Calling Conventions

This note describes the different calling conventions used by the compiler to transfer control between fragments (extended basic blocks). For the MLRISC-based code generator, the relevant code is in the files CodeGen/cpscompile/argPassing.{sig,sml} and CodeGen/main/mlrisc-gen-fn.sml.

Registers

The calling conventions pass all arguments in registers (note that on some targets, some registers are actually allocated in memory). There are four special registers used in the calling conventions:

1. STD_LINK
2. STD_CLOS
3. STD_ARG
4. STD_CONT

In addition, there are a target-specific number of miscellaneous registers (i.e., MISC1, ..., MISCn) and a set of available floating-point registers. We define the general-purpose argument registers to be the list STD_ARG, MISC1, , ..., MISCn.

The compiler defines a number of registers used to hold callee-save values. We use NCS to designate the number of general-purpose callee-save registers and NFCS to designate the number of floating-point callee-save registers. Currently the compiler uses NCS = 3 and NFCS = 0. The callee-save registers are assigned to the first few MISC registers, so typically MISC1, MISC2, and MISC3 are callee saves.

Standard function call

This convention is used to transfer control to an unkonwn or escaping function. Such a call must have at least NCS + NFCS + 3 arguments. The first three arguments are

1. STD_LINK
2. STD_CLOS
3. STD_CONT

Then come the callee-save and floating-point callee-save registers

The remaining registers are assigned to argument registers by type. Pointer and integer arguments are mapped to the general-purpose argument registers and floating-point arguments are mapped to the available floating-point registers.

Standard continuation call

This convention is used to transfer control to an unkonwn or escaping continuation. The convention depends on whether NCS is zero or not, but we first assume that it is at least one. In this case, the call must have at least NCS + NFCS + 1 arguments. The first argument is mapped to the STD_CONT register, then come the callee saves followed by the floating-point callee saves. Any remaining arguments are mapped by type to either the general-purpose argument registers or the available floating-point registers.

Call of a known function

When all of the calls to a function f occur from known call sites (and those call sites only call f), then an internal calling convention is used. Essentially, such a call is a goto with arguments. Arguments are assigned by type to MLRISC pseudo registers such that the same registers are used at the call sites and the function entry.

A call to a known function may also involve a heap limit check. For some targets (Alpha, PowerPC, and Sparc), this limit check is done at the call site and the result of the check is placed in the reserved EXHAUSTED condition-code register. It is unclear what the benefit of splitting the test across the call is.