[clang] [llvm] [BPF] Support Stack Arguments (PR #189060)

[via cfe-commits]

4ast wrote:


> For store, we could do hit the first store, do adjustment 'r12 += ' then for the rest stores, there is no need for adjustment hit a call. In this case, do 'r12 -= '.

to know 'first store' JIT would need to do data flow analysis, which is not feasible.
 
> My current thinking is there is no need to do explicit 12 adjustments, e.g., to accommodate with code sequences like the above. Second, for each store for x86, the offset adjustment is done in jit time. So at runtime, we have no overhead at all.

right, no extra overhead is mandatory, but I'm still missing how you think JIT will do the adjustment.
Like compute the maximum of all *(r12 - N) = .. insn and use that in the prologue ?
I guess it works, but then this function will reserve more stack then it needs.
like
foo()
{ 
   bar(); // with deep callchain
   
   meh(lots of args);
}
the space needed to call meh() will be reserved upfront, so unnecessary stack waste while calling bar() ?
For JIT to reserve it right before meh() it would need to know 'first store' and then consider all possible combinations of calling meh(). gets tricky.

https://github.com/llvm/llvm-project/pull/189060