[PATCH 0/1] RISC-V: Use faster hash table on disassembling

[Tsukasa OI]

This patch is postponed because I will merge this into a larger patchset
with general core disassembler improvements.

Thanks,
Tsukasa

On 2022/07/09 12:49, Tsukasa OI wrote:
> Hello,
> 
> This patchset intends to improve performance on disassembling RISC-V
> code (which may possibly contain invalid data).  It replaces riscv_hash
> (on opcodes/riscv-dis.c) with much faster data structure: sorted and
> partitioned hash table.
> 
> Tracker on GitHub:
> <https://github.com/a4lg/binutils-gdb/wiki/riscv_dis_opts_hashtable>
> 
>     Sidenote:
>     I started listing my Binutils submissions on my GitHub Wiki:
>     <https://github.com/a4lg/binutils-gdb/wiki/Patch-Queue>
>     hoping that current status and conflicting patches are clear.
> 
> ***WARNING***
> 
> This patchset conflicts with following patchset(s):
> -   <https://sourceware.org/pipermail/binutils/2022-June/121441.html>
>     (Tracker: <https://github.com/a4lg/binutils-gdb/wiki/riscv_dis_generics>)
> If either of them is merged, I will submit rebased patchset.
> 
> 
> 
> This is a technique actually used on SPARC architecture
> (opcodes/sparc-dis.c) and I simplified the algorithm even further.
> Unlike SPARC, RISC-V hashed opcode table is not a table to linked lists,
> it's just a table, pointing to "start" elements of the sorted opcode
> list (sorted by hash code) plus global tail.
> 
> I benchmarked some of the programs and I measure somewhat between 2%
> to 10% performance increase while disassembling code section of RISC-V
> ELF files (objdump -d $FILE).  That is not significant but not bad for
> such a small modification (with ~ 11KB heap memory allocation on 64-bit
> environment).
> 
> This is not the end.  This structure significantly improves plain binary
> file handling (on objdump, objdump -b binary -m riscv:rv[32|64] -D
> $FILE).  I tested on a big vmlinux image with debug symbols and I got
> over 50% performance boost.  This is due to the fact that, disassembling
> about one quarter of invalid "instruction" words required iterating over
> one thousand opcode entries (>= 348 being vector instructions with OP-V,
> that can be easily skipped with this new data structure).
> 
> Thanks,
> Tsukasa
> 
> 
> 
> 
> Tsukasa OI (1):
>   RISC-V: Use faster hash table on disassembling
> 
>  opcodes/riscv-dis.c | 214 ++++++++++++++++++++++++++++----------------
>  1 file changed, 136 insertions(+), 78 deletions(-)
> 
> 
> base-commit: d2acd4b0c5bab349aaa152d60268bc144634a844