Build compiler-rt for llvm riscv

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Summary

During a cross-compile of compiler-rt for RISC-V, assembly files failed due to missing -march=rv64gc in compiler flags. The CMake configuration explicitly set CMAKE_C_FLAGS and CMAKE_CXX Desarrollo_FLAGS but didn’t propagate to .S assembly files, causing FPU instruction failures.

Root Cause

  • CMake’s language-specific flag handling:
    • CMAKE_C_FLAGS/CMAKE_CXX_FLAGS apply to C/C++ sources only, not assembly (.S files).
    • Assembly compilation requires CMAKE_ASM_FLAGS for architecture/target flags.
  • Implicit behavior:
    • Build systems like CMake often separate compiler drivers per language.
    • When clang compiles assembly files, it defaults to the host’s target.

Why This Happens in Real Systems

  • Heterogeneous codebases: Projects mixing C, C++, and assembly necessitate per-language flags.
  • Toolchain abstraction: CMake abstracts toolchain details, requiring explicit configuration for each language.
  • Legacy assumptions: Engineers assume CMAKE_C_FLAGS propagates universally, causing oversights.

Real-World Impact

Failure to build compiler-rt halts downstream toolchain workflows:

  • Bootstrap failures: Inability to build critical runtime libraries like libclang_rt.
  • Dead toolchains: RISC-V targets lacking FPU instructions become unusable.
  • Debug paralysis: Non-obvious errors manifest at instruction-level diagnostics.

How Senior Engineers Fix It

  1. Explicitly set CMAKE_ASM_FLAGS: 
    set(CMAKE_ASM_FLAGS "-target riscv64-unknown-linux-gnu -march=rv64gc")
  2. Unify flags via toolchain files: 
    set(CMAKE_COMMON_FLAGS "-target riscv64-unknown-linux-gnu -march=rv64gc")  
set(CMAKE_C_FLAGS "${CMAKE_COMMON_FLAGS}")  
set(CMAKE_CXX_FLAGS "${CMAKE_COMMON_FLAGS}")  
set(CMAKE_ASM_FLAGS "${CMAKE_COMMON_FLAGS}")
  3. Override assembly extensions in CMake: 
    enable_language(ASM)

Why Juniors Miss It

  • Mental model mismatch: Assuming CMAKE_C_FLAGS affects all compilation phases.
  • Lack of granularity awareness: Not distinguishing compilation drivers for C vs. assembly.
  • Debugging shortcuts: Dependency on verbose builds (make VERBOSE=1) for diagnosis instead of inspecting CMake internals.
  • Undocumented edge cases: Toolchain behaviors for niche architectures like RISC-V are rarely taught.

(腻code omitted intentionally：No executable example required beyond CMake snippets)