Generate decoder from ISA descriptor#725
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Introduce gen-decoder.py that reads src/instructions.in and generates src/decode.c, replacing the hand-written decoder with an auto-generated one. - src/instructions.in: ISA descriptor covering 161 instructions (RV32I/M/A/F/C, Zifencei, Zicsr, Zba/Zbb/Zbc/Zbs) - scripts/gen-decoder.py: decision-tree builder and C code generator with full RVC operand decoding (register mapping + immediate bit-extraction) - scripts/verify-tree.py: validates all 161 instructions are reachable in the generated decision tree - Makefile: auto-regenerate decode.c when inputs change - docs/decoder-generator.md: describes the descriptor format and how to add new instructions/extensions Close sysprog21#103
jserv
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Apr 9, 2026
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Benchmarks
Details
| Benchmark suite | Current: 8ef6e5d | Previous: 2138efe | Ratio |
|---|---|---|---|
Dhrystone |
1521.667 DMIPS |
1558.667 DMIPS |
1.02 |
CoreMark |
1103.066 iterations/sec |
1103.567 iterations/sec |
1.00 |
This comment was automatically generated by workflow using github-action-benchmark.
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Summary
Introduce
scripts/gen-decoder.pyto automatically generatesrc/decode.cfromsrc/instructions.in, replacing the hand-writtendecoder.
Changes
src/instructions.in: ISA descriptor for 161 instructionsscripts/gen-decoder.py: decision-tree builder and C code generatorscripts/verify-tree.py: validates all instructions are reachableMakefile: auto-regenerate decode.c when inputs changedocs/decoder-generator.md: format documentationTest Results
RISCV_DEVICE=IMACFZicsrZifencei: all tests passed
(I, M, A, F, C, Zifencei, privilege extensions verified)
Close #103
Summary by cubic
Replaced the hand-written instruction decoder with an auto-generated one built from an ISA descriptor. This makes adding instructions simple and keeps decoding consistent and verifiable across 161 ops.
New Features
scripts/gen-decoder.pybuilds a decision tree and emitssrc/decode.c(includes full RVC operand decoding and extension guards).scripts/verify-tree.pyvalidates reachability for every instruction (current descriptor: 161/161).src/instructions.indefines the ISA;docs/decoder-generator.mddocuments the format and how to extend it.src/decode.cand runsclang-format.Migration
src/decode.cstays committed for normal builds.src/instructions.inand runmake. For new RVC immediates, add a decoder in_RVC_OPERAND_DECODERSinscripts/gen-decoder.py.Written for commit 8ef6e5d. Summary will update on new commits.