These function attributes are supported by the Nios II back end:
As discussed in Common Function Attributes, this attribute allows specification of target-specific compilation options.
When compiling for Nios II, the following options are allowed:
Each ‘custom-insn=N’ attribute locally enables use of a custom instruction with encoding N when generating code that uses insn. Similarly, ‘no-custom-insn’ locally inhibits use of the custom instruction insn. These target attributes correspond to the -mcustom-insn=N and -mno-custom-insn command-line options, and support the same set of insn keywords. See Nios II Options, for more information.
This attribute corresponds to the -mcustom-fpu-cfg=name command-line option, to select a predefined set of custom instructions named name. See Nios II Options, for more information.