Fix GCC 13+ optimization bug in BMW256 for LYRA2REv2 hashing

[melvincarvalho]

Summary

• Adds explicit build rules to compile LYRA2RE-related files with -O1 optimization
• Fixes GCC 13+ optimization bug that causes BMW256 hash corruption
• Resolves blockchain sync failures at Fork update to qt 4.8.6 #1 (block 450,947) and beyond

Problem

GCC 13 and later have optimization bugs affecting the BMW256 hash function used in the LYRA2REv2 hash chain. When compiled with -O2 (the default), the BMW256 function produces incorrect results, causing:

• All LYRA2REv2 block hashes to be computed incorrectly
• Proof-of-work verification failures for LYRA2REv2 blocks
• Node unable to sync past Fork update to qt 4.8.6 #1 (block 450,947)

Solution

Add explicit Makefile rules to compile the following files with -O1:

• bmw.c - BMW256 hash implementation (directly affected)
• Lyra2RE.c - LYRA2REv2 main implementation
• Lyra2.c - LYRA2 core functions
• Sponge.c - Sponge construction used by LYRA2

Testing

• Fresh sync from genesis to block 500,000+ successful
• All 22,199+ LYRA2REv2 blocks verified correctly
• Both Fork update to qt 4.8.6 #1 (450,947) and Fork Change the private key prefix to 128+pub #2 (511,111) passed
• Build tested with GCC 13.2.0 on Ubuntu 24.04

Fixes #141

[Copilot]

Pull request overview

This PR addresses a critical GCC 13+ compiler optimization bug that causes incorrect BMW256 hash calculations in the LYRA2REv2 algorithm, preventing blockchain synchronization past block 450,947.

Key changes:

• Adds custom Makefile rules to compile LYRA2RE-related files with -O1 optimization instead of the default -O2
• Targets four specific files affected by the compiler bug: bmw.c, Lyra2RE.c, Lyra2.c, and Sponge.c
• Includes documentation explaining the workaround and referencing issue #141

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[Copilot]

The custom compilation rules are missing the $(CFLAGS) variable that should be included in the compilation command. The standard automake rule for C files includes both $(AM_CFLAGS) and $(CFLAGS). Without $(CFLAGS), user-specified or configure-time CFLAGS will not be applied to these files.

The correct approach is to include $(CFLAGS) in the compilation command but place $(LYRA2RE_CFLAGS) after it to ensure the -O1 flag overrides any optimization level specified in CFLAGS. The compilation command should be:

$(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS) $(LYRA2RE_CFLAGS) -c -o $@ $<

This ensures that:

1. All standard flags are applied
2. The -O1 optimization override comes last and takes precedence
3. Other non-optimization CFLAGS are still applied to these files

Suggested change

	$(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) $(CPPFLAGS) $(LYRA2RE_CFLAGS) $(AM_CFLAGS) -c -o $@ $<
	Lyra2RE.o: Lyra2RE.c
	$(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) $(CPPFLAGS) $(LYRA2RE_CFLAGS) $(AM_CFLAGS) -c -o $@ $<
	Lyra2.o: Lyra2.c
	$(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) $(CPPFLAGS) $(LYRA2RE_CFLAGS) $(AM_CFLAGS) -c -o $@ $<
	Sponge.o: Sponge.c
	$(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) $(CPPFLAGS) $(LYRA2RE_CFLAGS) $(AM_CFLAGS) -c -o $@ $<
	$(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS) $(LYRA2RE_CFLAGS) -c -o $@ $<
	Lyra2RE.o: Lyra2RE.c
	$(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS) $(LYRA2RE_CFLAGS) -c -o $@ $<
	Lyra2.o: Lyra2.c
	$(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS) $(LYRA2RE_CFLAGS) -c -o $@ $<
	Sponge.o: Sponge.c
	$(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS) $(LYRA2RE_CFLAGS) -c -o $@ $<

[Copilot]

The comment states "to avoid undefined behavior" which is technically imprecise. This is a GCC optimizer bug that produces incorrect code, not undefined behavior in the C standard sense (which would indicate a problem with the source code). Consider rephrasing to be more accurate, such as:

"These files must be compiled with reduced optimization (-O1) to work around a GCC 13+ optimizer bug that produces incorrect code at higher optimization levels."

Suggested change

	# These files must be compiled with reduced optimization (-O1) to avoid undefined behavior.
	# These files must be compiled with reduced optimization (-O1) to work around a GCC 13+ optimizer bug that produces incorrect code at higher optimization levels.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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