docs/HistoricalNotes/2001-07-08-InstructionSelection.txt

Date: Sun, 8 Jul 2001 09:37:22 -0500
From: Vikram S. Adve <vadve@cs.uiuc.edu>
To: Ruchira Sasanka <sasanka@students.uiuc.edu>
Cc: Chris Lattner <lattner@cs.uiuc.edu>
Subject: machine instruction operands

Ruchira,

When generating machine instructions, I have to make several choices about
operands.  For cases were a register is required, there are 3 cases:

1. The register is for a Value* that is already in the VM code.

2. The register is for a value that is not in the VM code, usually because 2
machine instructions get generated for a single VM instruction (and the
register holds the result of the first m/c instruction and is used by the
second m/c instruction).

3. The register is a pre-determined machine register.

E.g, for this VM instruction:
        ptr = alloca type, numElements
I have to generate 2 machine instructions:
        reg = mul constant, numElements
        ptr = add %sp, reg

Each machine instruction is of class MachineInstr.
It has a vector of operands.  All register operands have type MO_REGISTER.
The 3 types of register operands are marked using this enum:

 enum VirtualRegisterType {
    MO_VMVirtualReg,            // virtual register for *value
    MO_MInstrVirtualReg,        // virtual register for result of *minstr
    MO_MachineReg               // pre-assigned machine register `regNum'
  } vregType;

Here's how this affects register allocation:

1. MO_VMVirtualReg is the standard case: you just do the register
allocation.

2. MO_MInstrVirtualReg is the case where there is a hidden register being
used.  You should decide how you want to handle it, e.g., do you want do
create a Value object during the preprocessing phase to make the value
explicit (like for address register for the RETURN instruction).

3. For case MO_MachineReg, you don't need to do anything, at least for
SPARC. The only machine regs I am using so far are %g0 and %sp.

--Vikram