cppinternals.info: Macro Expansion

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Macro Expansion Algorithm

   Macro expansion is a tricky operation, fraught with nasty corner
cases and situations that render what you thought was a nifty way to
optimize the preprocessor's expansion algorithm wrong in quite subtle
ways.
   I strongly recommend you have a good grasp of how the C and C++
standards require macros to be expanded before diving into this
section, let alone the code!.  If you don't have a clear mental picture
of how things like nested macro expansion, stringification and token
pasting are supposed to work, damage to your sanity can quickly result.

Internal representation of macros

   The preprocessor stores macro expansions in tokenized form.  This
saves repeated lexing passes during expansion, at the cost of a small
increase in memory consumption on average.  The tokens are stored
contiguously in memory, so a pointer to the first one and a token count
is all you need to get the replacement list of a macro.
   If the macro is a function-like macro the preprocessor also stores
its parameters, in the form of an ordered list of pointers to the hash
table entry of each parameter's identifier.  Further, in the macro's
stored expansion each occurrence of a parameter is replaced with a
special token of type `CPP_MACRO_ARG'.  Each such token holds the index
of the parameter it represents in the parameter list, which allows
rapid replacement of parameters with their arguments during expansion.
Despite this optimization it is still necessary to store the original
parameters to the macro, both for dumping with e.g., `-dD', and to warn
about non-trivial macro redefinitions when the parameter names have
changed.

Macro expansion overview

   The preprocessor maintains a "context stack", implemented as a
linked list of `cpp_context' structures, which together represent the
macro expansion state at any one time.  The `struct cpp_reader' member
variable `context' points to the current top of this stack.  The top
normally holds the unexpanded replacement list of the innermost macro
under expansion, except when cpplib is about to pre-expand an argument,
in which case it holds that argument's unexpanded tokens.
   When there are no macros under expansion, cpplib is in "base
context".  All contexts other than the base context contain a
contiguous list of tokens delimited by a starting and ending token.
When not in base context, cpplib obtains the next token from the list
of the top context.  If there are no tokens left in the list, it pops
that context off the stack, and subsequent ones if necessary, until an
unexhausted context is found or it returns to base context.  In base
context, cpplib reads tokens directly from the lexer.
   If it encounters an identifier that is both a macro and enabled for
expansion, cpplib prepares to push a new context for that macro on the
stack by calling the routine `enter_macro_context'.  When this routine
returns, the new context will contain the unexpanded tokens of the
replacement list of that macro.  In the case of function-like macros,
`enter_macro_context' also replaces any parameters in the replacement
list, stored as `CPP_MACRO_ARG' tokens, with the appropriate macro
argument.  If the standard requires that the parameter be replaced with
its expanded argument, the argument will have been fully macro expanded
first.
   `enter_macro_context' also handles special macros like `__LINE__'.
Although these macros expand to a single token which cannot contain any
further macros, for reasons of token spacing (*note Token Spacing::)
and simplicity of implementation, cpplib handles these special macros
by pushing a context containing just that one token.
   The final thing that `enter_macro_context' does before returning is
to mark the macro disabled for expansion (except for special macros
like `__TIME__').  The macro is re-enabled when its context is later
popped from the context stack, as described above.  This strict
ordering ensures that a macro is disabled whilst its expansion is being
scanned, but that it is _not_ disabled whilst any arguments to it are
being expanded.

Scanning the replacement list for macros to expand

   The C standard states that, after any parameters have been replaced
with their possibly-expanded arguments, the replacement list is scanned
for nested macros.  Further, any identifiers in the replacement list
that are not expanded during this scan are never again eligible for
expansion in the future, if the reason they were not expanded is that
the macro in question was disabled.
   Clearly this latter condition can only apply to tokens resulting from
argument pre-expansion.  Other tokens never have an opportunity to be
re-tested for expansion.  It is possible for identifiers that are
function-like macros to not expand initially but to expand during a
later scan.  This occurs when the identifier is the last token of an
argument (and therefore originally followed by a comma or a closing
parenthesis in its macro's argument list), and when it replaces its
parameter in the macro's replacement list, the subsequent token happens
to be an opening parenthesis (itself possibly the first token of an
argument).
   It is important to note that when cpplib reads the last token of a
given context, that context still remains on the stack.  Only when
looking for the _next_ token do we pop it off the stack and drop to a
lower context.  This makes backing up by one token easy, but more
importantly ensures that the macro corresponding to the current context
is still disabled when we are considering the last token of its
replacement list for expansion (or indeed expanding it).  As an
example, which illustrates many of the points above, consider
     #define foo(x) bar x
     foo(foo) (2)
which fully expands to `bar foo (2)'.  During pre-expansion of the
argument, `foo' does not expand even though the macro is enabled, since
it has no following parenthesis [pre-expansion of an argument only uses
tokens from that argument; it cannot take tokens from whatever follows
the macro invocation].  This still leaves the argument token `foo'
eligible for future expansion.  Then, when re-scanning after argument
replacement, the token `foo' is rejected for expansion, and marked
ineligible for future expansion, since the macro is now disabled.  It
is disabled because the replacement list `bar foo' of the macro is
still on the context stack.
   If instead the algorithm looked for an opening parenthesis first and
then tested whether the macro were disabled it would be subtly wrong.
In the example above, the replacement list of `foo' would be popped in
the process of finding the parenthesis, re-enabling `foo' and expanding
it a second time.

Looking for a function-like macro's opening parenthesis

   Function-like macros only expand when immediately followed by a
parenthesis.  To do this cpplib needs to temporarily disable macros and
read the next token.  Unfortunately, because of spacing issues (*note
Token Spacing::), there can be fake padding tokens in-between, and if
the next real token is not a parenthesis cpplib needs to be able to
back up that one token as well as retain the information in any
intervening padding tokens.
   Backing up more than one token when macros are involved is not
permitted by cpplib, because in general it might involve issues like
restoring popped contexts onto the context stack, which are too hard.
Instead, searching for the parenthesis is handled by a special
function, `funlike_invocation_p', which remembers padding information
as it reads tokens.  If the next real token is not an opening
parenthesis, it backs up that one token, and then pushes an extra
context just containing the padding information if necessary.

Marking tokens ineligible for future expansion

   As discussed above, cpplib needs a way of marking tokens as
unexpandable.  Since the tokens cpplib handles are read-only once they
have been lexed, it instead makes a copy of the token and adds the flag
`NO_EXPAND' to the copy.
   For efficiency and to simplify memory management by avoiding having
to remember to free these tokens, they are allocated as temporary tokens
from the lexer's current token run (*note Lexing a line::) using the
function `_cpp_temp_token'.  The tokens are then re-used once the
current line of tokens has been read in.
   This might sound unsafe.  However, tokens runs are not re-used at the
end of a line if it happens to be in the middle of a macro argument
list, and cpplib only wants to back-up more than one lexer token in
situations where no macro expansion is involved, so the optimization is
safe.