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

[Russell Wallace via llvm-dev]

Right, but when you say link time, the JIT compiler I'm writing works the
way openJDK or v8 do, it reads a script, JIT compiles it into memory and
runs the code in memory without ever writing anything to disk (an option
for ahead of time compilation may come later, but that will be a while down
the road), so we might be doing different things?

On Tue, Mar 29, 2016 at 2:59 AM, Philip Reames <listmail at philipreames.com>
wrote:

> 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>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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>
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