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

[Philip Reames via llvm-dev]

The option we use is to have a custom memory manager, override the 
getPointerToNamedFunction function, and provide the pointer to the 
external function at link time.  The inttoptr scheme works fairly well, 
but it does make for some pretty ugly and sometimes hard to analyze IR.  
I recommend leaving everything symbolic until link time if you can.

Philip

On 03/28/2016 06:33 PM, Russell Wallace via llvm-dev wrote:
> 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 <mailto:Chuck.Caldarale at unisys.com>> wrote:
>
>     > From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org
>     <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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