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\begin{Name}{3}{libunwind-ia64}{David Mosberger-Tang}{Programming Library}{IA-64-specific support in libunwind}libunwind-ia64 -- IA-64-specific support in libunwind
\end{Name}


\section{Introduction}

The IA-64 version of \Prog{libunwind} uses a platform-string of
\texttt{ia64} and, at least in theory, should be able to support all
operating systems adhering to the processor-specific ABI defined for
the Itanium Processor Family.  This includes both little-endian Linux
and big-endian HP-UX.  Furthermore, to make it possible for a single
library to unwind both 32- and 64-bit targets, the type
\Type{unw\_word\_t} is always defined to be 64 bits wide (independent
of the natural word-size of the host).  Having said that, the current
implementation has been tested only with IA-64 Linux.

When targeting IA-64, the \Prog{libunwind} header file defines the
macro \Const{UNW\_TARGET\_IA64} as 1 and the macro \Const{UNW\_TARGET}
as ``ia64'' (without the quotation marks).  The former makes it
possible for platform-dependent unwind code to use
conditional-compilation to select an appropriate implementation.  The
latter is useful for stringification purposes and to construct
target-platform-specific symbols.

One special feature of IA-64 is the use of NaT bits to support
speculative execution.  Often, NaT bits are thought of as the ``65-th
bit'' of a general register.  However, to make everything fit into
64-bit wide \Type{unw\_word\_t} values, \Prog{libunwind} treats the
NaT-bits like separate boolean registers, whose 64-bit value is either
TRUE (non-zero) or FALSE (zero).


\section{Machine-State}

The machine-state (set of registers) that is accessible through
\Prog{libunwind} depends on the type of stack frame that a cursor
points to.  For normal frames, all ``preserved'' (callee-saved)
registers are accessible.  For signal-trampoline frames, all registers
(including ``scratch'' (caller-saved) registers) are accessible.  Most
applications do not have to worry a-priori about which registers are
accessible when.  In case of doubt, it is always safe to \emph{try} to
access a register (via \Func{unw\_get\_reg}() or
\Func{unw\_get\_fpreg}()) and if the register isn't accessible, the
call will fail with a return-value of \texttt{-}\Const{UNW\_EBADREG}.

As a special exception to the above general rule, scratch registers
\texttt{r15}-\texttt{r18} are always accessible, even in normal
frames.  This makes it possible to pass arguments, e.g., to exception
handlers.

For a detailed description of the IA-64 register usage convention,
please see the ``Itanium Software Conventions and Runtime Architecture
Guide'', available at:
\begin{center}
  \URL{http://www.intel.com/design/itanium/downloads/245358.htm}
\end{center}


\section{Register Names}

The IA-64-version of \Prog{libunwind} defines three kinds of register
name macros: frame-register macros, normal register macros, and
convenience macros.  Below, we describe each kind in turn:


\subsection{Frame-register Macros}

Frame-registers are special (pseudo) registers because they always
have a valid value, even though sometimes they do not get saved
explicitly (e.g., if a memory stack frame is 16 bytes in size, the
previous stack-pointer value can be calculated simply as
\texttt{sp+16}, so there is no need to save the stack-pointer
explicitly).  Moreover, the set of frame register values uniquely
identifies a stack frame.  The IA-64 architecture defines two stacks
(a memory and a register stack). Including the instruction-pointer
(IP), this means there are three frame registers:
\begin{Description}
\item[\Const{UNW\_IA64\_IP}:] Contains the instruction pointer (IP, or
  ``program counter'') of the current stack frame.  Given this value,
  the remaining machine-state corresponds to the register-values that
  were present in the CPU when it was just about to execute the
  instruction pointed to by \Const{UNW\_IA64\_IP}.  Bits 0 and 1 of
  this frame-register encode the slot number of the instruction.
  \textbf{Note:} Due to the way the call instruction works on IA-64,
  the slot number is usually zero, but can be non-zero, e.g., in the
  stack-frame of a signal-handler trampoline.
\item[\Const{UNW\_IA64\_SP}:] Contains the (memory) stack-pointer
  value (SP).  This frame-register is read-only.
\item[\Const{UNW\_IA64\_BSP}:] Contains the register backing-store
  pointer (BSP).  This frame-register is read-only.  \textbf{Note:}
  the value in this register is equal to the contents of register
  \texttt{ar.bsp} at the time the instruction at \Const{UNW\_IA64\_IP}
  was about to begin execution.
\end{Description}


\subsection{Normal Register Macros}

The following normal register name macros are available:
\begin{Description}
\item[\Const{UNW\_IA64\_GR}:] The base-index for general (integer)
  registers.  Add an index in the range from 0..127 to get a
  particular general register.  For example, to access \texttt{r4},
  the index \Const{UNW\_IA64\_GR}\texttt{+4} should be used.
  Registers \texttt{r0} and \texttt{r1} (\texttt{gp}) are read-only,
  and any attempt to write them will result in an error
  (\texttt{-}\Const{UNW\_EREADONLYREG}).  Even though \texttt{r1} is
  read-only, \Prog{libunwind} will automatically adjust its value if
  the instruction-pointer (\Const{UNW\_IA64\_IP}) is modified.  For
  example, if \Const{UNW\_IA64\_IP} is set to a value inside a
  function \Func{func}(), then reading
  \Const{UNW\_IA64\_GR}\texttt{+1} will return the global-pointer
  value for this function.
\item[\Const{UNW\_IA64\_NAT}:] The base-index for the NaT bits of the
  general (integer) registers.  A non-zero value in these registers
  corresponds to a set NaT-bit.  Add an index in the range from 0..127
  to get a particular NaT-bit register.  For example, to access the
  NaT bit of \texttt{r4}, the index \Const{UNW\_IA64\_NAT}\texttt{+4}
  should be used.
\item[\Const{UNW\_IA64\_FR}:] The base-index for floating-point
  registers.  Add an index in the range from 0..127 to get a
  particular floating-point register.  For example, to access
  \texttt{f2}, the index \Const{UNW\_IA64\_FR}\texttt{+2} should be
  used.  Registers \texttt{f0} and \texttt{f1} are read-only, and any
  attempt to write to indices \Const{UNW\_IA64\_FR}\texttt{+0} or
  \Const{UNW\_IA64\_FR}\texttt{+1} will result in an error
  (\texttt{-}\Const{UNW\_EREADONLYREG}).
\item[\Const{UNW\_IA64\_AR}:] The base-index for application
  registers.  Add an index in the range from 0..127 to get a
  particular application register.  For example, to access
  \texttt{ar40}, the index \Const{UNW\_IA64\_AR}\texttt{+40} should be
  used.  The IA-64 architecture defines several application registers
  as ``reserved for future use''.  Attempting to access such registers
  results in an error (\texttt{-}\Const{UNW\_EBADREG}).
\item[\Const{UNW\_IA64\_BR}:] The base-index for branch registers.
  Add an index in the range from 0..7 to get a particular branch
  register.  For example, to access \texttt{b6}, the index
  \Const{UNW\_IA64\_BR}\texttt{+6} should be used.
\item[\Const{UNW\_IA64\_PR}:] Contains the set of predicate registers.
  This 64-bit wide register contains registers \texttt{p0} through
  \texttt{p63} in the ``broad-side'' format (i.e., \texttt{p0}
  corresponds to bit 0, \texttt{p1} to bit 1, and so on).
\item[\Const{UNW\_IA64\_CFM}:] Contains the current-frame-mask
  register.
\end{Description}


\subsection{Convenience Macros}

Convenience macros are simply aliases for certain frequently used
registers:
\begin{Description}
\item[\Const{UNW\_IA64\_GP}:] Alias for \Const{UNW\_IA64\_GR}\texttt{+1},
  the global-pointer register.
\item[\Const{UNW\_IA64\_TP}:] Alias for \Const{UNW\_IA64\_GR}\texttt{+13},
  the thread-pointer register.
\item[\Const{UNW\_IA64\_AR\_RSC}:] Alias for \Const{UNW\_IA64\_GR}\texttt{+16},
  the register-stack configuration register.
\item[\Const{UNW\_IA64\_AR\_BSP}:] Alias for
  \Const{UNW\_IA64\_GR}\texttt{+17}.  This register index accesses the
  value of register \texttt{ar.bsp} as of the time it was last saved
  explicitly.  This is rarely what you want.  Normally, you'll want to
  use \Const{UNW\_IA64\_BSP} instead.
\item[\Const{UNW\_IA64\_AR\_BSPSTORE}:] Alias for \Const{UNW\_IA64\_GR}\texttt{+18},
  the register-backing store write pointer.
\item[\Const{UNW\_IA64\_AR\_RNAT}:] Alias for \Const{UNW\_IA64\_GR}\texttt{+19},
  the register-backing store NaT-collection register.
\item[\Const{UNW\_IA64\_AR\_CCV}:] Alias for \Const{UNW\_IA64\_GR}\texttt{+32},
  the compare-and-swap value register.
\item[\Const{UNW\_IA64\_AR\_CSD}:] Alias for \Const{UNW\_IA64\_GR}\texttt{+25},
  the compare-and-swap-data register (used by 16-byte atomic operations).
\item[\Const{UNW\_IA64\_AR\_UNAT}:] Alias for \Const{UNW\_IA64\_GR}\texttt{+36},
  the user NaT-collection register.
\item[\Const{UNW\_IA64\_AR\_FPSR}:] Alias for \Const{UNW\_IA64\_GR}\texttt{+40},
  the floating-point status (and control) register.
\item[\Const{UNW\_IA64\_AR\_PFS}:] Alias for \Const{UNW\_IA64\_GR}\texttt{+64},
  the previous frame-state register.
\item[\Const{UNW\_IA64\_AR\_LC}:] Alias for \Const{UNW\_IA64\_GR}\texttt{+65}
  the loop-count register.
\item[\Const{UNW\_IA64\_AR\_EC}:] Alias for \Const{UNW\_IA64\_GR}\texttt{+66},
  the epilogue-count register.
\end{Description}


\section{The Unwind-Context Type}

On IA-64, \Type{unw\_context\_t} is simply an alias for
\Type{ucontext\_t} (as defined by the Single UNIX Spec).  This implies
that it is possible to initialize a value of this type not just with
\Func{unw\_getcontext}(), but also with \Func{getcontext}(), for
example.  However, since this is an IA-64-specific extension to
\Prog{libunwind}, portable code should not rely on this equivalence.


\section{See Also}

\SeeAlso{libunwind(3)}

\section{Author}

\noindent
David Mosberger-Tang\\
Hewlett-Packard Labs\\
Palo-Alto, CA 94304\\
Email: \Email{davidm@hpl.hp.com}\\
WWW: \URL{http://www.hpl.hp.com/research/linux/libunwind/}.
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