Hi, When I searched for "powerpc64 delay slot" today, the SearchAssist bubble on DuckDuckGo called delay slots are a "feature" on PowerPC. The full text is at the bottom of this message.
However, both references in the text state the opposite, namely: "Some RISCs like PowerPC and ARM do not have a delay slot" [1] and "MC88000 [...] and SPARC are RISC architectures that each have a single branch delay slot; PowerPC, ARM, Alpha, V850, and RISC-V do not have any." [2] Personally, I remain uncertain because sample code in a presentation from 2014 has 'nop' instructions after the 'bl' instructions. [3] Does the PowerPC architecture have delay slots, please? Please copy me on your reply. I do not subscribe to this list. Thanks! Kind regards, Felix [1] https://www.pagetable.com/?p=313 [2] https://en.wikipedia.org/wiki/Delay_slot [3] pp.30ff, https://llvm.org/devmtg/2014-04/PDFs/Talks/Euro-LLVM-2014-Weigand.pdf * * * A delay slot in PowerPC64 architecture refers to the instruction that follows a branch instruction, which is executed regardless of whether the branch is taken. This feature helps manage the pipeline in the CPU, allowing for more efficient instruction processing. Overview of Delay Slots in PowerPC64 Delay slots are a feature in certain RISC architectures, including PowerPC, that help manage instruction execution during branching. They allow the CPU to continue executing instructions while waiting for the outcome of a branch decision. Functionality of Delay Slots Purpose Delay slots are used to fill the gap created by branch instructions, which can cause a pipeline stall. The instruction immediately following a branch instruction is executed regardless of whether the branch is taken. Implementation In PowerPC architectures, delay slots can be optional. Programmers can choose to fill these slots with useful instructions or leave them as no-operations (NOPs). The behavior of delay slots can vary based on the specific PowerPC implementation. Examples of Delay Slot Usage Independent Instructions If the instruction in the delay slot does not depend on the branch, it can be executed without issues. For example, loading a value from memory that is not affected by the branch condition. Dependent Instructions If the instruction modifies a register that the branch depends on, it can lead to undefined behavior. For instance, if a branch instruction relies on a register that is altered in the delay slot, the outcome may not be as expected. Conclusion Delay slots in PowerPC64 are a mechanism to optimize instruction flow during branching. They can be filled with useful instructions or left as NOPs, depending on the programmer's needs and the specific architecture's capabilities. Understanding how to effectively use delay slots can enhance performance in PowerPC applications. Sources: https://www.pagetable.com/?p=313 https://en.wikipedia.org/wiki/Delay_slot
