[llvm-dev] TableGen operand question

Bruce Hoult via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 9 05:21:40 PDT 2016

That's going to be a really challenging architecture to target from LLVM,
and you'll be fighting LLVM assumptions all the way, It would be a
challenge for an LLVM expert, let alone an LLVM beginner.

Of course, if you succeed then you will be an expert :-)

Converting functions to use statically-allocated memory instead of stack
memory would be a cool feature to have anywhere, and would also be almost
essential on other stack-unfriendly ISAs such as 6502.

To do it properly, you'd want to have functions that can never be part of
same call chain (i.e. never active at the same time) share memory
locations. I guess the way to do that (once you've worked out the frame
size for each function) would be to walk the complete program call tree and
give each function's frame the next addresses after its deepest caller.

And then there's recursion. If a function calls itself, or a function up
towards the root of the call tree, then you'll have to save and restore all
the function frames between to somewhere else.

Re-entrancy (e.g. things called from interrupt handlers) is even nastier.

On Thu, Jun 9, 2016 at 12:41 PM, Dawid Retkiewicz via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
> As a way to learn LLVM, I'm trying to write a backend for the Microchip
> PIC18 8-bit microcontroller.
> On this device, the hardware stack is very small and is used only for
> storing function return addresses.
> A "real" software stack implementation is not very efficient because of
> the limited instruction set, so for all the non-reentrant functions I've
> decided to use a similar solution to the one used by Microchip's own XC8
> compiler, which is to simulate the stack by assigning constant memory
> locations to every function.
> Local variables/parameters could then be accessed by using constant memory
> addresses. Since this microcontroller can perform most instruction directly
> on such addresses, this seems the most efficient way to handle the stack.
> The issue I'm facing right now is how to resolve the stack addresses in
> the LLVM backend.
> For instance, when calling a function, I'd like to store its parameters
> directly in the memory area assigned to this function.
> If I understand correctly, the exact stack size (and thus the memory
> needed by each function) cannot be determined before the epilogue/prologue
> insertion pass.This means that the target memory location is not known
> before this step.
> My idea is to create an operand that would hold an identifier of the
> target function and the memory offset, and replace this identifier with a
> real memory address in a pass after emit prologue/epilogue.
> However, I'm not sure how to achieve such thing in TableGen, and whether
> it's at all possible.
> I would be grateful for any tips/directions
> Thanks,
> Dawid Retkiewicz
> _______________________________________________
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> llvm-dev at lists.llvm.org
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