On cygwin-1.7, clang and llvm can be built fine through 3 stages. With --disable-assertions (RC3's default), I saw a few warnings in llvm/lib/AsmParser/LLParser.cpp with g++-4.3.4. (and two warnings, PR9515) ...Takumi $ uname.exe -a CYGWIN_NT-6.1-WOW64 HEAVEN64 1.7.7(0.230/5/3) 2010-08-31 09:58 i686 Cygwin $ gcc --version gcc (GCC) 4.3.4 20090804 (release) 1 * Stage 1 configured by ../../llvm/configure, generated by GNU Autoconf 2.60, with options "'-C' '--enable-targets=all' '--enable-optimized' '--prefix=/cygdrive/e/llvm/build/cygwin-static/install' '--with-optimize-option=-O3 -Werror'" make: Entering directory `/cygdrive/e/llvm/build/cygwin-static/lib/AsmParser' llvm[0]: Compiling LLParser.cpp for Release build /cygdrive/e/llvm/llvm/lib/AsmParser/LLParser.cpp: In member function ‘bool llvm::LLParser::ParseSwitch(llvm::Instruction*&, llvm::LLParser::PerFunctionState&)’: /cygdrive/e/llvm/llvm/lib/AsmParser/LLParser.cpp:3231: warning: ‘DestBB’ may be used uninitialized in this function /cygdrive/e/llvm/llvm/lib/AsmParser/LLParser.cpp: In member function ‘bool llvm::LLParser::ParseIndirectBr(llvm::Instruction*&, llvm::LLParser::PerFunctionState&)’: /cygdrive/e/llvm/llvm/lib/AsmParser/LLParser.cpp:3273: warning: ‘DestBB’ may be used uninitialized in this function Testing Time: 5247.89s Expected Passes : 8144 Expected Failures : 69 Unsupported Tests : 546 * Stage 2 and 3 configured by ../../../llvm/configure, generated by GNU Autoconf 2.60, with options "'-C' '--enable-targets=all' '--enable-optimized' '--disable-assertions' '--prefix=/cygdrive/e/llvm/build/cygwin-static/install-stagen' '--with-optimize-option=-O3 -Werror' 'CC=/cygdrive/e/llvm/build/cygwin-static/install/bin/clang.exe' 'CXX=/cygdrive/e/llvm/build/cygwin-static/install/bin/clang++.exe'" Testing Time: 5708.07s Expected Passes : 8144 Expected Failures : 69 Unsupported Tests : 546 $ mv install install-stage1; mv install-stagen install