mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-23 19:23:23 +01:00
52 lines
1.9 KiB
Plaintext
52 lines
1.9 KiB
Plaintext
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
|
|
|