llvm写编译后端.pdf

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Understanding and writing an LLVM compiler back-endBruno Cardoso Lopesbruno.cardoso@gmail.comEmbedded Linux Conference 2009San Francisco, CA

• What’s LLVM? • LLVM design • The back-end • Why LLVM? • Who’s using Agenda

• Low Level Virtual Machine • A virtual instruction set • A compiler infrastructure suite with aggressive optimizations. What's LLVM ?Basics

information • Low-level representation, but with high-level type • 3-address-code-like representation • RISC based, language independent with SSA information. The on-disk representation is called bitcode. A virtual instruction set

int dummy(int a) { return a+3; } define i32 @dummy(i32 %a) nounwind readnone { entry: } %0 = add i32 %a, 3 ret i32 %0 A virtual instruction set

• Compiler front-end • llvm-gcc • clang • IR and tools to handle it • JIT and static back-ends. Front-endIRBack-endA compiler infrastructure suite

• GCC based front-end : llvm-gcc • GENERIC to LLVM instead of GIMPLE • GIMPLE is only an in-memory IR • GCC is not modular (intentionally) Front-end : llvm-gcc

C, C++ and ObjC. • Very mature and supports Java, Ada, FORTRAN, • Cross-compiler needed for a not native target. $ llvm-gcc -O2 -c clown.c -emit-llvm -o clown.bc $ llvm-extract -func=bozo < clown.bc | llvm-dis define float @bozo(i32 %lhs, i32 %rhs, float %w) nounwind { entry: } %0 = sdiv i32 %lhs, %rhs %1 = sitofp i32 %0 to float %2 = mul float %1, %w ret float %2 LLVM assemblyFront-end : llvm-gcc

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