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[Docs][CodeGenerator] Don't specify the number of operands in BuildMI 

Patch by Visoiu Mistrih Francis.

Differential Revision: http://reviews.llvm.org/D21819

llvm-svn: 274128
This commit is contained in:
Krzysztof Parzyszek 2016-06-29 14:14:59 +00:00
parent f9c5908ffd
commit 3da1078a9b
2 changed files with 13 additions and 16 deletions
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25
llvm/docs/CodeGenerator.rst
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@ -386,32 +386,27 @@ functions make it easy to build arbitrary machine instructions. Usage of the
.. code-block:: c++
// Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42')
// instruction. The '1' specifies how many operands will be added.
MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42);
// Create the same instr, but insert it at the end of a basic block.
// instruction and insert it at the end of the given MachineBasicBlock.
const TargetInstrInfo &TII = ...
MachineBasicBlock &MBB = ...
BuildMI(MBB, X86::MOV32ri, 1, DestReg).addImm(42);
DebugLoc DL;
MachineInstr *MI = BuildMI(MBB, DL, TII.get(X86::MOV32ri), DestReg).addImm(42);
// Create the same instr, but insert it before a specified iterator point.
MachineBasicBlock::iterator MBBI = ...
BuildMI(MBB, MBBI, X86::MOV32ri, 1, DestReg).addImm(42);
BuildMI(MBB, MBBI, DL, TII.get(X86::MOV32ri), DestReg).addImm(42);
// Create a 'cmp Reg, 0' instruction, no destination reg.
MI = BuildMI(X86::CMP32ri, 2).addReg(Reg).addImm(0);
MI = BuildMI(MBB, DL, TII.get(X86::CMP32ri8)).addReg(Reg).addImm(42);
// Create an 'sahf' instruction which takes no operands and stores nothing.
MI = BuildMI(X86::SAHF, 0);
MI = BuildMI(MBB, DL, TII.get(X86::SAHF));
// Create a self looping branch instruction.
BuildMI(MBB, X86::JNE, 1).addMBB(&MBB);
BuildMI(MBB, DL, TII.get(X86::JNE)).addMBB(&MBB);
The key thing to remember with the ``BuildMI`` functions is that you have to
specify the number of operands that the machine instruction will take. This
allows for efficient memory allocation. You also need to specify if operands
default to be uses of values, not definitions. If you need to add a definition
operand (other than the optional destination register), you must explicitly mark
it as such:
If you need to add a definition operand (other than the optional destination
register), you must explicitly mark it as such:
.. code-block:: c++

4
llvm/include/llvm/CodeGen/MachineInstrBuilder.h
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@ -10,7 +10,9 @@
// This file exposes a function named BuildMI, which is useful for dramatically
// simplifying how MachineInstr's are created. It allows use of code like this:
//
// M = BuildMI(X86::ADDrr8, 2).addReg(argVal1).addReg(argVal2);
// M = BuildMI(MBB, MI, DL, TII.get(X86::ADD8rr), Dst)
// .addReg(argVal1)
// .addReg(argVal2);
//
//===----------------------------------------------------------------------===//