GCC 13 optimization bug causes LYRA2REv2 hash failures - fix identified

[melvincarvalho]

Summary

The master branch fails to sync past Fork #1 (block 450,947) when compiled with GCC 13 due to an optimization bug affecting the BMW256 component of the LYRA2REv2 hash chain.

Root Cause

GCC 13's -O2 optimization introduces undefined behavior in sph_bmw.c, causing the BMW256 hash function to produce incorrect results. This affects all LYRA2REv2 blocks (both standalone and auxpow merge-mined blocks).

The LYRA2REv2 hash chain is:

Blake256 → Keccak256 → CubeHash256 → LYRA2 → Skein256 → CubeHash256 → BMW256

The BMW256 step produces incorrect output when compiled with -O2, causing valid blocks to fail PoW verification.

Key Finding: Same-Chain Merge Mining

Investigation revealed that LYRA2REv2 auxpow blocks use same-chain merge mining (LYRA2REv2 → LYRA2REv2), not cross-chain merge mining to Litecoin (LYRA2REv2 → SCRYPT). This means the BMW256 fix applies to both standalone and auxpow LYRA2REv2 blocks.

The Fix

Rebuild the hash-related object files with -O0 (no optimization):

# In src/ directory, rebuild affected files with -O0
gcc -c -O0 sph_bmw.c -o sph_bmw.o
gcc -c -O0 Lyra2RE.c -o Lyra2RE.o
gcc -c -O0 Lyra2.c -o Lyra2.o
gcc -c -O0 Sponge.c -o Sponge.o

# Then relink bitmarkd
make bitmarkd

Or modify the Makefile to compile these files with -O0 or -O1 instead of -O2.

Verification Results

After applying the fix, a fresh sync was performed:

Metric	Result
Blocks Synced	582,944+
Fork #1 (450,947)	✅ PASSED
Fork #2 (511,111)	✅ PASSED
LYRA2REv2 Blocks Verified	22,199+
PoW Errors	0

Environment

• OS: Ubuntu 24.04
• GCC: 13.3.0
• Affected Optimization Level: -O2
• Safe Optimization Levels: -O0, -O1

Recommended Permanent Fix

Update the Makefile to explicitly compile sph_bmw.c (and ideally all LYRA2RE-related C files) with -O1 or -O0:

# Add specific rule for BMW256 to avoid GCC optimization bug
sph_bmw.o: sph_bmw.c
	$(CC) -c -O1 $(CFLAGS_NO_O2) $< -o $@

Related

This issue supersedes the investigation in issue #71 with a confirmed root cause and fix.