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.