Add notes on instruction selection pass

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/docs/HistoricalNotes/2001-07-08-InstructionSelection.txt b/docs/HistoricalNotes/2001-07-08-InstructionSelection.txt
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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
+

diff --git a/docs/HistoricalNotes/2001-07-08-InstructionSelection2.txt b/docs/HistoricalNotes/2001-07-08-InstructionSelection2.txt
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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
+