The LLVM target-independent code generator is a framework that provides a suite of reusable components for translating the LLVM internal representation @@ -188,14 +198,12 @@ depend on the target-description and machine code representation classes, ensuring that it is portable.
-The two pieces of the LLVM code generator are the high-level interface to the code generator and the set of reusable components that can be used to build @@ -223,11 +231,11 @@
The LLVM target-independent code generator is designed to support efficient and quality code generation for standard register-based microprocessors. @@ -297,11 +305,11 @@
The target description classes require a detailed description of the target architecture. These target descriptions often have a large amount of common @@ -324,13 +332,15 @@
The LLVM target description classes (located in the include/llvm/Target directory) provide an abstract description of @@ -346,14 +356,12 @@ TargetMachine class provides accessors that should be implemented by the target.
-The TargetMachine class provides virtual methods that are used to access the target-specific implementations of the various target description @@ -369,11 +377,11 @@
The TargetData class is the only required target description class, and it is the only class that is not extensible (you cannot derived a new @@ -385,11 +393,11 @@
The TargetLowering class is used by SelectionDAG based instruction selectors primarily to describe how LLVM code should be lowered to @@ -411,11 +419,11 @@
The TargetRegisterInfo class is used to describe the register file of the target and any interactions between the registers.
@@ -445,11 +453,11 @@The TargetInstrInfo class is used to describe the machine instructions supported by the target. It is essentially an array of @@ -463,11 +471,11 @@
The TargetFrameInfo class is used to provide information about the stack frame layout of the target. It holds the direction of stack growth, the @@ -479,11 +487,11 @@
The TargetSubtarget class is used to provide information about the specific chip set being targeted. A sub-target informs code generation of @@ -495,11 +503,11 @@ -
+ -The TargetJITInfo class exposes an abstract interface used by the Just-In-Time code generator to perform target-specific activities, such as @@ -509,13 +517,15 @@
At the high-level, LLVM code is translated to a machine specific representation formed out of @@ -528,14 +538,12 @@ SSA representation for machine code, as well as a register allocated, non-SSA form.
-Target machine instructions are represented as instances of the MachineInstr class. This class is an extremely abstract way of @@ -576,14 +584,12 @@
Also if the first operand is a def, it is easier to create instructions whose only def is the first operand.
-Machine instructions are created by using the BuildMI functions, located in the include/llvm/CodeGen/MachineInstrBuilder.h file. The @@ -630,11 +636,11 @@ MI.addReg(Reg, RegState::Define);
One important issue that the code generator needs to be aware of is the presence of fixed registers. In particular, there are often places in the @@ -702,11 +708,26 @@ ret
Some machine instructions, like calls, clobber a large number of physical
+ registers. Rather than adding <def,dead> operands for
+ all of them, it is possible to use an MO_RegisterMask operand
+ instead. The register mask operand holds a bit mask of preserved registers,
+ and everything else is considered to be clobbered by the instruction.
MachineInstr's are initially selected in SSA-form, and are maintained in SSA-form until register allocation happens. For the most part, @@ -719,12 +740,14 @@ ret
The MachineBasicBlock class contains a list of machine instructions (MachineInstr instances). It roughly @@ -737,11 +760,11 @@ ret
The MachineFunction class contains a list of machine basic blocks (MachineBasicBlock instances). It @@ -754,14 +777,99 @@ ret
LLVM code generator can model sequences of instructions as MachineInstr + bundles. A MI bundle can model a VLIW group / pack which contains an + arbitrary number of parallel instructions. It can also be used to model + a sequential list of instructions (potentially with data dependencies) that + cannot be legally separated (e.g. ARM Thumb2 IT blocks).
+ +Conceptually a MI bundle is a MI with a number of other MIs nested within: +
+ ++-------------- +| Bundle | --------- +-------------- \ + | ---------------- + | | MI | + | ---------------- + | | + | ---------------- + | | MI | + | ---------------- + | | + | ---------------- + | | MI | + | ---------------- + | +-------------- +| Bundle | -------- +-------------- \ + | ---------------- + | | MI | + | ---------------- + | | + | ---------------- + | | MI | + | ---------------- + | | + | ... + | +-------------- +| Bundle | -------- +-------------- \ + | + ... ++
MI bundle support does not change the physical representations of + MachineBasicBlock and MachineInstr. All the MIs (including top level and + nested ones) are stored as sequential list of MIs. The "bundled" MIs are + marked with the 'InsideBundle' flag. A top level MI with the special BUNDLE + opcode is used to represent the start of a bundle. It's legal to mix BUNDLE + MIs with indiviual MIs that are not inside bundles nor represent bundles. +
+ +MachineInstr passes should operate on a MI bundle as a single unit. Member + methods have been taught to correctly handle bundles and MIs inside bundles. + The MachineBasicBlock iterator has been modified to skip over bundled MIs to + enforce the bundle-as-a-single-unit concept. An alternative iterator + instr_iterator has been added to MachineBasicBlock to allow passes to + iterate over all of the MIs in a MachineBasicBlock, including those which + are nested inside bundles. The top level BUNDLE instruction must have the + correct set of register MachineOperand's that represent the cumulative + inputs and outputs of the bundled MIs.
+ +Packing / bundling of MachineInstr's should be done as part of the register + allocation super-pass. More specifically, the pass which determines what + MIs should be bundled together must be done after code generator exits SSA + form (i.e. after two-address pass, PHI elimination, and copy coalescing). + Bundles should only be finalized (i.e. adding BUNDLE MIs and input and + output register MachineOperands) after virtual registers have been + rewritten into physical registers. This requirement eliminates the need to + add virtual register operands to BUNDLE instructions which would effectively + double the virtual register def and use lists.
+ +The MC Layer is used to represent and process code at the raw machine code @@ -770,7 +878,7 @@ level, devoid of "high level" information like "constant pools", "jump tables", like label names, machine instructions, and sections in the object file. The code in this layer is used for a number of important purposes: the tail end of the code generator uses it to write a .s or .o file, and it is also used by the -llvm-mc tool to implement standalone machine codeassemblers and disassemblers. +llvm-mc tool to implement standalone machine code assemblers and disassemblers.
@@ -779,15 +887,12 @@ of important subsystems that interact at this layer, they are described later in this manual.
-MCStreamer is best thought of as an assembler API. It is an abstract API which @@ -817,11 +922,11 @@ MCObjectStreamer implements a full assembler.
The MCContext class is the owner of a variety of uniqued data structures at the @@ -832,11 +937,11 @@ interact with to create symbols and sections. This class can not be subclassed.
The MCSymbol class represents a symbol (aka label) in the assembly file. There @@ -864,11 +969,11 @@ like this to the .s file:
The MCSection class represents an object-file specific section. It is subclassed @@ -882,11 +987,11 @@ directive in a .s file).
The MCInst class is a target-independent representation of an instruction. It @@ -904,27 +1009,26 @@ printer, and the type generated by the assembly parser and disassembler.
This section documents the phases described in the high-level design of the code generator. It explains how they work and some of the rationale behind their design.
-Instruction Selection is the process of translating LLVM code presented to the code generator into target-specific machine instructions. There are @@ -936,14 +1040,12 @@ printer, and the type generated by the assembly parser and disassembler. selector to be generated from these .td files, though currently there are still things that require custom C++ code.
-The SelectionDAG provides an abstraction for code representation in a way that is amenable to instruction selection using automatic techniques @@ -1001,11 +1103,11 @@ printer, and the type generated by the assembly parser and disassembler.
SelectionDAG-based instruction selection consists of the following steps:
@@ -1082,11 +1184,11 @@ printer, and the type generated by the assembly parser and disassembler.The initial SelectionDAG is naïvely peephole expanded from the LLVM input by the SelectionDAGLowering class in the @@ -1102,11 +1204,11 @@ printer, and the type generated by the assembly parser and disassembler.
The Legalize phase is in charge of converting a DAG to only use the types that are natively supported by the target.
@@ -1135,11 +1237,11 @@ printer, and the type generated by the assembly parser and disassembler.The Legalize phase is in charge of converting a DAG to only use the operations that are natively supported by the target.
@@ -1167,12 +1269,13 @@ printer, and the type generated by the assembly parser and disassembler.The SelectionDAG optimization phase is run multiple times for code generation, immediately after the DAG is built and once after each @@ -1202,11 +1305,11 @@ printer, and the type generated by the assembly parser and disassembler.
The Select phase is the bulk of the target-specific code for instruction selection. This phase takes a legal SelectionDAG as input, pattern matches @@ -1363,11 +1466,11 @@ def : Pat<(i32 imm:$imm),
The scheduling phase takes the DAG of target instructions from the selection phase and assigns an order. The scheduler can pick an order depending on @@ -1384,11 +1487,11 @@ def : Pat<(i32 imm:$imm),
To Be Written
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Live Intervals are the ranges (intervals) where a variable is live. They are used by some register allocator passes to @@ -1417,14 +1522,12 @@ def : Pat<(i32 imm:$imm), register are live at the same point in the program (i.e., they conflict). When this situation occurs, one virtual register must be spilled.
-The first step in determining the live intervals of variables is to calculate the set of registers that are immediately dead after the instruction (i.e., @@ -1466,11 +1569,11 @@ def : Pat<(i32 imm:$imm),
We now have the information available to perform the live intervals analysis and build the live intervals themselves. We start off by numbering the basic @@ -1485,12 +1588,14 @@ def : Pat<(i32 imm:$imm),
The Register Allocation problem consists in mapping a program Pv, that can use an unbounded number of virtual registers, @@ -1500,23 +1605,21 @@ def : Pat<(i32 imm:$imm), accommodate all the virtual registers, some of them will have to be mapped into memory. These virtuals are called spilled virtuals.
-In LLVM, physical registers are denoted by integer numbers that normally range from 1 to 1023. To see how this numbering is defined for a particular architecture, you can read the GenRegisterNames.inc file for that architecture. For instance, by - inspecting lib/Target/X86/X86GenRegisterNames.inc we see that the - 32-bit register EAX is denoted by 15, and the MMX register - MM0 is mapped to 48.
+ inspecting lib/Target/X86/X86GenRegisterInfo.inc we see that the + 32-bit register EAX is denoted by 43, and the MMX register + MM0 is mapped to 65.Some architectures contain registers that share the same physical location. A notable example is the X86 platform. For instance, in the X86 architecture, @@ -1524,7 +1627,7 @@ def : Pat<(i32 imm:$imm), bits. These physical registers are marked as aliased in LLVM. Given a particular architecture, you can check which registers are aliased by inspecting its RegisterInfo.td file. Moreover, the method - TargetRegisterInfo::getAliasSet(p_reg) returns an array containing + MCRegisterInfo::getAliasSet(p_reg) returns an array containing all the physical registers aliased to the register p_reg.
Physical registers, in LLVM, are grouped in Register Classes. @@ -1617,11 +1720,11 @@ bool RegMapping_Fer::compatible_class(MachineFunction &mf, -
+ -There are two ways to map virtual registers to physical registers (or to memory slots). The first way, that we will call direct mapping, is @@ -1667,11 +1770,11 @@ bool RegMapping_Fer::compatible_class(MachineFunction &mf,
With very rare exceptions (e.g., function calls), the LLVM machine code instructions are three address instructions. That is, each instruction is @@ -1703,11 +1806,11 @@ bool RegMapping_Fer::compatible_class(MachineFunction &mf,
An important transformation that happens during register allocation is called the SSA Deconstruction Phase. The SSA form simplifies many analyses @@ -1727,11 +1830,11 @@ bool RegMapping_Fer::compatible_class(MachineFunction &mf,
Instruction folding is an optimization performed during register allocation that removes unnecessary copy instructions. For instance, a @@ -1764,32 +1867,38 @@ bool RegMapping_Fer::compatible_class(MachineFunction &mf, -
+ -The LLVM infrastructure provides the application developer with three different register allocators:
The type of register allocator used in llc can be chosen with the @@ -1805,23 +1914,143 @@ $ llc -regalloc=pbqp file.bc -o pbqp.s;
Throwing an exception requires unwinding out of a function. The + information on how to unwind a given function is traditionally expressed in + DWARF unwind (a.k.a. frame) info. But that format was originally developed + for debuggers to backtrace, and each Frame Description Entry (FDE) requires + ~20-30 bytes per function. There is also the cost of mapping from an address + in a function to the corresponding FDE at runtime. An alternative unwind + encoding is called compact unwind and requires just 4-bytes per + function.
+ +The compact unwind encoding is a 32-bit value, which is encoded in an
+ architecture-specific way. It specifies which registers to restore and from
+ where, and how to unwind out of the function. When the linker creates a final
+ linked image, it will create a __TEXT,__unwind_info
+ section. This section is a small and fast way for the runtime to access
+ unwind info for any given function. If we emit compact unwind info for the
+ function, that compact unwind info will be encoded in
+ the __TEXT,__unwind_info section. If we emit DWARF unwind info,
+ the __TEXT,__unwind_info section will contain the offset of the
+ FDE in the __TEXT,__eh_frame section in the final linked
+ image.
For X86, there are three modes for the compact unwind encoding:
+ +EBP or RBP)EBP/RBP-based frame, where EBP/RBP is pushed
+ onto the stack immediately after the return address,
+ then ESP/RSP is moved to EBP/RBP. Thus to
+ unwind, ESP/RSP is restored with the
+ current EBP/RBP value, then EBP/RBP is restored
+ by popping the stack, and the return is done by popping the stack once
+ more into the PC. All non-volatile registers that need to be restored must
+ have been saved in a small range on the stack that
+ starts EBP-4 to EBP-1020 (RBP-8
+ to RBP-1020). The offset (divided by 4 in 32-bit mode and 8
+ in 64-bit mode) is encoded in bits 16-23 (mask: 0x00FF0000).
+ The registers saved are encoded in bits 0-14
+ (mask: 0x00007FFF) as five 3-bit entries from the following
+ table:
| Compact Number | +i386 Register | +x86-64 Regiser | +
|---|---|---|
| 1 | +EBX |
+ RBX |
+
| 2 | +ECX |
+ R12 |
+
| 3 | +EDX |
+ R13 |
+
| 4 | +EDI |
+ R14 |
+
| 5 | +ESI |
+ R15 |
+
| 6 | +EBP |
+ RBP |
+
EBP
+ or RBP is not used as a frame pointer)To return, a constant (encoded in the compact unwind encoding) is added
+ to the ESP/RSP. Then the return is done by popping the stack
+ into the PC. All non-volatile registers that need to be restored must have
+ been saved on the stack immediately after the return address. The stack
+ size (divided by 4 in 32-bit mode and 8 in 64-bit mode) is encoded in bits
+ 16-23 (mask: 0x00FF0000). There is a maximum stack size of
+ 1024 bytes in 32-bit mode and 2048 in 64-bit mode. The number of registers
+ saved is encoded in bits 9-12 (mask: 0x00001C00). Bits 0-9
+ (mask: 0x000003FF) contain which registers were saved and
+ their order. (See
+ the encodeCompactUnwindRegistersWithoutFrame() function
+ in lib/Target/X86FrameLowering.cpp for the encoding
+ algorithm.)
EBP
+ or RBP is not used as a frame pointer)This case is like the "Frameless with a Small Constant Stack Size"
+ case, but the stack size is too large to encode in the compact unwind
+ encoding. Instead it requires that the function contains "subl
+ $nnnnnn, %esp" in its prolog. The compact encoding contains the
+ offset to the $nnnnnn value in the function in bits 9-12
+ (mask: 0x00001C00).
To Be Written
To Be Written
To Be Written
The code emission step of code generation is responsible for lowering from the code generator abstractions (like
In a Very Long Instruction Word (VLIW) architecture, the compiler is + responsible for mapping instructions to functional-units available on + the architecture. To that end, the compiler creates groups of instructions + called packets or bundles. The VLIW packetizer in LLVM is + a target-independent mechanism to enable the packetization of machine + instructions.
+ + + +Instructions in a VLIW target can typically be mapped to multiple functional +units. During the process of packetizing, the compiler must be able to reason +about whether an instruction can be added to a packet. This decision can be +complex since the compiler has to examine all possible mappings of instructions +to functional units. Therefore to alleviate compilation-time complexity, the +VLIW packetizer parses the instruction classes of a target and generates tables +at compiler build time. These tables can then be queried by the provided +machine-independent API to determine if an instruction can be accommodated in a +packet.
+The packetizer reads instruction classes from a target's itineraries and +creates a deterministic finite automaton (DFA) to represent the state of a +packet. A DFA consists of three major elements: inputs, states, and +transitions. The set of inputs for the generated DFA represents the instruction +being added to a packet. The states represent the possible consumption +of functional units by instructions in a packet. In the DFA, transitions from +one state to another occur on the addition of an instruction to an existing +packet. If there is a legal mapping of functional units to instructions, then +the DFA contains a corresponding transition. The absence of a transition +indicates that a legal mapping does not exist and that the instruction cannot +be added to the packet.
+ +To generate tables for a VLIW target, add TargetGenDFAPacketizer.inc +as a target to the Makefile in the target directory. The exported API provides +three functions: DFAPacketizer::clearResources(), +DFAPacketizer::reserveResources(MachineInstr *MI), and +DFAPacketizer::canReserveResources(MachineInstr *MI). These functions +allow a target packetizer to add an instruction to an existing packet and to +check whether an instruction can be added to a packet. See +llvm/CodeGen/DFAPacketizer.h for more information.
+ +Though you're probably reading this because you want to write or maintain a compiler backend, LLVM also fully supports building a native assemblers too. @@ -1896,20 +2193,18 @@ We've tried hard to automate the generation of the assembler from the .td files part of the manual and repetitive data entry can be factored and shared with the compiler.
-To Be Written
To Be Written
Once the instruction is parsed, it enters the MatchInstructionImpl function. The MatchInstructionImpl function performs alias processing and then does actual matching.
@@ -1922,12 +2217,10 @@ complex/powerful). Generally you want to use the first alias mechanism that meets the needs of your instruction, because it will allow a more concise description. -The first phase of alias processing is simple instruction mnemonic remapping for classes of instructions which are allowed with two different @@ -1965,9 +2258,9 @@ on the current instruction set.
The most general phase of alias processing occurs while matching is happening: it provides new forms for the matcher to match along with a specific @@ -2026,38 +2319,40 @@ def : InstAlias<"fcomi $reg", (COM_FIr RST:$reg)>;
Instruction aliases can also have a Requires clause to make them subtarget specific.
-If the back-end supports it, the instruction printer can automatically emit + the alias rather than what's being aliased. It typically leads to better, + more readable code. If it's better to print out what's being aliased, then + pass a '0' as the third parameter to the InstAlias definition.
+To Be Written
To Be Written
This section of the document explains features or design decisions that are specific to the code generator for a particular target. First we start with a table that summarizes what features are supported by each target.
-Note that this table does not include the C backend or Cpp backends, since they do not use the target independent code generator infrastructure. It also @@ -2092,16 +2387,14 @@ is the key:
This box indicates whether the target is considered to be production quality. This indicates that the target has been used as a static compiler to compile large amounts of code by a variety of different people and is in @@ -2241,9 +2533,9 @@ continuous use.
This box indicates whether the target supports parsing target specific .s files by implementing the MCAsmParser interface. This is required for llvm-mc to be able to act as a native assembler and is required for inline assembly @@ -2253,30 +2545,27 @@ support in the native .o file writer.
-This box indicates whether the target supports the MCDisassembler API for disassembling machine opcode bytes into MCInst's.
This box indicates whether the target supports most popular inline assembly constraints and modifiers.
-X86 lacks reliable support for inline assembly -constraints relating to the X86 floating point stack.
-This box indicates whether the target supports the JIT compiler through the ExecutionEngine interface.
@@ -2286,9 +2575,9 @@ in ARM codegen mode, but lacks NEON and full Thumb support.This box indicates whether the target supports writing .o files (e.g. MachO, ELF, and/or COFF) files directly from the target. Note that the target also @@ -2302,9 +2591,9 @@ file to a .o file (as is the case for many C compilers).
This box indicates whether the target supports guaranteed tail calls. These are calls marked "tail" and use the fastcc @@ -2313,15 +2602,30 @@ more more details.
This box indicates whether the target supports segmented stacks. This +replaces the traditional large C stack with many linked segments. It +is compatible with the gcc +implementation used by the Go front end.
+Basic support exists on the X86 backend. Currently +vararg doesn't work and the object files are not marked the way the gold +linker expects, but simple Go programs can be built by dragonegg.
+Tail call optimization, callee reusing the stack of the caller, is currently supported on x86/x86-64 and PowerPC. It is performed if:
@@ -2383,11 +2687,11 @@ define fastcc i32 @tailcaller(i32 %in1, i32 %in2) {Sibling call optimization is a restricted form of tail call optimization. Unlike tail call optimization described in the previous section, it can be @@ -2427,24 +2731,22 @@ entry:
The X86 code generator lives in the lib/Target/X86 directory. This code generator is capable of targeting a variety of x86-32 and x86-64 processors, and includes support for ISA extensions such as MMX and SSE.
-The following are the known target triples that are supported by the X86 backend. This is not an exhaustive list, and it would be useful to add those @@ -2469,31 +2771,34 @@ entry:
The following target-specific calling conventions are known to backend:
The x86 has a very flexible way of accessing memory. It is capable of forming memory addresses of the following expression directly in integer @@ -2526,11 +2831,11 @@ OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm PhysReg
x86 has a feature which provides the ability to perform loads and stores to different address spaces @@ -2571,11 +2876,11 @@ OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm PhysReg
An instruction name consists of the base name, a default operand size, and a a character per operand with an optional special size. For example:
@@ -2591,25 +2896,25 @@ MOVSX32rm16 -> movsx, 32-bit register, 16-bit memoryThe PowerPC code generator lives in the lib/Target/PowerPC directory. The code generation is retargetable to several variations or subtargets of the PowerPC ISA; including ppc32, ppc64 and altivec.
-LLVM follows the AIX PowerPC ABI, with two deviations. LLVM uses a PC relative (PIC) or static addressing for accessing global values, so no TOC @@ -2625,11 +2930,11 @@ MOVSX32rm16 -> movsx, 32-bit register, 16-bit memory
The size of a PowerPC frame is usually fixed for the duration of a function's invocation. Since the frame is fixed size, all references @@ -2772,11 +3077,11 @@ MOVSX32rm16 -> movsx, 32-bit register, 16-bit memory
The llvm prolog and epilog are the same as described in the PowerPC ABI, with the following exceptions. Callee saved registers are spilled after the frame @@ -2789,16 +3094,83 @@ MOVSX32rm16 -> movsx, 32-bit register, 16-bit memory
TODO - More to come.
The PTX code generator lives in the lib/Target/PTX directory. It is + currently a work-in-progress, but already supports most of the code + generation functionality needed to generate correct PTX kernels for + CUDA devices.
+ +The code generator can target PTX 2.0+, and shader model 1.0+. The + PTX ISA Reference Manual is used as the primary source of ISA + information, though an effort is made to make the output of the code + generator match the output of the NVidia nvcc compiler, whenever + possible.
+ +Code Generator Options:
+| Option | +Description | +
|---|---|
double |
+ If enabled, the map_f64_to_f32 directive is + disabled in the PTX output, allowing native double-precision + arithmetic | +
no-fma |
+ Disable generation of Fused-Multiply Add + instructions, which may be beneficial for some devices | +
smxy / computexy |
+ Set shader model/compute capability to x.y, + e.g. sm20 or compute13 | +
Working:
+In Progress:
+