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.TH "LIBUNWIND\-IA64" "3" "16 August 2007" "Programming Library " "Programming Library "
.SH NAME
libunwind\-ia64
\-\- IA\-64\-specific support in libunwind 
.PP
.SH INTRODUCTION

.PP
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 IA\-64 Linux. 
.PP
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. 
.PP
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). 
.PP
.SH MACHINE\-STATE

.PP
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 \fItry\fP
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\&.
.PP
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. 
.PP
For a detailed description of the IA\-64 register usage convention, 
please see the ``Itanium Software Conventions and Runtime Architecture 
Guide\&'', available at: 
.ce 100
\fBhttp://www.intel.com/design/itanium/downloads/245358.htm\fP
.ce 0

.PP
.SH REGISTER NAMES

.PP
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: 
.PP
.SS FRAME\-REGISTER MACROS
.PP
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 frame registers: 
.TP
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 UNW_IA64_IP\&.
Bits 0 and 1 of 
this frame\-register encode the slot number of the instruction. 
\fBNote:\fP
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. 
.TP
UNW_IA64_SP:
 Contains the (memory) stack\-pointer 
value (SP). 
.TP
UNW_IA64_BSP:
 Contains the register backing\-store 
pointer (BSP). \fBNote:\fP
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 begin execution. 
.PP
.SS NORMAL REGISTER MACROS
.PP
The following normal register name macros are available: 
.TP
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 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. 
.TP
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 r4,
the index UNW_IA64_NAT+4
should be used. 
.TP
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 
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).
.TP
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 
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).
.TP
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 b6,
the index 
UNW_IA64_BR+6
should be used. 
.TP
UNW_IA64_PR:
 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\&.
.TP
UNW_IA64_CFM:
 Contains the current\-frame\-mask 
register. 
.PP
.SS CONVENIENCE MACROS
.PP
Convenience macros are simply aliases for certain frequently used 
registers: 
.TP
UNW_IA64_GP:
 Alias for UNW_IA64_GR+1,
the global\-pointer register. 
.TP
UNW_IA64_TP:
 Alias for UNW_IA64_GR+13,
the thread\-pointer register. 
.TP
UNW_IA64_AR_RSC:
 Alias for UNW_IA64_GR+16,
the register\-stack configuration register. 
.TP
UNW_IA64_AR_BSP:
 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. 
.TP
UNW_IA64_AR_BSPSTORE:
 Alias for UNW_IA64_GR+18,
the register\-backing store write pointer. 
.TP
UNW_IA64_AR_RNAT:
 Alias for UNW_IA64_GR+19,
the register\-backing store NaT\-collection register. 
.TP
UNW_IA64_AR_CCV:
 Alias for UNW_IA64_GR+32,
the compare\-and\-swap value register. 
.TP
UNW_IA64_AR_CSD:
 Alias for UNW_IA64_GR+25,
the compare\-and\-swap\-data register (used by 16\-byte atomic operations). 
.TP
UNW_IA64_AR_UNAT:
 Alias for UNW_IA64_GR+36,
the user NaT\-collection register. 
.TP
UNW_IA64_AR_FPSR:
 Alias for UNW_IA64_GR+40,
the floating\-point status (and control) register. 
.TP
UNW_IA64_AR_PFS:
 Alias for UNW_IA64_GR+64,
the previous frame\-state register. 
.TP
UNW_IA64_AR_LC:
 Alias for UNW_IA64_GR+65
the loop\-count register. 
.TP
UNW_IA64_AR_EC:
 Alias for UNW_IA64_GR+66,
the epilogue\-count register. 
.PP
.SH THE UNWIND\-CONTEXT TYPE

.PP
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 this equivalence. 
.PP
.SH SEE ALSO

.PP
libunwind(3)
.PP
.SH AUTHOR

.PP
David Mosberger\-Tang
.br
Email: \fBdmosberger@gmail.com\fP
.br
WWW: \fBhttp://www.nongnu.org/libunwind/\fP\&.
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