13 – .SYMBOL ALIGNMENT
This directive associates an alignment attribute with a symbol
definition for a register offset. You can use this directive
when you know the alignment of the base register. This attribute
guarantees to the compiler that the base register has the same
alignment, which enables the compiler to generate optimal code.
Format
.SYMBOL_ALIGNMENT argument-list
13.1 – Parameters
argument-list
One of the arguments listed in the following table:
Option Description
long Declares longword alignment for any symbol that you
declare after this directive.
quad Declares quadword alignment for any symbol that you
declare after this directive.
none Turns off the alignment specified by the preceding
.SYMBOL_ALIGNMENT directive.
13.2 – Description
The .SYMBOL_ALIGNMENT directive is used to associate an alignment
attribute with the fields in a structure when you know the base
alignment. It is used in pairs. The first .SYMBOL_ALIGNMENT
directive associates either longword (long) or quadword (quad)
alignment with the symbol or symbols that follow. The second
directive, .SYMBOL_ALIGNMENT none, turns it off.
Any time a reference is made with a symbol with an alignment
attribute, the base register of that reference, in effect,
inherits the symbol's alignment. The compiler also resets the
base register's alignment to longword for subsequent alignment
tracking. This alignment guarantee enables the compiler to
produce more efficient code sequences.
13.3 – Example
OFFSET1 = 4
.SYMBOL_ALIGNMENT LONG
OFFSET2 = 8
OFFSET3 = 12
.SYMBOL_ALIGNMENT QUAD
OFFSET4 = 16
.SYMBOL_ALIGNMENT NONE
OFFSET5 = 20
.
.
.
CLR1 OFFSET2(R8)
.
.
.
MOVL R2, OFFSET4(R6)
For OFFSET1 and OFFSET5, the compiler will use only its
tracking information for deciding if Rn in OFFSET1(Rn) is
aligned or not. For the other references, the base register
will be treated as longword (OFFSET2 and OFFSET3) or quadword
(OFFSET4) aligned.
After each use of OFFSET2 or OFFSET4, the base register in the
reference is reset to longword alignment. In this example, the
alignment of R8 and R6 will be reset to longword, although the
reference to OFFSET4 will use the stronger quadword alignment.
14 – .USE LINKAGE
OpenVMS I64 only.
Establishes a temporary named or anonymous linkage that will
be used by the compiler for the next CALLS, CALLG, JSB, BSBB,
or BSBW instruction processed in lexical order. This directive
is used when the target of the next CALLS, CALLG, JSB, BSBB,
or BSBW instruction is not a name, but a run-time value (for
example, CALLS #0, (R6)). When the compiler sees the next
CALLS, CALLG, JSB, BSBB, or BSBW instruction, it will use the
associated linkage to decide which registers need to be saved and
restored around the call. After the instruction is processed, the
temporary linkage is reset to null.
Format
.USE_LINKAGE [linkage_name] [,input] [,output] [,scratch]
[,preserve]
14.1 – Parameters
linkage_name
The name of a linkage previously defined with the .DEFINE_LINKAGE
directive. If you specify a linkage_name, you cannot specify an
input, output, scratch or preserve clause.
input=<>
Register set that indicates those registers from which the
routine called by the next CALLS, CALLG, JSB, BSBB, or BSBW
instruction receives input values.
output=<>
Register set that indicates those registers to which the routine
called by the next CALLS, CALLG, JSB, BSBB, or BSBW instruction
assigns values that are returned to the routine's caller.
Registers included in this register set are not saved and
restored around the call.
scratch=<>
Register set that indicates registers that are used within the
routine called by the next CALLS, CALLG, JSB, BSBB, or BSBW
instruction. This parameter is for documentation purposes only.
preserve=<>
Register set that indicates those registers which the routine
called by the next CALLS, CALLG, JSB, BSBB, or BSBW instruction
will preserve. Registers included in this register set are not
saved and restored around a call to the routine, since the called
routine will perform that task.