[cfe-dev] Nested functions in Clang?

Rodney M. Bates via cfe-dev cfe-dev at lists.llvm.org
Sun Sep 13 14:03:11 PDT 2015

I want to avoid trampolines for two reasons.  They have to be built in memory that is
both writable and executable, which has already been a problem on some targets.  Also,
a debugger needs to both construct and interpret them, which creates difficult target
dependencies and compiler code-generation dependencies.

I intend to pass some kind of environment pointer, like a static link.  The real question
is, what does it point to?  As I understand, llvm locals can not be used outside the
function they are created in, so using a static link from outside can't access the
alloca values.  I am thinking the best way is to wrap at least up-level referenceable
formals and locals  in a struct, and have the static link point to that.  This would
include the next outer static link.

On 09/13/2015 04:39 AM, David Chisnall wrote:
> On 13 Sep 2015, at 01:18, Rodney M. Bates via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>> Does Clang have an option that allows C nested functions, as in gcc?
>> So far, I can't find it.  I just want to look at generated llvm IR, for
>> ideas on the best way to produce it from another front end.
> Clang doesn’t support it, but llvm-gcc did (and DragonEgg, possibly?), and this is what the trampoline intrinsics are for.  The way that GCC implements it is very dangerous - it requires having an executable stack, which makes various attacks significantly easier.
> The nested function approach is *only* required if you need the ability for unmodified C code (or code using a C FFI) to be able to call the generated functions.  If you do not have this requirement, then using an explicit argument in the LLVM IR to contain the state for the closure (as [Objective-]C blocks and C++ lambdas do) is probably a better option.
> David

Rodney Bates
rodney.m.bates at acm.org

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