Restrictions on inline assembly statements

This section describes the restrictions on the use of inline assembly statements.

The following restrictions are on the use of inline assembly statements:
  • The assembler instructions must be self-contained within an asm statement. The asm statement can only be used to generate instructions. All connections to the rest of the program must be established through the output and input operand list.
  • Referencing an external symbol directly without going through the operand list is not supported.
  • Assembler instructions requiring a pair of registers are not specifiable by any constraints, and are therefore not supported. For example, you cannot use the %f constraint for a long double operand.
  • The shared register file between the floating-point scalar and the vector registers are not modelled as shared in inline assembly statements. You must specify registers f0-f31 and v0-v31 in the clobbers list. There is no combined x0-x63.
  • Operand replacements (such as %0, %1, and so on) can use an optional x before the number or symbolic name to indicate that a vsx register reference must be used. For example, a vector operand %1 allocated to register v0 is replaced with 0 (for use in VMX instructions). The same operand used as %x1 in the assembly text is replaced with 32 (for use in VSX instructions). Note that this restriction applies only for architectures that support VSX architecture extension.