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Alejandro Cabeza Romero 2026-05-18 18:54:24 +02:00
parent e5a31cb8bc
commit 73a6ea5c9c
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2 changed files with 28 additions and 33 deletions
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23
.github/resources/witness-generator/Makefile vendored
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@ -52,15 +52,10 @@ windows: $(BIN)
windows-lib: CXXFLAGS=$(CXXFLAGS_COMMON) -fPIC -I/include -Duint="unsigned int"
windows-lib: $(LIB)
# Localizes circuit-specific symbols so multiple circuit libraries can coexist in the
# same binary without symbol conflicts. See CONTRIBUTING.md § "Symbol Isolation".
# Override with LOCALIZE_SYMS= to skip localization.
MERGE_OBJS := $(PROJECT).o $(PROJECT)/ffi.o circom_adapter.o
LOCALIZE_SYMS ?= loadCircuit writeBinWitness \
get_size_of_witness get_size_of_constants get_size_of_input_hashmap \
get_main_input_signal_no get_main_input_signal_start get_total_signal_no \
get_number_of_components get_size_of_io_map get_size_of_bus_field_map
LOCAL_OBJ := $(PROJECT)_local.o
# Localizes all circuit-specific code to prevent conflicts when multiple circuit
# libraries are linked into the same binary. See CONTRIBUTING.md § "Symbol Isolation".
PUBLIC_SYMS := $(PROJECT)_generate_witness $(PROJECT)_generate_witness_from_files
LOCAL_OBJ := $(PROJECT)_local.o
UNAME := $(shell uname -s)
@ -69,14 +64,12 @@ $(BIN): $(COMMON_OBJS)
$(CXX) $(LDFLAGS) $^ $(LDLIBS) -o $@
$(LIB): $(LIB_OBJS)
ifeq ($(strip $(LOCALIZE_SYMS)),)
ar rcs $@ $^ # Localization disabled
else ifeq ($(UNAME),Darwin)
ifeq ($(UNAME),Darwin)
ar rcs $@ $^ # On macOS two-level namespace, conflicts don't arise
else
ld -r -o $(LOCAL_OBJ) $(MERGE_OBJS)
objcopy $(foreach s,$(LOCALIZE_SYMS),--localize-symbol=$(s)) $(LOCAL_OBJ)
ar rcs $@ $(filter-out $(MERGE_OBJS),$^) $(LOCAL_OBJ)
ld -r -o $(LOCAL_OBJ) $(filter-out fr.o,$^)
llvm-objcopy $(foreach s,$(PUBLIC_SYMS),--keep-global-symbol=$(s)) $(LOCAL_OBJ)
ar rcs $@ fr.o $(LOCAL_OBJ)
rm $(LOCAL_OBJ)
endif

38
CONTRIBUTING.md
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@ -57,32 +57,34 @@ page, which results in a **SIGSEGV**.
### The Fix
The Makefile's `$(LIB)` rule uses a two-step process on Linux and Windows to localize all internal symbols before
The Makefile's `$(LIB)` rule uses a two-step process on Linux and Windows to localize all circuit-specific code before
archiving:
1. **Partial link** (`ld -r`): merges all `.o` files into a single relocatable object without producing a final
executable.
No symbols are resolved yet; this is consolidation only.
2. **Symbol localization** (`objcopy --keep-global-symbol`): demotes every global symbol to local *except* the circuit's
two public FFI entry points.
Local symbols are invisible to the final linker, so each archive retains a private copy of every internal symbol. This
means no conflict is possible regardless of how many circuits are linked together.
1. **Partial link** (`ld -r`): merges all circuit-specific `.o` files — everything except `fr.o` (pure field
arithmetic, no circuit-specific calls) — into a single relocatable object. No symbols are resolved yet; this is
consolidation only.
2. **Symbol localization** (`llvm-objcopy --keep-global-symbol`): demotes every global symbol to local *except* the
circuit's two public FFI entry points (`$(PROJECT)_generate_witness` and `$(PROJECT)_generate_witness_from_files`).
Local symbols are invisible to the final linker, so each archive retains a private copy of every internal symbol — no
conflict is possible regardless of how many circuits are linked together.
The public symbols are derived automatically from `PROJECT`: a circuit built with `PROJECT=poq` keeps
`poq_generate_witness` and `poq_generate_witness_from_files` global and localizes everything else.
`llvm-objcopy` is required rather than GNU `objcopy`. GNU `objcopy` only changes the binding of COMDAT signature
symbols to local, which confuses the linker's deduplication logic and causes "relocation refers to symbol in discarded
section" errors. `llvm-objcopy` additionally clears the `GRP_COMDAT` flag on affected section groups, turning them into
regular non-COMDAT sections that are simply kept as-is rather than deduplicated. The result is a slightly larger binary
(each circuit keeps its own copy of shared template instantiations), but no linker errors.
> To skip localization for a specific build (e.g. for debugging), pass `PUBLIC_SYMBOLS=` explicitly on the `make` command
> line.
`fr.o` is excluded from the merge because it contains only field arithmetic (`Fr_*`) with no circuit-specific calls.
It is safe to deduplicate across circuits — the linker picks one copy, which is correct since the code is identical.
On macOS, localization is skipped because `objcopy` is a GNU Binutils tool unavailable by default there.
This is safe: macOS uses a two-level namespace by default, meaning symbols are qualified by which library they come
from, so the conflict does not arise.
On macOS, localization is skipped. macOS uses a two-level namespace by default, meaning symbols are qualified by which
library they come from, so the conflict does not arise.
### Maintenance
`PUBLIC_SYMBOLS` defaults to `$(PROJECT)_generate_witness` and `$(PROJECT)_generate_witness_from_files`.
If the public FFI API ever changes, meaning the entrypoints are renamed or new ones added, the Makefile default must be
updated, otherwise the affected symbols will be localized and linking will fail.
`PUBLIC_SYMS` is hardcoded to `$(PROJECT)_generate_witness` and `$(PROJECT)_generate_witness_from_files` in the
Makefile. If the public FFI API ever changes — entry points renamed or new ones added — update that variable,
otherwise the affected symbols will be localized and linking will fail.
---