code-generation-and-backends

Code Generation and Backends

Purpose

Overview LLVM (and general compiler) backend code generation: IR legalization, SelectionDAG instruction selection, register allocation, assembly emission, and what adding a new target entails — complementing IR skills and skills/compiler-internals/llvm-passes.

When to Use

Reading llc output for a specific target
Understanding why IR type legalizer inserted extra ops
Evaluating porting compiler to new architecture (high level)
Debugging wrong code at asm layer (not IR)

Workflow

1. Backend pipeline (LLVM)

LLVM IR per function
├── IR legalizer (types/ops target supports)
├── SelectionDAGBuilder
├── LegalizeTypes / LegalizeOps
├── Instruction selection (pattern match TableGen)
├── Scheduling (pre-RA)
├── Register allocation
├── Prolog/epilog insertion
└── AsmPrinter → .s object

2. llc usage

clang -c -emit-llvm -O2 -o foo.bc foo.c
llc -march=aarch64 -O2 foo.bc -o foo.s
llc -march=riscv64 -O2 foo.bc -o foo-rv.s

3. TableGen patterns (conceptual)

def ADD32rr : Pat<(add i32 GPR:$a, GPR:$b),
                  (ADD32rr GPR:$a, GPR:$b)>;

Patterns map DAG nodes to machine instructions. .td files define registers, calling conv, instr formats.

4. Calling convention lowering

ABI rules become CC_AArch64 / CC_X86_64 in TableGen — ties to skills/computer-architecture/abi-and-calling-conventions.

5. Target triple

clang --target=arm-none-eabi -c -O2 foo.c
llc -mtriple=thumbv7em-none-eabi foo.bc

Mismatch between triple and CPU features (+neon, +crc) causes legalizer failures or suboptimal code.

6. Adding a target (outline)

Define register classes and instr formats in TableGen
Implement lowering hooks (TargetLowering)
AsmPrinter and MC layer for relocations
Builtin calling convention and ELF/COFF object writer

Full port is large — reuse existing backend closest to arch.

7. Agent usage

/code-generation-and-backends Trace how this IR add becomes AArch64 ADD instruction

Common Problems

Symptom	Cause	Fix
`Cannot select` fatal	Unsupported IR op on target	Legalize or expand op
Wrong soft-float	ABI mismatch	Set `-mfloat-abi` / triple
Huge stack frame	Many spills post-RA	IR-level pressure reduction
llc vs clang differ	Different targets passed	Same `-mtriple`
TableGen build fail	Syntax in .td	`llvm-tblgen` error line

Related Skills

skills/compiler-internals/llvm-ir-and-passes — pre-codegen IR
skills/compiler-internals/compiler-optimizations-deep — RA intuition
skills/compilers/cross-gcc — embedded triples
skills/computer-architecture/abi-and-calling-conventions — call lowering
skills/low-level-programming/assembly-arm — read emitted asm