[llvm-dev] JIT compiler and calls to existing functions

[Russell Wallace via llvm-dev]

That seems to work, thanks! The specific code I ended up with to call
int64_t print(int64_t) looks like:

        auto f = builder.CreateIntToPtr(
            ConstantInt::get(builder.getInt64Ty(), uintptr_t(print)),
            PointerType::getUnqual(FunctionType::get(
                builder.getInt64Ty(), {builder.getInt64Ty()}, false)));
        return builder.CreateCall(f, args);


On Mon, Mar 28, 2016 at 1:40 PM, Caldarale, Charles R <
Chuck.Caldarale at unisys.com> wrote:

> > From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org]
> > On Behalf Of Russell Wallace via llvm-dev
> > Subject: [llvm-dev] JIT compiler and calls to existing functions
>
> > In the context of a JIT compiler, what's the recommended way to generate
> a call to an
> > existing function, that is, not one that you are generating on-the-fly
> with LLVM, but
> > one that's already linked into your program? For example the cosine
> function (from the
> > standard math library); the Kaleidoscope tutorial recommends looking it
> up by name with
> > dlsym("cos"), but it seems to me that it should be possible to use a
> more efficient and
> > portable solution that takes advantage of the fact that you already have
> an actual pointer
> > to cos, even if you haven't linked with debugging symbols.
>
> Perhaps not the most elegant, but we simply use the
> IRBuilder.CreateIntToPtr() method to construct the Callee argument for
> IRBuilder.CreateCall().  The first argument for CreateIntToPtr() comes from
> ConstantInt::get(I64, uintptr_t(ptr)), while the second is a function type
> pointer defined by using PointerType::get() on the result of
> FunctionType::get() with the appropriate function signature.
>
>  - Chuck
>
>
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