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Add notes on instruction selection pass

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2001-07-15 00:23:59 +00:00
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docs/HistoricalNotes/2001-07-08-InstructionSelection.txt Normal file
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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

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Date: Sun, 8 Jul 2001 10:02:20 -0500
From: Vikram S. Adve <vadve@cs.uiuc.edu>
To: vadve@cs.uiuc.edu, Ruchira Sasanka <sasanka@students.uiuc.edu>
Cc: Chris Lattner <lattner@cs.uiuc.edu>
Subject: RE: machine instruction operands
I got interrupted and forgot to explain the example. In that case:
reg will be the 3rd operand of MUL and it will be of type
MO_MInstrVirtualReg. The field MachineInstr* minstr will point to the
instruction that computes reg.
numElements will be an immediate constant, not a register.
%sp will be operand 1 of ADD and it will be of type MO_MachineReg. The
field regNum identifies the register.
numElements will be operand 2 of ADD and it will be of type
MO_VMVirtualReg. The field Value* value identifies the value.
ptr will be operand 3 of ADD will also be %sp, i.e., of
type MO_MachineReg. regNum identifies the register.
--Vikram