[LLVMdev] ocaml build system

[Gordon Henriksen]

On 2009-01-19, at 13:50, Nick Lewycky wrote:

> I have an issue with the OCaml build system and the ExecutionEngine.
>
> PR2128 has a patch to change the Interpreter to use libffi. This  
> breaks test/Bindings/Ocaml/executionengine.ml because OCaml doesn't  
> try to link with libffi, even though llvm-config knows that we should:
>
>  $ Debug/bin/llvm-config --ldflags interpreter
>  -L/home/nicholas/llvm-commit/Debug/lib  -lpthread -lffi -ldl -lm - 
> lelf
>
> If I run 'ocamlc' by hand and pass it '-cclib -lffi', the test  
> succeeds, so this is the only remaining problem. Is anyone familiar  
> with how Ocaml knows what libraries to link against? We can't just  
> pass `llvm-config --ldflags interpreter` to ocamlc because it  
> doesn't support the -L argument.
>
> Does anyone have any ideas how this ought to work?


The linker flags for each .cma (ocaml library) are baked in by ocaml.  
Here's the relevant section from Makefile.ocaml:

# Info from llvm-config and similar
ifdef UsedComponents
UsedLibs = $(shell $(LLVM_CONFIG) --libs $(UsedComponents))
UsedLibNames = $(shell $(LLVM_CONFIG) --libnames $(UsedComponents))
endif

# Tools
OCAMLCFLAGS += -I $(OcamlDir) -I $(ObjDir)
ifneq ($(ObjectsO),)
OCAMLAFLAGS += $(patsubst %,-cclib %, \
                  $(filter-out -L$(LibDir),-l$(LIBRARYNAME) \
                                           $(shell $(LLVM_CONFIG) -- 
ldflags)) \
                                           $(UsedLibs))
else
OCAMLAFLAGS += $(patsubst %,-cclib %, \
                  $(filter-out -L$(LibDir),$(shell $(LLVM_CONFIG) -- 
ldflags)) \
                                           $(UsedLibs))
endif

This transforms "x y z" from llvm-config into "-cclib x -cclib y - 
cclib z", which instructs ocaml to pass "x y z" down to the linker  
when creating an executable. The UsedComponents variable originates in  
this case from bindings/ocaml/executionengine/Makefile:

LEVEL := ../../..
LIBRARYNAME := llvm_executionengine
DONT_BUILD_RELINKED := 1
UsedComponents := executionengine jit interpreter native
UsedOcamlInterfaces := llvm llvm_target

include ../Makefile.ocaml

I don't see anything obviously wrong here; it seems that llvm-config - 
ldflags executionengine jit interpreter native should be invoked,  
which should give you the -cclib -lffi you seek. make VERBOSE=1 and/or  
TOO_VERBOSE=1 should show you the full ocaml command lines; maybe  
that'll show where -lffi is getting dropped on the floor.

— Gordon

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