[llvm-dev] conceiving of a frontend feature which requires LLVM support: builtin to determine stack size upper bound

[Hal Finkel via llvm-dev]

On 10/21/2017 12:43 PM, Andrew Kelley via llvm-dev wrote:
> Here is a feature I would like to introduce to my frontend:
>
> A builtin function to calculate the upper bound of the stack size of 
> any given function.
>
> The idea here is that, if we do not have any:
>  * external library calls
>  * alloca
>  * indirect function calls
>  * non-tail-recursion
>  * inline assembly that modifies the stack pointer
>
> Then, we can look at a given function and determine precisely how much 
> stack space is needed to spawn a thread to run that function. This 
> provides two benefits:
>  * Statically guarantee that stack overflow will not occur
>  * Threads can have smaller stack sizes, lowering the cost of creating 
> a thread
>
> This feature requires tight coupling with LLVM, because:
>  * We need to know the stack size post-optimizations
>  * Frontend needs to make a compile error if non-tail-recursion 
> occurs, but LLVM might introduce (or remove?) tail recursion, and so 
> we would need to capture this error from LLVM and report it back in 
> the frontend.
>
>
> What do you think? Is this feature reasonable to achieve with LLVM and 
> in scope of the LLVM project?
>
> I would propose an LLVM builtin function to perform this calculation, 
> and I can work on a proof-of-concept if llvm-dev thinks this idea is 
> worth pursuing.

Would a builtin function be the best way to expose this information? 
Maybe generating a side table would be best? As I recall, we already 
produce some stack-size information as part of our StackMap table (see 
https://llvm.org/docs/StackMaps.html). Another option could be some 
special form of prefix/prologue data 
(http://llvm.org/docs/LangRef.html#prefix-data / 
http://llvm.org/docs/LangRef.html#prologue-data).

The "@stack_size(function)" you've discussed on the zig issue may not be 
implementable in general (because we'd need some way of making sure that 
we always generate code for "@foo" before any other function containing 
a call to "@stack_size(@foo)", and that's probably not possible in general).

  -Hal

>
>
> Upstream issue: https://github.com/zig-lang/zig/issues/157
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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