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9.3.4 ARM Machine Directives

.align expression [, expression ] This is the generic .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.

name .req register name This creates an alias for register name called name. For example:

                  foo .req r0

.unreq alias-name This undefines a register alias which was previously defined using the req, dn or qn directives. For example:

                  foo .req r0
                  .unreq foo

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.

name .dn register name [ .type ] [[ index ]]
name .qn register name [ .type ] [[ index ]] The dn and 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:

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

This is equivalent to writing the following:

                  vmul.f32 d2,d3,d4[1]

Aliases created using dn or qn can be destroyed using unreq.

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

.thumb This performs the same action as .code 16.

.arm This performs the same action as .code 32.

.force_thumb This directive forces the selection of Thumb instructions, even if the target processor does not support those instructions

.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 .thumb

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

.thumb_set This performs the equivalent of a .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 .thumb_func directive does.

.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). GAS maintains a separate literal pool for each section and each sub-section. The .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 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.

.pool This is a synonym for .ltorg.

.fnstart Marks the start of a function with an unwind table entry.

.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.

.cantunwind Prevents unwinding through the current function. No personality routine or exception table data is required or permitted.

.personality name Sets the personality routine for the current function to name.

.personalityindex index Sets the personality routine for the current function to the EABI standard routine number index

.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 .fnend directive will be added to the exception table entry.

Must be preceded by a .personality or .personalityindex directive.

.save reglist Generate unwinder annotations to restore the registers in reglist. The format of reglist is the same as the corresponding store-multiple instruction.



core registers


            .save {r4, r5, r6, lr}
            stmfd sp!, {r4, r5, r6, lr}
     

FPA registers


            .save f4, 2
            sfmfd f4, 2, [sp]!
     

VFP registers


            .save {d8, d9, d10}
            fstmdx sp!, {d8, d9, d10}
     

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]!

.vsave vfp-reglist Generate unwinder annotations to restore the VFP registers in vfp-reglist using FLDMD. Also works for VFPv3 registers that are to be restored using VLDM. The format of vfp-reglist is the same as the corresponding store-multiple instruction.



VFP registers


            .vsave {d8, d9, d10}
            fstmdd sp!, {d8, d9, d10}
     

VFPv3 registers


            .vsave {d15, d16, d17}
            vstm sp!, {d15, d16, d17}

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

.pad # count Generate unwinder annotations for a stack adjustment of count bytes. A positive value indicates the function prologue allocated stack space by decrementing the stack pointer.

.movsp reg [, # offset ] Tell the unwinder that reg contains an offset from the current stack pointer. If offset is not specified then it is assumed to be zero.

.setfp fpreg , spreg [, # 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 sub or mov instruction used to set the frame pointer. spreg must be either sp or mentioned in a previous .movsp directive.

          .movsp ip
          mov ip, sp
          ...
          .setfp fp, ip, #4
          sub fp, ip, #4

.raw offset , byte1 , ... Insert one of more arbitary unwind opcode bytes, which are known to adjust the stack pointer by offset bytes.

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

.cpu name Select the target processor. Valid values for name are the same as for the -mcpu commandline option.

.arch name Select the target architecture. Valid values for name are the same as for the -march commandline option.

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

.fpu name Select the floating point unit to assemble for. Valid values for name are the same as for the -mfpu commandline option.

.eabi_attribute tag , value Set the EABI object attribute number tag to value. The value is either a number, "string", or number, "string" depending on the tag.

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