On 17/6/2025 20:42, Jeff Law wrote:
On 6/16/25 10:08 PM, Dongyan Chen wrote:
Hi, I've come across a question regarding the branch cost of gcc. In
the link
https://gcc.godbolt.org/z/hnddevd5h, gcc fails to recognize the
optimization
branch judgment, while llvm does. I eventually discovered that the
value of the branch
cost was too small. Moreover, in that link, if I add "-mbranch-
cost=4" (a larger
number can also be used) for gcc, the zicond extension functions
properly. So, is
it necessary to modify the branch cost for gcc? According to the
source code, the
default mtun is rocket, which has a branch cost of 3. I think it
should be set to 4.
gcc/ChangeLog:
* config/riscv/riscv.cc: Change the branch cost.
gcc/testsuite/ChangeLog:
* gcc.target/riscv/zicond-
primitiveSemantics_compare_reg_reg_return_reg_reg.c: New test.
So I'd be a lot more comfortable with this if someone that knows the
rocket uarch could chime in or if we had wider data on how this behaves
in general. One pico-sized benchmark isn't a great way to evaluate
something like this.
The rocket core is quite simple, utilizing a five-stage in-order scalar
pipeline with a 3-cycle branch mis-predict penalty.
However, there is a trade-off here:
- Use branch
- 2-3 dynamic instructions reduced for each loop
- 3 cycles penalty when branch can be predicted
- Use Zicond
- No branch mis-predict penalty
- 2-3 dynamic instructions overhead for each loop
I agree that this might not be helpful for rocket-chip. However, since
rocket-chip is the default tune information for RISC-V, and AFAIK every
rocket core that has been taped out lacks a zicond extension. I think
it's acceptable to adjust this for better RISC-V ecosystems, as branch
misprediction on large OoO cores usually incurs a penalty of about 10
cycles.
Thanks,
Yangyu Chen
WRT a followup from Yangyu which touches on the BPI. My son is
currently putting together a tuning and scheduler model for the spacemit
x60 chip in that system under my guidance. I expect we'll start
benchmarking that work next week. We can look at the branch costing
model for that design as a part of that work.
Jeff
