On Wed, Mar 3, 2021 at 3:07 PM Fangrui Song <maskray@xxxxxxxxxx> wrote: > > > On 2021-03-03, Masahiro Yamada wrote: > >Hi. > > > >On Wed, Mar 3, 2021 at 6:44 AM Fangrui Song <maskray@xxxxxxxxxx> wrote: > >> > >> Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx> > >> > >> Thanks for the clean-up! > >> --gcc-toolchain= is an obsscure way searching for GCC installation prefixes (--prefix). > >> The logic is complex and different for different distributions/architectures. > >> > >> If we specify --prefix= (-B) explicitly, --gcc-toolchain is not needed. > > > > > >I tested this, and worked for me too. > > > >Before applying this patch, could you please > >help me understand the logic? > > > > > > > > > >I checked the manual > >(https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-b-dir) > > > > > > > >-B<dir>, --prefix <arg>, --prefix=<arg> > > Add <dir> to search path for binaries and object files used implicitly > > > >--gcc-toolchain=<arg>, -gcc-toolchain <arg> > > Use the gcc toolchain at the given directory > > > > > >Hmm, this description is too concise > >to understand how it works... > > > > > > > >I use Ubuntu 20.10. > > > >I use distro's default clang > >located in /usr/bin/clang. > > > >I place my aarch64 linaro toolchain in > >/home/masahiro/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu-gcc, > >which is not in my PATH environment. > > > > > > > > > >From my some experiments, > > > >clang --target=aarch64-linux-gnu -no-integrated-as \ > >--prefix=/home/masahiro/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu- ... > > > >works almost equivalent to > > > >PATH=/home/masahiro/tools/aarch64-linaro-7.5/bin:$PATH \ > >clang --target=aarch64-linux-gnu -no-integrated-as ... > > > > > >Then, clang will pick up aarch64-linux-gnu-as > >found in the search path. > > > >Is this correct? > > > > > >On the other hand, I could not understand > >what the purpose of --gcc-toolchain= is. > > > > > >Even if I add --gcc-toolchain=/home/masahiro/tools/aarch64-linaro-7.5, > >it does not make any difference, and it is completely useless. > > > > > >I read the comment from stephenhines: > >https://github.com/ClangBuiltLinux/linux/issues/78 > > > >How could --gcc-toolchain be used > >in a useful way? > > --gcc-toolchain was introduced in > https://reviews.llvm.org/rG1af7c219c7113a35415651127f05cdf056b63110 > to provide a flexible alternative to autoconf configure-time --with-gcc-toolchain (now cmake variable GCC_INSTALL_PREFIX). > > I agree the option is confusing, the documentation is poor, and probably very few people understand what it does. > I apologize that my previous reply is not particular correct. > So the more correct answer is below: > > > A --prefix option can specify either of > > 1) A directory (for something like /a/b/lib/gcc/arm-linux-androideabi, this should be /a/b, the parent directory of 'lib') > 2) A path fragment like /usr/bin/aarch64-linux-gnu- > > The directory values of the --prefix list and --gcc-toolchain are used to detect GCC installation directories. The directory is used to fetch include directories, system library directories and binutils directories (as, objcopy). > (See below, Linux kernel only needs the binutils executables, so the include/library logic is really useless to us) > > The logic is around https://github.com/llvm/llvm-project/blob/main/clang/lib/Driver/ToolChains/Gnu.cpp#L1910 > > Prefixes = --prefix/-B list (only the directory subset is effective) > StringRef GCCToolchainDir = --gcc-toolchain= or CMake variable GCC_INSTALL_PREFIX > if (GCCToolchainDir != "") { > Prefixes.push_back(std::string(GCCToolchainDir)); > } else { > if (!D.SysRoot.empty()) { > Prefixes.push_back(D.SysRoot); > // Add D.SysRoot+"/usr" to Prefixes. Some distributions add more directories. > AddDefaultGCCPrefixes(TargetTriple, Prefixes, D.SysRoot); > } > > // D.InstalledDir is the directory of the clang executable, e.g. /usr/bin > Prefixes.push_back(D.InstalledDir + "/.."); > > if (D.SysRoot.empty()) > AddDefaultGCCPrefixes(TargetTriple, Prefixes, D.SysRoot); > } > > // Gentoo / ChromeOS specific logic. > // I think this block is misplaced. > if (GCCToolchainDir == "" || GCCToolchainDir == D.SysRoot + "/usr") { > ... > } > > // Loop over the various components which exist and select the best GCC > // installation available. GCC installs are ranked by version number. > Version = GCCVersion::Parse("0.0.0"); > for (const std::string &Prefix : Prefixes) { > auto &VFS = D.getVFS(); > if (!VFS.exists(Prefix)) > continue; > > // CandidateLibDirs is a subset of {/lib64, /lib32, /lib}. > for (StringRef Suffix : CandidateLibDirs) { > const std::string LibDir = Prefix + Suffix.str(); > if (!VFS.exists(LibDir)) > continue; > bool GCCDirExists = VFS.exists(LibDir + "/gcc"); > bool GCCCrossDirExists = VFS.exists(LibDir + "/gcc-cross"); > > // Precise match. Detect $Prefix/lib/$--target > ScanLibDirForGCCTriple(TargetTriple, Args, LibDir, TargetTriple.str(), > false, GCCDirExists, GCCCrossDirExists); > // Usually empty. > for (StringRef Candidate : ExtraTripleAliases) // Try these first. > ScanLibDirForGCCTriple(TargetTriple, Args, LibDir, Candidate, false, > GCCDirExists, GCCCrossDirExists); > // CandidateTripleAliases is a set with "x86_64-linux-gnu", "x86_64-unknown-linux-gnu", ... > // This loop detects directories like $Prefix/lib/x86_64-linux-gnu. > for (StringRef Candidate : CandidateTripleAliases) > ScanLibDirForGCCTriple(TargetTriple, Args, LibDir, Candidate, false, > GCCDirExists, GCCCrossDirExists); > } > for (StringRef Suffix : CandidateBiarchLibDirs) { > const std::string LibDir = Prefix + Suffix.str(); > if (!VFS.exists(LibDir)) > continue; > bool GCCDirExists = VFS.exists(LibDir + "/gcc"); > bool GCCCrossDirExists = VFS.exists(LibDir + "/gcc-cross"); > for (StringRef Candidate : CandidateBiarchTripleAliases) > ScanLibDirForGCCTriple(TargetTriple, Args, LibDir, Candidate, true, > GCCDirExists, GCCCrossDirExists); > } > } > > > The comment > // Loop over the various components which exist and select the best GCC > // installation available. GCC installs are ranked by version number. > > is important. If you specify --prefix=$dir but not --gcc-toolchain, > the system cross toolchains (/usr/lib/gcc-cross) are also candidates and they may win. > > Specifying just --gcc-toolchain (due to if (GCCToolchainDir != "")) can effectively ignore system cross toolchains. > > > > In the Linux kernel use case, We specify -nostdinc and -nostdlib so GCC include/library directories are not used. > We seem to prefer the non-directory use of --prefix: CROSS_COMPILE=arm-linux-gnueabi- > So all the directory detection logic can be dropped. > > > A better commit message is along the lines of: > --gcc-toolchain specified directory is used to detect GCC installations > for include/library directories and binutils executables. > > We already specify something like --prefix=aarch64-linux-gnu- to inform > Clang of the binutils executables, and we do not need include/library > directories, so we can drop --gcc-toolchain. > > > > > > > > > > > > > > > >> On 2021-03-02, Nathan Chancellor wrote: > >> >This is not necessary anymore now that we specify '--prefix=', which > >> >tells clang exactly where to find the GNU cross tools. This has been > >> >verified with self compiled LLVM 10.0.1 and LLVM 13.0.0 as well as a > >> >distribution version of LLVM 11.1.0 without binutils in the LLVM > >> >toolchain locations. > >> > > >> >Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx> > >> >--- > >> > Makefile | 4 ---- > >> > 1 file changed, 4 deletions(-) > >> > > >> >diff --git a/Makefile b/Makefile > >> >index f9b54da2fca0..c20f0ad8be73 100644 > >> >--- a/Makefile > >> >+++ b/Makefile > >> >@@ -568,10 +568,6 @@ ifneq ($(CROSS_COMPILE),) > >> > CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%)) > >> > GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit)) > >> > CLANG_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE)) > >> >-GCC_TOOLCHAIN := $(realpath $(GCC_TOOLCHAIN_DIR)/..) > >> >-endif > >> >-ifneq ($(GCC_TOOLCHAIN),) > >> >-CLANG_FLAGS += --gcc-toolchain=$(GCC_TOOLCHAIN) > >> > endif > >> > ifneq ($(LLVM_IAS),1) > >> > CLANG_FLAGS += -no-integrated-as > >> > > >> >base-commit: 7a7fd0de4a9804299793e564a555a49c1fc924cb > >> >-- > >> >2.31.0.rc0.75.gec125d1bc1 > >> > > >> >-- > >> >You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > >> >To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@xxxxxxxxxxxxxxxx. > >> >To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20210302210646.3044738-1-nathan%40kernel.org. > > > > > > > >-- > >Best Regards > > > >Masahiro Yamada I've sent https://reviews.llvm.org/D97902 to improve the -B and --gcc-toolchain documentation, and posted https://lists.llvm.org/pipermail/cfe-dev/2021-March/067820.html (Message-Id: CAFP8O3JZBWx6OYm14KhP0f+RNU_OHLPLZnyX7jDtCL_yvnJDfA@xxxxxxxxxxxxxx) to discuss Clang's current weird behaviors. * --gcc-toolchain suppresses GCC detection under sysroot, -B doesn't. I'd like that -B suppresses GCC detection under sysroot as well but unsure whether some folks are dependent on this behavior. * With -B -B -B --gcc-toolchain, the largest version instead of the first wins. I think it makes sense to select the first option where a GCC installation is found. Hey, this list has a lot developers using cross compilers. Do you have thoughts to share? :)
