Recently C23 added two new attributes: unsequenced and reproducible, which are now supported by GCC alongside the existing pure and const.
I am slightly confused by the exact differences between all of them, and all of the sources I could find only compared one or two of them, not all four at once.
Is the following understanding I have of them correct?
| Attribute | Side Effects | Output | Read Globals [0] | Write Globals [0] | Read Pointer Args | Write Pointer Args | Examples | Optimisations |
|---|---|---|---|---|---|---|---|---|
| Pure | no | return | yes [1] | no | yes | no | strlen, memcpy |
Elide calls where globals and args have not changed |
| Const | no | return | const [2] | no | const [2] | no | abs |
All calls with the same arguments have the same value; evaluate at compile-time |
| Unsequenced | yes | return | const | no | yes | no | strlen |
Reorder calls |
| Reproducible | yes | return or args | const | no | yes | yes | memcpy |
Elide repeated calls with the same arguments |
| None | yes | return or args | yes | yes | yes | yes | printf |
None |