don't read CSR in csrrw(i) with rd=0 #1628
Open
+8
−2
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
According to
![螢幕擷取畫面 2024-03-19 110524](https://private-user-images.githubusercontent.com/162972686/313913794-1c05349e-ae75-4615-8c85-519e2c2906c9.png?jwt=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpc3MiOiJnaXRodWIuY29tIiwiYXVkIjoicmF3LmdpdGh1YnVzZXJjb250ZW50LmNvbSIsImtleSI6ImtleTUiLCJleHAiOjE3MjE3MDY3MjgsIm5iZiI6MTcyMTcwNjQyOCwicGF0aCI6Ii8xNjI5NzI2ODYvMzEzOTEzNzk0LTFjMDUzNDllLWFlNzUtNDYxNS04Yzg1LTUxOWUyYzI5MDZjOS5wbmc_WC1BbXotQWxnb3JpdGhtPUFXUzQtSE1BQy1TSEEyNTYmWC1BbXotQ3JlZGVudGlhbD1BS0lBVkNPRFlMU0E1M1BRSzRaQSUyRjIwMjQwNzIzJTJGdXMtZWFzdC0xJTJGczMlMkZhd3M0X3JlcXVlc3QmWC1BbXotRGF0ZT0yMDI0MDcyM1QwMzQ3MDhaJlgtQW16LUV4cGlyZXM9MzAwJlgtQW16LVNpZ25hdHVyZT04MDkwOTRmNTEyOTBiNTQ3YmE4NjQ5ZGVjNTBiMWEyMGQ4MzlkZDc4ZTAyOWEzNzg1YjU5ZDMxYWE1MDFlNjQxJlgtQW16LVNpZ25lZEhlYWRlcnM9aG9zdCZhY3Rvcl9pZD0wJmtleV9pZD0wJnJlcG9faWQ9MCJ9.6VeSfLmgkU2o4YRTTd9Oo53VbgGWu32Sl1Kr78TVMZA)
The RISC-V Instruction Set Manual Volume I: Unprivileged ISA Document Version 20191213 Table 9.1,
there are some conditional with rd = 0 that inhibit read on CSRs.
Though original implementation is correct in most cases, this always-read implementation may cause some effects. For example, some CSRs that read-write to it may change the architectural state.