marfrit
00d655187a
Three small functions extracted from the v1.19 conservative blob with
ground-truth C and per-tool (Ghidra / retdec / decomp.me) docs:
01_memset — byte memset, 28 B
02_memcpy32 — word-aligned memcpy, 36 B
03_magic_memset — magic check + tail-call to memset, 40 B
04_train_phy_block — first real poll-site function (104 B, 26 insts),
contains poll sites 12-15
Results in RESULTS.md:
- Ghidra: A on all four. Auto-decompile is close to final.
- retdec: A on #3, F on #1 and #2 (no register-arg inference on raw),
C on #4 (mistakes & 0xF0000000 for < 0x10000000).
GRIND_LOG.md (in 04_train_phy_block/) records the matching-decomp
iteration: 116-byte candidate.c at -Os vs vendor 104 bytes = 89.7%
size match on first real iteration. Remaining gap is GCC's choice of
`cmp w, w_const; b.ls` over vendor's `tst w, #imm; b.eq` for the
mask tests.
gdb_debug/ holds a native-aarch64 GDB single-stepper for the three
benchmark functions — boltzmann smoke test passed (memset:
buf[10] 0x00→0xab).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
59 lines
1.5 KiB
Markdown
59 lines
1.5 KiB
Markdown
# decomp.me recipe — 01_memset
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## Create a scratch
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Open https://decomp.me/ (or your self-hosted instance at
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http://192.168.88.64 if available). Click **New scratch**.
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- **Platform / Compiler:** `gcc 12.x aarch64-linux-gnu` (or whatever
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aarch64-gcc is offered — exact version doesn't matter much for this
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size).
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- **Compiler flags:** `-O2 -ffreestanding -nostdlib`
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- **Diff label:** `memset_byte`
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## Target asm
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Paste the following into the **"Target asm"** box:
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```asm
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memset_byte:
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mov x3, #0x0
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.Lloop:
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cmp x2, x3
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b.ne .Lbody
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ret
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.Lbody:
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strb w1, [x0, x3]
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add x3, x3, #0x1
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b .Lloop
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```
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## Context (headers/decls)
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```c
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#include <stddef.h>
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#include <stdint.h>
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```
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## Source
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Paste the ground-truth C from `reference.c` (or write your own first
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and iterate).
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## Expected workflow
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- First compile: scorer usually reports a high similarity (>= 80%) if
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the compiler picks the same `while (i != n)` pattern.
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- Fine-tune: try `i++` vs `i+=1`, try `while` vs `for`, try `uint8_t *`
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cast placement. Each yields a distinct register-allocation order the
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scorer rewards or punishes.
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- Perfect match possible if you hit the exact code shape GCC chose.
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## Benchmark notes
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- decomp.me's strength is the **compile-and-diff** feedback loop — every
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edit immediately shows the byte-delta against the target.
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- Weakness for this target: the real blob was likely built with a
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different compiler (ARMCC / Keil / vendor LLVM?). GCC may never match
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exactly even with perfect C. Similarity >= 90% is the realistic ceiling.
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