Using as: MIPS Symbol Sizes

Next: MIPS Small Data, Previous: MIPS Macros, Up: MIPS-Dependent [Contents][Index]

#### 9.27.3 Directives to override the size of symbols

The n64 ABI allows symbols to have any 64-bit value. Although this provides a great deal of flexibility, it means that some macros have much longer expansions than their 32-bit counterparts. For example, the non-PIC expansion of ‘`dla $4,sym`’ is usually:

    lui     $4,%highest(sym)
    lui     $1,%hi(sym)
    daddiu  $4,$4,%higher(sym)
    daddiu  $1,$1,%lo(sym)
    dsll32  $4,$4,0
    daddu   $4,$4,$1

whereas the 32-bit expansion is simply:

    lui     $4,%hi(sym)
    daddiu  $4,$4,%lo(sym)

n64 code is sometimes constructed in such a way that all symbolic constants are known to have 32-bit values, and in such cases, it’s preferable to use the 32-bit expansion instead of the 64-bit expansion.

You can use the `.set sym32` directive to tell the assembler that, from this point on, all expressions of the form ‘`symbol`’ or ‘`symbol + offset`’ have 32-bit values. For example:

    .set sym32
    dla     $4,sym
    lw      $4,sym+16
    sw      $4,sym+0x8000($4)

will cause the assembler to treat ‘`sym`’, `sym+16` and `sym+0x8000` as 32-bit values. The handling of non-symbolic addresses is not affected.

The directive `.set nosym32` ends a `.set sym32` block and reverts to the normal behavior. It is also possible to change the symbol size using the command-line options `-msym32` and `-mno-sym32`.

These options and directives are always accepted, but at present, they have no effect for anything other than n64.