Bit Manipulation C code style testing

[Rohan Ahmed]

Hi, I am trying to emit the orc.b instruction with a c program. I wrote different scalar optmized c code but it didn't generate the orc.b instruction and I have to provide the pure scalar code which emits this instruction. can you suggest something

Also I am able to generate all others zbb type instruction using scalar code, you can found in this sheet.
https://docs.google.com/spreadsheets/d/1A5dQQVhP_8RrnpvLiUgXO3rc5Q2yjKwAMJNZpheK2RM/edit#gid=556068285

[Christoph Müllner]

What are real-world code examples where orc.b could be used?
GCC has inline expansion for str(n)cmp/strlen, which utilizes orc.b
(if the requirements are met):
https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/config/riscv/riscv-string.cc;h=b09b51d7526b9085cdd20706d10f95ecff91db69;hb=HEAD#l147

BR
Christoph

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[Rohan Ahmed]

orc.b appear to be well-suited for the byte mask generation

Regards,
Rohan Ahmed

[Rohan Ahmed]

is there any way, we can emit orc.b instruction using scalar c code like others in sheet?

Thanks,
Rohan Ahmed

[Christoph Müllner]

Regarding the byte mask generation:
Where would orc.b be superior over existing approaches?
You still have to set a bit in each byte.

The two examples in your spreadsheet are attempts to synthesize an
orc.b in C code.
However, where would this code then be used?

As mentioned, GCC only emits orc.b via str*/mem* inline expansion (if
the requirements for that are fulfilled).
Other than that you can just use the __riscv_orc_b_* intrinsics.

BR
Christoph

[Tommy Murphy]

> Other than that you can just use the __riscv_orc_b_* intrinsics.

Or inline assembly code. 

https://gcc.gnu.org/onlinedocs/gcc/extensions-to-the-c-language-family/how-to-use-inline-assembly-language-in-c-code.html 

[Bruce Hoult]

I have already explained to you, at length, that there is no guarantee
that any particular instruction can be generated from pattern matching
plain C code in any given compiler. I have also given you the kind of
code that might be matched in some compiler, if any is.

https://new.reddit.com/r/RISCV/comments/syu3s3/comment/kzybuy0/?context=3

[Rohan Ahmed]

Thanks to all of you for giving your insights.

BR,
Rohan