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