Segmented Stacks in LLVM

The runtime functionality is already there in libgcc.

Implementation Details

The size of function arguments on the stack needs to be passed to__morestack (this function is implemented in libgcc) since that numberof bytes has to be copied from the previous stacklet to the current one. This isso that SP (and FP) relative addressing of function arguments work as expected.

Variable Sized Allocas

The section on automatically assumes that every stackframe will be of fixed size. However, LLVM allows the use of the llvm.allocaintrinsic to allocate dynamically sized blocks of memory on the stack. Whenfaced with such a variable-sized alloca, code is generated to:

• Check if the current stacklet has enough space. If yes, just bump the SP, likein the normal case.