* doc/c-arm.texi (): Document -mwarn-deprecated.

[deliverable/binutils-gdb.git] / gas / doc / c-arm.texi

@c Copyright 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2008
@c Free Software Foundation, Inc.
@c This is part of the GAS manual.
@c For copying conditions, see the file as.texinfo.

@ifset GENERIC
@page
@node ARM-Dependent
@chapter ARM Dependent Features
@end ifset

@ifclear GENERIC
@node Machine Dependencies
@chapter ARM Dependent Features
@end ifclear

@cindex ARM support
@cindex Thumb support
@menu
* ARM Options::              Options
* ARM Syntax::               Syntax
* ARM Floating Point::       Floating Point
* ARM Directives::           ARM Machine Directives
* ARM Opcodes::              Opcodes
* ARM Mapping Symbols::      Mapping Symbols
* ARM Unwinding Tutorial::   Unwinding
@end menu

@node ARM Options
@section Options
@cindex ARM options (none)
@cindex options for ARM (none)

@table @code

@cindex @code{-mcpu=} command line option, ARM
@item -mcpu=@var{processor}[+@var{extension}@dots{}]
This option specifies the target processor.  The assembler will issue an
error message if an attempt is made to assemble an instruction which
will not execute on the target processor.  The following processor names are
recognized: 
@code{arm1},
@code{arm2},
@code{arm250},
@code{arm3},
@code{arm6},
@code{arm60},
@code{arm600},
@code{arm610},
@code{arm620},
@code{arm7},
@code{arm7m},
@code{arm7d},
@code{arm7dm},
@code{arm7di},
@code{arm7dmi},
@code{arm70},
@code{arm700},
@code{arm700i},
@code{arm710},
@code{arm710t},
@code{arm720},
@code{arm720t},
@code{arm740t},
@code{arm710c},
@code{arm7100},
@code{arm7500},
@code{arm7500fe},
@code{arm7t},
@code{arm7tdmi},
@code{arm7tdmi-s},
@code{arm8},
@code{arm810},
@code{strongarm},
@code{strongarm1},
@code{strongarm110},
@code{strongarm1100},
@code{strongarm1110},
@code{arm9},
@code{arm920},
@code{arm920t},
@code{arm922t},
@code{arm940t},
@code{arm9tdmi},
@code{fa526} (Faraday FA526 processor),
@code{fa626} (Faraday FA626 processor),
@code{arm9e},
@code{arm926e},
@code{arm926ej-s},
@code{arm946e-r0},
@code{arm946e},
@code{arm946e-s},
@code{arm966e-r0},
@code{arm966e},
@code{arm966e-s},
@code{arm968e-s},
@code{arm10t},
@code{arm10tdmi},
@code{arm10e},
@code{arm1020},
@code{arm1020t},
@code{arm1020e},
@code{arm1022e},
@code{arm1026ej-s},
@code{fa626te} (Faraday FA626TE processor),
@code{fa726te} (Faraday FA726TE processor),
@code{arm1136j-s},
@code{arm1136jf-s},
@code{arm1156t2-s},
@code{arm1156t2f-s},
@code{arm1176jz-s},
@code{arm1176jzf-s},
@code{mpcore},
@code{mpcorenovfp},
@code{cortex-a8},
@code{cortex-a9},
@code{cortex-r4},
@code{cortex-m3},
@code{ep9312} (ARM920 with Cirrus Maverick coprocessor),
@code{i80200} (Intel XScale processor)
@code{iwmmxt} (Intel(r) XScale processor with Wireless MMX(tm) technology coprocessor)
and
@code{xscale}.  
The special name @code{all} may be used to allow the
assembler to accept instructions valid for any ARM processor.

In addition to the basic instruction set, the assembler can be told to 
accept various extension mnemonics that extend the processor using the 
co-processor instruction space.  For example, @code{-mcpu=arm920+maverick}
is equivalent to specifying @code{-mcpu=ep9312}.  The following extensions
are currently supported: 
@code{+maverick}
@code{+iwmmxt}
and
@code{+xscale}.

@cindex @code{-march=} command line option, ARM
@item -march=@var{architecture}[+@var{extension}@dots{}]
This option specifies the target architecture.  The assembler will issue
an error message if an attempt is made to assemble an instruction which
will not execute on the target architecture.  The following architecture 
names are recognized: 
@code{armv1},
@code{armv2},
@code{armv2a},
@code{armv2s},
@code{armv3},
@code{armv3m},
@code{armv4},
@code{armv4xm},
@code{armv4t},
@code{armv4txm},
@code{armv5},
@code{armv5t},
@code{armv5txm},
@code{armv5te},
@code{armv5texp},
@code{armv6},
@code{armv6j},
@code{armv6k},
@code{armv6z},
@code{armv6zk},
@code{armv7},
@code{armv7-a},
@code{armv7-r},
@code{armv7-m},
@code{iwmmxt}
and
@code{xscale}.
If both @code{-mcpu} and
@code{-march} are specified, the assembler will use
the setting for @code{-mcpu}.

The architecture option can be extended with the same instruction set
extension options as the @code{-mcpu} option.

@cindex @code{-mfpu=} command line option, ARM
@item -mfpu=@var{floating-point-format}

This option specifies the floating point format to assemble for.  The
assembler will issue an error message if an attempt is made to assemble
an instruction which will not execute on the target floating point unit.  
The following format options are recognized:
@code{softfpa},
@code{fpe},
@code{fpe2},
@code{fpe3},
@code{fpa},
@code{fpa10},
@code{fpa11},
@code{arm7500fe},
@code{softvfp},
@code{softvfp+vfp},
@code{vfp},
@code{vfp10},
@code{vfp10-r0},
@code{vfp9},
@code{vfpxd},
@code{vfpv2}
@code{vfpv3}
@code{vfpv3-d16}
@code{arm1020t},
@code{arm1020e},
@code{arm1136jf-s},
@code{maverick}
and
@code{neon}.

In addition to determining which instructions are assembled, this option
also affects the way in which the @code{.double} assembler directive behaves
when assembling little-endian code.

The default is dependent on the processor selected.  For Architecture 5 or 
later, the default is to assembler for VFP instructions; for earlier 
architectures the default is to assemble for FPA instructions.

@cindex @code{-mthumb} command line option, ARM
@item -mthumb
This option specifies that the assembler should start assembling Thumb
instructions; that is, it should behave as though the file starts with a 
@code{.code 16} directive.

@cindex @code{-mthumb-interwork} command line option, ARM
@item -mthumb-interwork
This option specifies that the output generated by the assembler should
be marked as supporting interworking.

@cindex @code{-mapcs} command line option, ARM
@item -mapcs @code{[26|32]}
This option specifies that the output generated by the assembler should
be marked as supporting the indicated version of the Arm Procedure.
Calling Standard.

@cindex @code{-matpcs} command line option, ARM
@item -matpcs
This option specifies that the output generated by the assembler should 
be marked as supporting the Arm/Thumb Procedure Calling Standard.  If
enabled this option will cause the assembler to create an empty
debugging section in the object file called .arm.atpcs.  Debuggers can
use this to determine the ABI being used by.

@cindex @code{-mapcs-float} command line option, ARM
@item -mapcs-float
This indicates the floating point variant of the APCS should be
used.  In this variant floating point arguments are passed in FP
registers rather than integer registers.

@cindex @code{-mapcs-reentrant} command line option, ARM
@item -mapcs-reentrant
This indicates that the reentrant variant of the APCS should be used.
This variant supports position independent code.

@cindex @code{-mfloat-abi=} command line option, ARM
@item -mfloat-abi=@var{abi}
This option specifies that the output generated by the assembler should be
marked as using specified floating point ABI.
The following values are recognized:
@code{soft},
@code{softfp}
and
@code{hard}.

@cindex @code{-eabi=} command line option, ARM
@item -meabi=@var{ver}
This option specifies which EABI version the produced object files should
conform to.
The following values are recognized:
@code{gnu},
@code{4}
and
@code{5}.

@cindex @code{-EB} command line option, ARM
@item -EB
This option specifies that the output generated by the assembler should
be marked as being encoded for a big-endian processor.

@cindex @code{-EL} command line option, ARM
@item -EL
This option specifies that the output generated by the assembler should
be marked as being encoded for a little-endian processor.

@cindex @code{-k} command line option, ARM
@cindex PIC code generation for ARM
@item -k
This option specifies that the output of the assembler should be marked
as position-independent code (PIC).

@cindex @code{--fix-v4bx} command line option, ARM
@item --fix-v4bx
Allow @code{BX} instructions in ARMv4 code.  This is intended for use with
the linker option of the same name.

@cindex @code{-mwarn-deprecated} command line option, ARM
@item -mwarn-deprecated
@itemx -mno-warn-deprecated
Enable or disable warnings about using deprecated options or
features.  The default is to warn.

@end table


@node ARM Syntax
@section Syntax
@menu
* ARM-Chars::                Special Characters
* ARM-Regs::                 Register Names
* ARM-Relocations::          Relocations
@end menu

@node ARM-Chars
@subsection Special Characters

@cindex line comment character, ARM
@cindex ARM line comment character
The presence of a @samp{@@} on a line indicates the start of a comment
that extends to the end of the current line.  If a @samp{#} appears as
the first character of a line, the whole line is treated as a comment.

@cindex line separator, ARM
@cindex statement separator, ARM
@cindex ARM line separator
The @samp{;} character can be used instead of a newline to separate
statements.

@cindex immediate character, ARM
@cindex ARM immediate character
Either @samp{#} or @samp{$} can be used to indicate immediate operands.

@cindex identifiers, ARM
@cindex ARM identifiers
*TODO* Explain about /data modifier on symbols.

@node ARM-Regs
@subsection Register Names

@cindex ARM register names
@cindex register names, ARM
*TODO* Explain about ARM register naming, and the predefined names.

@node ARM Floating Point
@section Floating Point

@cindex floating point, ARM (@sc{ieee})
@cindex ARM floating point (@sc{ieee})
The ARM family uses @sc{ieee} floating-point numbers.

@node ARM-Relocations
@subsection ARM relocation generation

@cindex data relocations, ARM
@cindex ARM data relocations
Specific data relocations can be generated by putting the relocation name
in parentheses after the symbol name.  For example:

@smallexample
        .word foo(TARGET1)
@end smallexample

This will generate an @samp{R_ARM_TARGET1} relocation against the symbol
@var{foo}.
The following relocations are supported:
@code{GOT},
@code{GOTOFF},
@code{TARGET1},
@code{TARGET2},
@code{SBREL},
@code{TLSGD},
@code{TLSLDM},
@code{TLSLDO},
@code{GOTTPOFF}
and
@code{TPOFF}.

For compatibility with older toolchains the assembler also accepts
@code{(PLT)} after branch targets.  This will generate the deprecated
@samp{R_ARM_PLT32} relocation.

@cindex MOVW and MOVT relocations, ARM
Relocations for @samp{MOVW} and @samp{MOVT} instructions can be generated
by prefixing the value with @samp{#:lower16:} and @samp{#:upper16}
respectively.  For example to load the 32-bit address of foo into r0:

@smallexample
        MOVW r0, #:lower16:foo
        MOVT r0, #:upper16:foo
@end smallexample

@node ARM Directives
@section ARM Machine Directives

@cindex machine directives, ARM
@cindex ARM machine directives
@table @code

@cindex @code{align} directive, ARM
@item .align @var{expression} [, @var{expression}]
This is the generic @var{.align} directive.  For the ARM however if the
first argument is zero (ie no alignment is needed) the assembler will
behave as if the argument had been 2 (ie pad to the next four byte
boundary).  This is for compatibility with ARM's own assembler.

@cindex @code{req} directive, ARM
@item @var{name} .req @var{register name}
This creates an alias for @var{register name} called @var{name}.  For
example:

@smallexample
        foo .req r0
@end smallexample

@cindex @code{unreq} directive, ARM
@item .unreq @var{alias-name}
This undefines a register alias which was previously defined using the
@code{req}, @code{dn} or @code{qn} directives.  For example:

@smallexample
        foo .req r0
        .unreq foo
@end smallexample

An error occurs if the name is undefined.  Note - this pseudo op can
be used to delete builtin in register name aliases (eg 'r0').  This
should only be done if it is really necessary.

@cindex @code{dn} and @code{qn} directives, ARM
@item @var{name} .dn @var{register name} [@var{.type}] [[@var{index}]]
@item @var{name} .qn @var{register name} [@var{.type}] [[@var{index}]]

The @code{dn} and @code{qn} directives are used to create typed
and/or indexed register aliases for use in Advanced SIMD Extension
(Neon) instructions.  The former should be used to create aliases
of double-precision registers, and the latter to create aliases of
quad-precision registers.

If these directives are used to create typed aliases, those aliases can
be used in Neon instructions instead of writing types after the mnemonic
or after each operand.  For example:

@smallexample
        x .dn d2.f32
        y .dn d3.f32
        z .dn d4.f32[1]
        vmul x,y,z
@end smallexample

This is equivalent to writing the following:

@smallexample
        vmul.f32 d2,d3,d4[1]
@end smallexample

Aliases created using @code{dn} or @code{qn} can be destroyed using
@code{unreq}.

@cindex @code{code} directive, ARM
@item .code @code{[16|32]}
This directive selects the instruction set being generated. The value 16
selects Thumb, with the value 32 selecting ARM.

@cindex @code{thumb} directive, ARM
@item .thumb
This performs the same action as @var{.code 16}.

@cindex @code{arm} directive, ARM
@item .arm
This performs the same action as @var{.code 32}.

@cindex @code{force_thumb} directive, ARM
@item .force_thumb
This directive forces the selection of Thumb instructions, even if the
target processor does not support those instructions

@cindex @code{thumb_func} directive, ARM
@item .thumb_func
This directive specifies that the following symbol is the name of a
Thumb encoded function.  This information is necessary in order to allow
the assembler and linker to generate correct code for interworking
between Arm and Thumb instructions and should be used even if
interworking is not going to be performed.  The presence of this
directive also implies @code{.thumb}

This directive is not neccessary when generating EABI objects.  On these
targets the encoding is implicit when generating Thumb code.

@cindex @code{thumb_set} directive, ARM
@item .thumb_set
This performs the equivalent of a @code{.set} directive in that it
creates a symbol which is an alias for another symbol (possibly not yet
defined).  This directive also has the added property in that it marks
the aliased symbol as being a thumb function entry point, in the same
way that the @code{.thumb_func} directive does.

@cindex @code{.ltorg} directive, ARM
@item .ltorg
This directive causes the current contents of the literal pool to be
dumped into the current section (which is assumed to be the .text
section) at the current location (aligned to a word boundary).
@code{GAS} maintains a separate literal pool for each section and each
sub-section.  The @code{.ltorg} directive will only affect the literal
pool of the current section and sub-section.  At the end of assembly
all remaining, un-empty literal pools will automatically be dumped.

Note - older versions of @code{GAS} would dump the current literal
pool any time a section change occurred.  This is no longer done, since
it prevents accurate control of the placement of literal pools.

@cindex @code{.pool} directive, ARM
@item .pool
This is a synonym for .ltorg.

@anchor{arm_fnstart}
@cindex @code{.fnstart} directive, ARM
@item .fnstart
Marks the start of a function with an unwind table entry.

@anchor{arm_fnend}
@cindex @code{.fnend} directive, ARM
@item .fnend
Marks the end of a function with an unwind table entry.  The unwind index
table entry is created when this directive is processed.

If no personality routine has been specified then standard personality
routine 0 or 1 will be used, depending on the number of unwind opcodes
required.

@cindex @code{.cantunwind} directive, ARM
@item .cantunwind
Prevents unwinding through the current function.  No personality routine
or exception table data is required or permitted.

@cindex @code{.personality} directive, ARM
@item .personality @var{name}
Sets the personality routine for the current function to @var{name}.

@cindex @code{.personalityindex} directive, ARM
@item .personalityindex @var{index}
Sets the personality routine for the current function to the EABI standard
routine number @var{index}

@cindex @code{.handlerdata} directive, ARM
@item .handlerdata
Marks the end of the current function, and the start of the exception table
entry for that function.  Anything between this directive and the
@code{.fnend} directive will be added to the exception table entry.

Must be preceded by a @code{.personality} or @code{.personalityindex}
directive.

@anchor{arm_save}
@cindex @code{.save} directive, ARM
@item .save @var{reglist}
Generate unwinder annotations to restore the registers in @var{reglist}.
The format of @var{reglist} is the same as the corresponding store-multiple
instruction.

@smallexample
@exdent @emph{core registers}
  .save @{r4, r5, r6, lr@}
  stmfd sp!, @{r4, r5, r6, lr@}
@exdent @emph{FPA registers}
  .save f4, 2
  sfmfd f4, 2, [sp]!
@exdent @emph{VFP registers}
  .save @{d8, d9, d10@}
  fstmdx sp!, @{d8, d9, d10@}
@exdent @emph{iWMMXt registers}
  .save @{wr10, wr11@}
  wstrd wr11, [sp, #-8]!
  wstrd wr10, [sp, #-8]!
or
  .save wr11
  wstrd wr11, [sp, #-8]!
  .save wr10
  wstrd wr10, [sp, #-8]!
@end smallexample

@cindex @code{.vsave} directive, ARM
@item .vsave @var{vfp-reglist}
Generate unwinder annotations to restore the VFP registers in @var{vfp-reglist}
using FLDMD.  Also works for VFPv3 registers
that are to be restored using VLDM.
The format of @var{vfp-reglist} is the same as the corresponding store-multiple
instruction.

@smallexample
@exdent @emph{VFP registers}
  .vsave @{d8, d9, d10@}
  fstmdd sp!, @{d8, d9, d10@}
@exdent @emph{VFPv3 registers}
  .vsave @{d15, d16, d17@}
  vstm sp!, @{d15, d16, d17@}
@end smallexample

Since FLDMX and FSTMX are now deprecated, this directive should be
used in favour of @code{.save} for saving VFP registers for ARMv6 and above.

@anchor{arm_pad}
@cindex @code{.pad} directive, ARM
@item .pad #@var{count}
Generate unwinder annotations for a stack adjustment of @var{count} bytes.
A positive value indicates the function prologue allocated stack space by
decrementing the stack pointer.

@anchor{arm_movsp}
@cindex @code{.movsp} directive, ARM
@item .movsp @var{reg} [, #@var{offset}]
Tell the unwinder that @var{reg} contains an offset from the current
stack pointer.  If @var{offset} is not specified then it is assumed to be
zero.

@anchor{arm_setfp}
@cindex @code{.setfp} directive, ARM
@item .setfp @var{fpreg}, @var{spreg} [, #@var{offset}]
Make all unwinder annotations relaive to a frame pointer.  Without this
the unwinder will use offsets from the stack pointer.

The syntax of this directive is the same as the @code{sub} or @code{mov}
instruction used to set the frame pointer.  @var{spreg} must be either
@code{sp} or mentioned in a previous @code{.movsp} directive.

@smallexample
.movsp ip
mov ip, sp
@dots{}
.setfp fp, ip, #4
sub fp, ip, #4
@end smallexample

@cindex @code{.unwind_raw} directive, ARM
@item .raw @var{offset}, @var{byte1}, @dots{}
Insert one of more arbitary unwind opcode bytes, which are known to adjust
the stack pointer by @var{offset} bytes.

For example @code{.unwind_raw 4, 0xb1, 0x01} is equivalent to
@code{.save @{r0@}}

@cindex @code{.cpu} directive, ARM
@item .cpu @var{name}
Select the target processor.  Valid values for @var{name} are the same as
for the @option{-mcpu} commandline option.

@cindex @code{.arch} directive, ARM
@item .arch @var{name}
Select the target architecture.  Valid values for @var{name} are the same as
for the @option{-march} commandline option.

@cindex @code{.object_arch} directive, ARM
@item .object_arch @var{name}
Override the architecture recorded in the EABI object attribute section.
Valid values for @var{name} are the same as for the @code{.arch} directive.
Typically this is useful when code uses runtime detection of CPU features.

@cindex @code{.fpu} directive, ARM
@item .fpu @var{name}
Select the floating point unit to assemble for.  Valid values for @var{name}
are the same as for the @option{-mfpu} commandline option.

@cindex @code{.eabi_attribute} directive, ARM
@item .eabi_attribute @var{tag}, @var{value}
Set the EABI object attribute @var{tag} to @var{value}.

The @var{tag} is either an attribute number, or one of the following:
@code{Tag_CPU_raw_name}, @code{Tag_CPU_name}, @code{Tag_CPU_arch},
@code{Tag_CPU_arch_profile}, @code{Tag_ARM_ISA_use},
@code{Tag_THUMB_ISA_use}, @code{Tag_VFP_arch}, @code{Tag_WMMX_arch},
@code{Tag_Advanced_SIMD_arch}, @code{Tag_PCS_config},
@code{Tag_ABI_PCS_R9_use}, @code{Tag_ABI_PCS_RW_data},
@code{Tag_ABI_PCS_RO_data}, @code{Tag_ABI_PCS_GOT_use},
@code{Tag_ABI_PCS_wchar_t}, @code{Tag_ABI_FP_rounding},
@code{Tag_ABI_FP_denormal}, @code{Tag_ABI_FP_exceptions},
@code{Tag_ABI_FP_user_exceptions}, @code{Tag_ABI_FP_number_model},
@code{Tag_ABI_align8_needed}, @code{Tag_ABI_align8_preserved},
@code{Tag_ABI_enum_size}, @code{Tag_ABI_HardFP_use},
@code{Tag_ABI_VFP_args}, @code{Tag_ABI_WMMX_args},
@code{Tag_ABI_optimization_goals}, @code{Tag_ABI_FP_optimization_goals},
@code{Tag_compatibility}, @code{Tag_CPU_unaligned_access},
@code{Tag_VFP_HP_extension}, @code{Tag_ABI_FP_16bit_format},
@code{Tag_nodefaults}, @code{Tag_also_compatible_with},
@code{Tag_conformance}, @code{Tag_T2EE_use},
@code{Tag_Virtualization_use}, @code{Tag_MPextension_use}

The @var{value} is either a @code{number}, @code{"string"}, or
@code{number, "string"} depending on the tag.

@end table

@node ARM Opcodes
@section Opcodes

@cindex ARM opcodes
@cindex opcodes for ARM
@code{@value{AS}} implements all the standard ARM opcodes.  It also
implements several pseudo opcodes, including several synthetic load
instructions. 

@table @code

@cindex @code{NOP} pseudo op, ARM
@item NOP
@smallexample
  nop
@end smallexample

This pseudo op will always evaluate to a legal ARM instruction that does
nothing.  Currently it will evaluate to MOV r0, r0.

@cindex @code{LDR reg,=<label>} pseudo op, ARM
@item LDR 
@smallexample
  ldr <register> , = <expression>
@end smallexample

If expression evaluates to a numeric constant then a MOV or MVN
instruction will be used in place of the LDR instruction, if the
constant can be generated by either of these instructions.  Otherwise
the constant will be placed into the nearest literal pool (if it not
already there) and a PC relative LDR instruction will be generated.

@cindex @code{ADR reg,<label>} pseudo op, ARM
@item ADR
@smallexample
  adr <register> <label>
@end smallexample

This instruction will load the address of @var{label} into the indicated
register.  The instruction will evaluate to a PC relative ADD or SUB
instruction depending upon where the label is located.  If the label is
out of range, or if it is not defined in the same file (and section) as
the ADR instruction, then an error will be generated.  This instruction
will not make use of the literal pool.

@cindex @code{ADRL reg,<label>} pseudo op, ARM
@item ADRL 
@smallexample
  adrl <register> <label>
@end smallexample

This instruction will load the address of @var{label} into the indicated
register.  The instruction will evaluate to one or two PC relative ADD
or SUB instructions depending upon where the label is located.  If a
second instruction is not needed a NOP instruction will be generated in
its place, so that this instruction is always 8 bytes long.

If the label is out of range, or if it is not defined in the same file
(and section) as the ADRL instruction, then an error will be generated.
This instruction will not make use of the literal pool.

@end table

For information on the ARM or Thumb instruction sets, see @cite{ARM
Software Development Toolkit Reference Manual}, Advanced RISC Machines
Ltd.

@node ARM Mapping Symbols
@section Mapping Symbols

The ARM ELF specification requires that special symbols be inserted
into object files to mark certain features:

@table @code

@cindex @code{$a}
@item $a
At the start of a region of code containing ARM instructions.

@cindex @code{$t}
@item $t
At the start of a region of code containing THUMB instructions.

@cindex @code{$d}
@item $d
At the start of a region of data.

@end table

The assembler will automatically insert these symbols for you - there
is no need to code them yourself.  Support for tagging symbols ($b,
$f, $p and $m) which is also mentioned in the current ARM ELF
specification is not implemented.  This is because they have been
dropped from the new EABI and so tools cannot rely upon their
presence.

@node ARM Unwinding Tutorial
@section Unwinding

The ABI for the ARM Architecture specifies a standard format for
exception unwind information.  This information is used when an
exception is thrown to determine where control should be transferred.
In particular, the unwind information is used to determine which
function called the function that threw the exception, and which
function called that one, and so forth.  This information is also used
to restore the values of callee-saved registers in the function
catching the exception.

If you are writing functions in assembly code, and those functions
call other functions that throw exceptions, you must use assembly
pseudo ops to ensure that appropriate exception unwind information is
generated.  Otherwise, if one of the functions called by your assembly
code throws an exception, the run-time library will be unable to
unwind the stack through your assembly code and your program will not
behave correctly.

To illustrate the use of these pseudo ops, we will examine the code
that G++ generates for the following C++ input:

@verbatim
void callee (int *);

int 
caller () 
{
  int i;
  callee (&i);
  return i; 
}
@end verbatim

This example does not show how to throw or catch an exception from
assembly code.  That is a much more complex operation and should
always be done in a high-level language, such as C++, that directly
supports exceptions.

The code generated by one particular version of G++ when compiling the
example above is:

@verbatim
_Z6callerv:
        .fnstart
.LFB2:
        @ Function supports interworking.
        @ args = 0, pretend = 0, frame = 8
        @ frame_needed = 1, uses_anonymous_args = 0
        stmfd   sp!, {fp, lr}
        .save {fp, lr}
.LCFI0:
        .setfp fp, sp, #4
        add     fp, sp, #4
.LCFI1:
        .pad #8
        sub     sp, sp, #8
.LCFI2:
        sub     r3, fp, #8
        mov     r0, r3
        bl      _Z6calleePi
        ldr     r3, [fp, #-8]
        mov     r0, r3
        sub     sp, fp, #4
        ldmfd   sp!, {fp, lr}
        bx      lr
.LFE2:
        .fnend
@end verbatim

Of course, the sequence of instructions varies based on the options
you pass to GCC and on the version of GCC in use.  The exact
instructions are not important since we are focusing on the pseudo ops
that are used to generate unwind information.

An important assumption made by the unwinder is that the stack frame
does not change during the body of the function.  In particular, since
we assume that the assembly code does not itself throw an exception,
the only point where an exception can be thrown is from a call, such
as the @code{bl} instruction above.  At each call site, the same saved
registers (including @code{lr}, which indicates the return address)
must be located in the same locations relative to the frame pointer.

The @code{.fnstart} (@pxref{arm_fnstart,,.fnstart pseudo op}) pseudo
op appears immediately before the first instruction of the function
while the @code{.fnend} (@pxref{arm_fnend,,.fnend pseudo op}) pseudo
op appears immediately after the last instruction of the function.
These pseudo ops specify the range of the function.  

Only the order of the other pseudos ops (e.g., @code{.setfp} or
@code{.pad}) matters; their exact locations are irrelevant.  In the
example above, the compiler emits the pseudo ops with particular
instructions.  That makes it easier to understand the code, but it is
not required for correctness.  It would work just as well to emit all
of the pseudo ops other than @code{.fnend} in the same order, but
immediately after @code{.fnstart}.

The @code{.save} (@pxref{arm_save,,.save pseudo op}) pseudo op
indicates registers that have been saved to the stack so that they can
be restored before the function returns.  The argument to the
@code{.save} pseudo op is a list of registers to save.  If a register
is ``callee-saved'' (as specified by the ABI) and is modified by the
function you are writing, then your code must save the value before it
is modified and restore the original value before the function
returns.  If an exception is thrown, the run-time library restores the
values of these registers from their locations on the stack before
returning control to the exception handler.  (Of course, if an
exception is not thrown, the function that contains the @code{.save}
pseudo op restores these registers in the function epilogue, as is
done with the @code{ldmfd} instruction above.)

You do not have to save callee-saved registers at the very beginning
of the function and you do not need to use the @code{.save} pseudo op
immediately following the point at which the registers are saved.
However, if you modify a callee-saved register, you must save it on
the stack before modifying it and before calling any functions which
might throw an exception.  And, you must use the @code{.save} pseudo
op to indicate that you have done so.

The @code{.pad} (@pxref{arm_pad,,.pad}) pseudo op indicates a
modification of the stack pointer that does not save any registers.
The argument is the number of bytes (in decimal) that are subtracted
from the stack pointer.  (On ARM CPUs, the stack grows downwards, so
subtracting from the stack pointer increases the size of the stack.)

The @code{.setfp} (@pxref{arm_setfp,,.setfp pseudo op}) pseudo op
indicates the register that contains the frame pointer.  The first
argument is the register that is set, which is typically @code{fp}.
The second argument indicates the register from which the frame
pointer takes its value.  The third argument, if present, is the value
(in decimal) added to the register specified by the second argument to
compute the value of the frame pointer.  You should not modify the
frame pointer in the body of the function.

If you do not use a frame pointer, then you should not use the
@code{.setfp} pseudo op.  If you do not use a frame pointer, then you
should avoid modifying the stack pointer outside of the function
prologue.  Otherwise, the run-time library will be unable to find
saved registers when it is unwinding the stack.

The pseudo ops described above are sufficient for writing assembly
code that calls functions which may throw exceptions.  If you need to
know more about the object-file format used to represent unwind
information, you may consult the @cite{Exception Handling ABI for the
ARM Architecture} available from @uref{http://infocenter.arm.com}.