent

Neither of these registers is directly visible to the bytecode language

itself, but they are useful for defining the meanings of the bytecode

operations.

There are no instructions to perform side effects on the running

program, or call the program's functions; we assume that these

expressions are only used for unobtrusive debugging, not for patching

the running code.

Most bytecode instructions do not distinguish between the various sizes

of values, and operate on full-width values; the upper bits of the

values are simply ignored, since they do not usually make a difference

to the value computed.  The exceptions to this rule are:

@table @asis

@item memory reference instructions (@code{ref}@var{n})

There are distinct instructions to fetch different word sizes from

memory.  Once on the stack, however, the values are treated as full-size

integers.  They may need to be sign-extended; the @code{ext} instruction

exists for this purpose.

@item the sign-extension instruction (@code{ext} @var{n})

These clearly need to know which portion of their operand is to be

extended to occupy the full length of the word.

@end table

If the interpreter is unable to evaluate an expression completely for

some reason (a memory location is inaccessible, or a divisor is zero,

for example), we say that interpretation ``terminates with an error''.

This means that the problem is reported back to the interpreter's caller

in some helpful way.  In general, code using agent expressions should

assume that they may attempt to divide by zero, fetch arbitrary memory

locations, and misbehave in other ways.

Even complicated C expressions compile to a few bytecode instructions;

for example, the expression @code{x + y * z} would typically produce

code like the following, assuming that @code{x} and @code{y} live in

registers, and @code{z} is a global variable holding a 32-bit

@code{int}:

@example

reg 1

reg 2

const32 @i{address of z}

ref32

ext 32

mul

add

end

@end example

In detail, these mean:

@table @code

@item reg 1

Push the value of register 1 (presumably holding @code{x}) onto the

stack.

@item reg 2

Push the value of register 2 (holding @code{y}).

@item const32 @i{address of z}

Push the address of @code{z} onto the stack.

@item ref32

Fetch a 32-bit word from the address at the top of the stack; replace

the address on the stack with the value.  Thus, we replace the address

of @code{z} with @code{z}'s value.

@item ext 32

Sign-extend the value on the top of the stack from 32 bits to full

length.  This is necessary because @code{z} is a signed integer.

@item mul

Pop the top two numbers on the stack, multiply them, and push their

product.  Now the top of the stack contains the value of the expression

@code{y * z}.

@item add

Pop the top two numbers, add them, and push the sum.  Now the top of the

stack contains the value of @code{x + y * z}.

@item end

Stop executing; the value left on the stack top is the value to be

recorded.

@end table

@node Bytecode Descriptions

@section Bytecode Descriptions

Each bytecode description has the following form:

@table @asis

@item @code{add} (0x02): @var{a} @var{b} @result{} @var{a+b}

Pop the top two stack items, @var{a} and @var{b}, as integers; push

their sum, as an integer.

@end table

In this example, @code{add} is the name of the bytecode, and

@code{(0x02)} is the one-byte value used to encode the bytecode, in

hexadecimal.  The phrase ``@var{a} @var{b} @result{} @var{a+b}'' shows

the stack before and after the bytecode executes.  Beforehand, the stack

must contain at least two values, @var{a} and @var{b}; since the top of

the stack is to the right, @var{b} is on the top of the stack, and

@var{a} is underneath it.  After execution, the bytecode will have

popped @var{a} and @var{b} from the stack, and replaced them with a

single value, @var{a+b}.  There may be other values on the stack below

those shown, but the bytecode affects only those shown.

Here is another example:

@table @asis

@item @code{const8} (0x22) @var{n}: @result{} @var{n}

Push the 8-bit integer constant @var{n} on the stack, without sign

extension.

@end table

In this example, the bytecode @code{const8} takes an operand @var{n}

directly from the bytecode stream; the operand follows the @code{const8}

bytecode itself.  We write any such operands immediately after the name

of the bytecode, before the colon, and describe the exact encoding of

the operand in the bytecode stream in the body of the bytecode

description.

For the @code{const8} bytecode, there are no stack items given before

the @result{}; this simply means that the bytecode consumes no values

from the stack.  If a bytecode consumes no values, or produces no

values, the list on either side of the @result{} may be empty.

If a value is written as @var{a}, @var{b}, or @var{n}, then the bytecode

treats it as an integer.  If a value is written is @var{addr}, then the

bytecode treats it as an address.

We do not fully describe the floating point operations here; although

this design can be extended in a clean way to handle floating point

values, they are not of immediate interest to the customer, so we avoid

describing them, to save time.

@table @asis

@item @code{float} (0x01): @result{}

Prefix for floating-point bytecodes.  Not implemented yet.

@item @code{add} (0x02): @var{a} @var{b} @result{} @var{a+b}

Pop two integers from the stack, and push their sum, as an integer.

@item @code{sub} (0x03): @var{a} @var{b} @result{} @var{a-b}

Pop two integers from the stack, subtract the top value from the

next-to-top value, and push the difference.

@item @code{mul} (0x04): @var{a} @var{b} @result{} @var{a*b}

Pop two integers from the stack, multiply them, and push the product on

the stack.  Note that, when one multiplies two @var{n}-bit numbers

yielding another @var{n}-bit number, it is irrelevant whether the

numbers are signed or not; the results are the same.

@item @code{div_signed} (0x05): @var{a} @var{b} @result{} @var{a/b}

Pop two signed integers from the stack; divide the next-to-top value by

the top value, and push the quotient.  If the divisor is zero, terminate

with an error.

@item @code{div_unsigned} (0x06): @var{a} @var{b} @result{} @var{a/b}

Pop two unsigned integers from the stack; divide the next-to-top value

by the top value, and push the quotient.  If the divisor is zero,

terminate with an error.

@item @code{rem_signed} (0x07): @var{a} @var{b} @result{} @var{a modulo b}

Pop two signed integers from the stack; divide the next-to-top value by

the top value, and push the remainder.  If the divisor is zero,

terminate with an error.

@item @code{rem_unsigned} (0x08): @var{a} @var{b} @result{} @var{a modulo b}

Pop two unsigned integers from the stack; divide the next-to-top value

by the top value, and push the remainder.  If the divisor is zero,

terminate with an error.

@item @code{lsh} (0x09): @var{a} @var{b} @result{} @var{a<<b}

Pop two integers from the stack; let @var{a} be the next-to-top value,

and @var{b} be the top value.  Shift @var{a} left by @var{b} bits, and

push the result.

@item @code{rsh_signed} (0x0a): @var{a} @var{b} @result{} @code{(signed)}@var{a>>b}

Pop two integers from the stack; let @var{a} be the next-to-top value,

and @var{b} be the top value.  Shift @var{a} right by @var{b} bits,

inserting copies of the top bit at the high end, and push the result.

@item @code{rsh_unsigned} (0x0b): @var{a} @var{b} @result{} @var{a>>b}

Pop two integers from the stack; let @var{a} be the next-to-top value,

and @var{b} be the top value.  Shift @var{a} right by @var{b} bits,

inserting zero bits at the high end, and push the result.

@item @code{log_not} (0x0e): @var{a} @result{} @var{!a}

Pop an integer from the stack; if it is zero, push the value one;

otherwise, push the value zero.

@item @code{bit_and} (0x0f): @var{a} @var{b} @result{} @var{a&b}

Pop two integers from the stack, and push their bitwise @code{and}.

@item @code{bit_or} (0x10): @var{a} @var{b} @result{} @var{a|b}

Pop two integers from the stack, and push their bitwise @code{or}.

@item @code{bit_xor} (0x11): @var{a} @var{b} @result{} @var{a^b}

Pop two integers from the stack, and push their bitwise

exclusive-@code{or}.

@item @code{bit_not} (0x12): @var{a} @result{} @var{~a}

Pop an integer from the stack, and push its bitwise complement.

@item @code{equal} (0x13): @var{a} @var{b} @result{} @var{a=b}

Pop two integers from the stack; if they are equal, push the value one;

otherwise, push the value zero.

@item @code{less_signed} (0x14): @var{a} @var{b} @result{} @var{a<b}

Pop two signed integers from the stack; if the next-to-top value is less

than the top value, push the value one; otherwise, push the value zero.

@item @code{less_unsigned} (0x15): @var{a} @var{b} @result{} @var{a<b}

Pop two unsigned integers from the stack; if the next-to-top value is less

than the top value, push the value one; otherwise, push the value zero.

@item @code{ext} (0x16) @var{n}: @var{a} @result{} @var{a}, sign-extended from @var{n} bits

Pop an unsigned value from the stack; treating it as an @var{n}-bit

twos-complement value, extend it to full length.  This means that all

bits to the left of bit @var{n-1} (where the least significant bit is bit

0) are set to the value of bit @var{n-1}.  Note that @var{n} may be

larger than or equal to the width of the stack elements of the bytecode

engine; in this case, the bytecode should have no effect.

The number of source bits to preserve, @var{n}, is encoded as a single

byte unsigned integer following the @code{ext} bytecode.

@item @code{zero_ext} (0x2a) @var{n}: @var{a} @result{} @var{a}, zero-extended from @var{n} bits

Pop an unsigned value from the stack; zero all but the bottom @var{n}

bits.  This means that all bits to the left of bit @var{n-1} (where the

least significant bit is bit 0) are set to the value of bit @var{n-1}.

The number of source bits to preserve, @var{n}, is encoded as a single

byte unsigned integer following the @code{zero_ext} bytecode.

@item @code{ref8} (0x17): @var{addr} @result{} @var{a}

@itemx @code{ref16} (0x18): @var{addr} @result{} @var{a}

@itemx @code{ref32} (0x19): @var{addr} @result{} @var{a}

@itemx @code{ref64} (0x1a): @var{addr} @result{} @var{a}

Pop an address @var{addr} from the stack.  For bytecode

@code{ref}@var{n}, fetch an @var{n}-bit value from @var{addr}, using the

natural target endianness.  Push the fetched value as an unsigned

integer.

Note that @var{addr} may not be aligned in any particular way; the

@code{ref@var{n}} bytecodes should operate correctly for any address.

If attempting to access memory at @var{addr} would cause a processor

exception of some sort, terminate with an error.

@item @code{ref_float} (0x1b): @var{addr} @result{} @var{d}

@itemx @code{ref_double} (0x1c): @var{addr} @result{} @var{d}

@itemx @code{ref_long_double} (0x1d): @var{addr} @result{} @var{d}

@itemx @code{l_to_d} (0x1e): @var{a} @result{} @var{d}

@itemx @code{d_to_l} (0x1f): @var{d} @result{} @var{a}

Not implemented yet.

@item @code{dup} (0x28): @var{a} => @var{a} @var{a}

Push another copy of the stack's top element.

@item @code{swap} (0x2b): @var{a} @var{b} => @var{b} @var{a}

Exchange the top two items on the stack.

@item @code{pop} (0x29): @var{a} =>

Discard the top value on the stack.

@item @code{if_goto} (0x20) @var{offset}: @var{a} @result{}

Pop an integer off the stack; if it is non-zero, branch to the given

offset in the bytecode string.  Otherwise, continue to the next

instruction in the bytecode stream.  In other words, if @var{a} is

non-zero, set the @code{pc} register to @code{start} + @var{offset}.

Thus, an offset of zero denotes the beginning of the expression.

The @var{offset} is stored as a sixteen-bit unsigned value, stored

immediately following the @code{if_goto} bytecode.  It is always stored

most significant byte first, regardless of the target's normal

endianness.  The offset is not guaranteed to fall at any particular

alignment within the bytecode stream; thus, on machines where fetching a

16-bit on an unaligned address raises an exception, you should fetch the

offset one byte at a time.

@item @code{goto} (0x21) @var{offset}: @result{}

Branch unconditionally to @var{offset}; in other words, set the

@code{pc} register to @code{start} + @var{offset}.

The offset is stored in the same way as for the @code{if_goto} bytecode.

@item @code{const8} (0x22) @var{n}: @result{} @var{n}

@itemx @code{const16} (0x23) @var{n}: @result{} @var{n}

@itemx @code{const32} (0x24) @var{n}: @result{} @var{n}

@itemx @code{const64} (0x25) @var{n}: @result{} @var{n}

Push the integer constant @var{n} on the stack, without sign extension.

To produce a small negative value, push a small twos-complement value,

and then sign-extend it using the @code{ext} bytecode.

The constant @var{n} is stored in the appropriate number of bytes

following the @code{const}@var{b} bytecode.  The constant @var{n} is

always stored most significant byte first, regardless of the target's

normal endianness.  The constant is not guaranteed to fall at any

particular alignment within the bytecode stream; thus, on machines where

fetching a 16-bit on an unaligned address raises an exception, you

should fetch @var{n} one byte at a time.

@item @code{reg} (0x26) @var{n}: @result{} @var{a}

Push the value of register number @var{n}, without sign extension.  The

registers are numbered following GDB's conventions.

The register number @var{n} is encoded as a 16-bit unsigned integer

immediately following the @code{reg} bytecode.  It is always stored most

significant byte first, regardless of the target's normal endianness.

The register number is not guaranteed to fall at any particular

alignment within the bytecode stream; thus, on machines where fetching a

16-bit on an unaligned address raises an exception, you should fetch the

register number one byte at a time.

@item @code{getv} (0x2c) @var{n}: @result{} @var{v}

Push the value of trace state variable number @var{n}, without sign

extension.

The variable number @var{n} is encoded as a 16-bit unsigned integer

immediately following the @code{getv} bytecode.  It is always stored most

significant byte first, regardless of the target's normal endianness.

The variable number is not guaranteed to fall at any particular

alignment within the bytecode stream; thus, on machines where fetching a

16-bit on an unaligned address raises an exception, you should fetch the

register number one byte at a time.

@item @code{setv} (0x2d) @var{n}: @result{} @var{v}

Set trace state variable number @var{n} to the value found on the top

of the stack.  The stack is unchanged, so that the value is readily

available if the assignment is part of a larger expression.  The

handling of @var{n} is as described for @code{getv}.

@item @code{trace} (0x0c): @var{addr} @var{size} @result{}

Record the contents of the @var{size} bytes at @var{addr} in a trace

buffer, for later retrieval by GDB.

@item @code{trace_quick} (0x0d) @var{size}: @var{addr} @result{} @var{addr}

Record the contents of the @var{size} bytes at @var{addr} in a trace

buffer, for later retrieval by GDB.  @var{size} is a single byte

unsigned integer following the @code{trace} opcode.

This bytecode is equivalent to the sequence @code{dup const8 @var{size}

trace}, but we provide it anyway to save space in bytecode strings.

@item @code{trace16} (0x30) @var{size}: @var{addr} @result{} @var{addr}

Identical to trace_quick, except that @var{size} is a 16-bit big-endian

unsigned integer, not a single byte.  This should probably have been

named @code{trace_quick16}, for consistency.

@item @code{tracev} (0x2e) @var{n}: @result{} @var{a}

Record the value of trace state variable number @var{n} in the trace

buffer.  The handling of @var{n} is as described for @code{getv}.

@item @code{end} (0x27): @result{}

Stop executing bytecode; the result should be the top element of the

stack.  If the purpose of the expression was to compute an lvalue or a

range of memory, then the next-to-top of the stack is the lvalue's

address, and the top of the stack is the lvalue's size, in bytes.

@end table

@node Using Agent Expressions

@section Using Agent Expressions

Agent expressions can be used in several different ways by @value{GDBN},

and the debugger can generate different bytecode sequences as appropriate.

One possibility is to do expression evaluation on the target rather

than the host, such as for the conditional of a conditional

tracepoint.  In such a case, @value{GDBN} compiles the source

expression into a bytecode sequence that simply gets values from

registers or memory, does arithmetic, and returns a result.

Another way to use agent expressions is for tracepoint data

collection.  @value{GDBN} generates a different bytecode sequence for

collection; in addition to bytecodes that do the calculation,

@value{GDBN} adds @code{trace} bytecodes to save the pieces of

memory that were used.

@itemize @bullet

@item

The user selects trace points in the program's code at which GDB should

collect data.

@item

The user specifies expressions to evaluate at each trace point.  These

expressions may denote objects in memory, in which case those objects'

contents are recorded as the program runs, or computed values, in which

case the values themselves are recorded.

@item

GDB transmits the tracepoints and their associated expressions to the

GDB agent, running on the debugging target.

@item

The agent arranges to be notified when a trace point is hit.

@item

When execution on the target reaches a trace point, the agent evaluates

the expressions associated with that trace point, and records the

resulting values and memory ranges.

@item

Later, when the user selects a given trace event and inspects the

objects and expression values recorded, GDB talks to the agent to

retrieve recorded data as necessary to meet the user's requests.  If the

user asks to see an object whose contents have not been recorded, GDB

reports an error.

@end itemize

@node Varying Target Capabilities

@section Varying Target Capabilities

Some targets don't support floating-point, and some would rather not

have to deal with @code{long long} operations.  Also, different targets

will have different stack sizes, and different bytecode buffer lengths.

Thus, GDB needs a way to ask the target about itself.  We haven't worked

out the details yet, but in general, GDB should be able to send the

target a packet asking it to describe itself.  The reply should be a

packet whose length is explicit, so we can add new information to the

packet in future revisions of the agent, without confusing old versions

of GDB, and it should contain a version number.  It should contain at

least the following information:

@itemize @bullet

@item

whether floating point is supported

@item

whether @code{long long} is supported

@item

maximum acceptable size of bytecode stack

@item

maximum acceptable length of bytecode expressions

@item

which registers are actually available for collection

@item

whether the target supports disabled tracepoints

@end itemize

@node Rationale

@section Rationale

Some of the design decisions apparent above are arguable.

@table @b

@item What about stack overflow/underflow?

GDB should be able to query the target to discover its stack size.

Given that information, GDB can determine at translation time whether a

given expression will overflow the stack.  But this spec isn't about

what kinds of error-checking GDB ought to do.

@item Why are you doing everything in LONGEST?

Speed isn't important, but agent code size is; using LONGEST brings in a

bunch of support code to do things like division, etc.  So this is a

serious concern.

First, note that you don't need different bytecodes for different

operand sizes.  You can generate code without @emph{knowing} how big the

stack elements actually are on the target.  If the target only supports

32-bit ints, and you don't send any 64-bit bytecodes, everything just

works.  The observation here is that the MIPS and the Alpha have only

fixed-size registers, and you can still get C's semantics even though

most instructions only operate on full-sized words.  You just need to

make sure everything is properly sign-extended at the right times.  So

there is no need for 32- and 64-bit variants of the bytecodes.  Just

implement everything using the largest size you support.

GDB should certainly check to see what sizes the target supports, so the

user can get an error earlier, rather than later.  But this information

is not necessary for correctness.

@item Why don't you have @code{>} or @code{<=} operators?

I want to keep the interpreter small, and we don't need them.  We can

combine the @code{less_} opcodes with @code{log_not}, and swap the order

of the operands, yielding all four asymmetrical comparison operators.

For example, @code{(x <= y)} is @code{! (x > y)}, which is @code{! (y <

x)}.

@item Why do you have @code{log_not}?

@itemx Why do you have @code{ext}?

@itemx Why do you have @code{zero_ext}?

These are all easily synthesized from other instructions, but I expect

them to be used frequently, and they're simple, so I include them to

keep bytecode strings short.

@code{log_not} is equivalent to @code{const8 0 equal}; it's used in half

the relational operators.

@code{ext @var{n}} is equivalent to @code{const8 @var{s-n} lsh const8

@var{s-n} rsh_signed}, where @var{s} is the size of the stack elements;

it follows @code{ref@var{m}} and @var{reg} bytecodes when the value

should be signed.  See the next bulleted item.

@code{zero_ext @var{n}} is equivalent to @code{const@var{m} @var{mask}

log_and}; it's used whenever we push the value of a register, because we

can't assume the upper bits of the register aren't garbage.

@item Why not have sign-extending variants of the @code{ref} operators?

Because that would double the number of @code{ref} operators, and we

need the @code{ext} bytecode anyway for accessing bitfields.

@item Why not have constant-address variants of the @code{ref} operators?

Because that would double the number of @code{ref} operators again, and

@code{const32 @var{address} ref32} is only one byte longer.

@item Why do the @code{ref@var{n}} operators have to support unaligned fetches?

GDB will generate bytecode that fetches multi-byte values at unaligned

addresses whenever the executable's debugging information tells it to.

Furthermore, GDB does not know the value the pointer will have when GDB

generates the bytecode, so it cannot determine whether a particular

fetch will be aligned or not.

In particular, structure bitfields may be several bytes long, but follow

no alignment rules; members of packed structures are not necessarily

aligned either.

In general, there are many cases where unaligned references occur in

correct C code, either at the programmer's explicit request, or at the

compiler's discretion.  Thus, it is simpler to make the GDB agent

bytecodes work correctly in all circumstances than to make GDB guess in

each case whether the compiler did the usual thing.

@item Why are there no side-effecting operators?

Because our current client doesn't want them?  That's a cheap answer.  I

think the real answer is that I'm afraid of implementing function

calls.  We should re-visit this issue after the present contract is

delivered.

@item Why aren't the @code{goto} ops PC-relative?

The interpreter has the base address around anyway for PC bounds

checking, and it seemed simpler.

@item Why is there only one offset size for the @code{goto} ops?

Offsets are currently sixteen bits.  I'm not happy with this situation

either:

Suppose we have multiple branch ops with different offset sizes.  As I

generate code left-to-right, all my jumps are forward jumps (there are

no loops in expressions), so I never know the target when I emit the

jump opcode.  Thus, I have to either always assume the largest offset

size, or do jump relaxation on the code after I generate it, which seems

like a big waste of time.

I can imagine a reasonable expression being longer than 256 bytes.  I

can't imagine one being longer than 64k.  Thus, we need 16-bit offsets.

This kind of reasoning is so bogus, but relaxation is pathetic.

The other approach would be to generate code right-to-left.  Then I'd

always know my offset size.  That might be fun.

@item Where is the function call bytecode?

When we add side-effects, we should add this.

@item Why does the @code{reg} bytecode take a 16-bit register number?

Intel's IA-64 architecture has 128 general-purpose registers,

and 128 floating-point registers, and I'm sure it has some random

control registers.

@item Why do we need @code{trace} and @code{trace_quick}?

Because GDB needs to record all the memory contents and registers an

expression touches.  If the user wants to evaluate an expression

@code{x->y->z}, the agent must record the values of @code{x} and

@code{x->y} as well as the value of @code{x->y->z}.

@item Don't the @code{trace} bytecodes make the interpreter less general?

They do mean that the interpreter contains special-purpose code, but

that doesn't mean the interpreter can only be used for that purpose.  If

an expression doesn't use the @code{trace} bytecodes, they don't get in

its way.

@item Why doesn't @code{trace_quick} consume its arguments the way everything else does?

In general, you do want your operators to consume their arguments; it's

consistent, and generally reduces the amount of stack rearrangement

necessary.  However, @code{trace_quick} is a kludge to save space; it

only exists so we needn't write @code{dup const8 @var{SIZE} trace}

before every memory reference.  Therefore, it's okay for it not to

consume its arguments; it's meant for a specific context in which we

know exactly what it should do with the stack.  If we're going to have a

kludge, it should be an effective kludge.

@item Why does @code{trace16} exist?

That opcode was added by the customer that contracted Cygnus for the

data tracing work.  I personally think it is unnecessary; objects that

large will be quite rare, so it is okay to use @code{dup const16

@var{size} trace} in those cases.

Whatever we decide to do with @code{trace16}, we should at least leave

opcode 0x30 reserved, to remain compatible with the customer who added

it.

@end table