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LIBUNWIND-IA64(3) Programming Library LIBUNWIND-IA64(3)
libunwind-ia64 -- IA-64-specific support in libunwind
The IA-64 version of libunwind uses a platform-string of 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 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
When targeting IA-64, the libunwind header file defines the macro
UNW_TARGET_IA64 as 1 and the macro 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 unw_word_t values, libunwind treats the NaT-bits like
separate boolean registers, whose 64-bit value is either TRUE
(non-zero) or FALSE (zero).
The machine-state (set of registers) that is accessible through
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 try to access a register
(via unw_get_reg() or unw_get_fpreg()) and if the register isn't
accessible, the call will fail with a return-value of -UNW_EBADREG.
As a special exception to the above general rule, scratch registers
r15-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:
The IA-64-version of libunwind defines three kinds of register name
macros: frame-register macros, normal register macros, and convenience
macros. Below, we describe each kind in turn:
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 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
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 UNW_IA64_IP. Bits 0 and 1 of this
frame-register encode the slot number of the instruction. 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.
Contains the (memory) stack-pointer value (SP).
Contains the register backing-store pointer (BSP). Note: the
value in this register is equal to the contents of register
ar.bsp at the time the instruction at UNW_IA64_IP was about to
NORMAL REGISTER MACROS
The following normal register name macros are available:
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 r4, the index UNW_IA64_GR+4 should be used.
Registers r0 and r1 (gp) are read-only, and any attempt to write
them will result in an error (-UNW_EREADONLYREG). Even though
r1 is read-only, libunwind will automatically adjust its value
if the instruction-pointer (UNW_IA64_IP) is modified. For
example, if UNW_IA64_IP is set to a value inside a function
func(), then reading UNW_IA64_GR+1 will return the
global-pointer value for this function.
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 r4, the index UNW_IA64_NAT+4 should be used.
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 f2, the index UNW_IA64_FR+2
should be used. Registers f0 and f1 are read-only, and any
attempt to write to indices UNW_IA64_FR+0 or UNW_IA64_FR+1 will
result in an error (-UNW_EREADONLYREG).
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 ar40, the index UNW_IA64_AR+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 (-UNW_EBADREG).
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 b6, the index UNW_IA64_BR+6 should be used.
Contains the set of predicate registers. This 64-bit wide
register contains registers p0 through p63 in the ``broad-side''
format. Just like with the ``move predicates'' instruction, the
registers are mapped as if CFM.rrb.pr were set to 0. Thus, in
general the value of predicate register pN with N>=16 can be
found in bit 16 + ((N-16)+CFM.rrb.pr) % 48.
Contains the current-frame-mask register.
Convenience macros are simply aliases for certain frequently used
Alias for UNW_IA64_GR+1, the global-pointer register.
Alias for UNW_IA64_GR+13, the thread-pointer register.
Alias for UNW_IA64_GR+16, the register-stack configuration
Alias for UNW_IA64_GR+17. This register index accesses the
value of register ar.bsp as of the time it was last saved
explicitly. This is rarely what you want. Normally, you'll want
to use UNW_IA64_BSP instead.
Alias for UNW_IA64_GR+18, the register-backing store write
Alias for UNW_IA64_GR+19, the register-backing store
Alias for UNW_IA64_GR+32, the compare-and-swap value register.
Alias for UNW_IA64_GR+25, the compare-and-swap-data register
(used by 16-byte atomic operations).
Alias for UNW_IA64_GR+36, the user NaT-collection register.
Alias for UNW_IA64_GR+40, the floating-point status (and
Alias for UNW_IA64_GR+64, the previous frame-state register.
Alias for UNW_IA64_GR+65 the loop-count register.
Alias for UNW_IA64_GR+66, the epilogue-count register.
THE UNWIND-CONTEXT TYPE
On IA-64, unw_context_t is simply an alias for 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 unw_getcontext(), but
also with getcontext(), for example. However, since this is an
IA-64-specific extension to libunwind, portable code should not rely on
Programming Library 16 August 2007 LIBUNWIND-IA64(3)