Annotation of wikisrc/pkgsrc/gcc.mdwn, revision 1.1
1.1 ! gdt 1: Using gcc in pkgsrc
! 2:
! 3: On many systems pkgsrc supports, gcc is the standard compiler. In
! 4: general, different versions of each OS have different gcc versions,
! 5: and some packages require newer GCC versions, in order to support
! 6: newer language standards (e.g. c++11, written in the style of
! 7: USE_LANGUAGES), or because older versions don't work (infrequently).
! 8:
! 9: This page discusses issues related to version selection, and intends
! 10: to be a design document for how pkgsrc should address this problem, to
! 11: be converted into historical design rationale once implemented. It
! 12: freely takes content from extensive mailinglist discussions, and
! 13: attempts to follow the rough consensus that has emerged.
! 14:
! 15: ## Base system gcc vs pkgsrc gcc
! 16:
! 17: Systems using gcc (e.g. NetBSD) have a compiler as /usr/bin/gcc, and
! 18: this is usable by pkgsrc without any bootstrapping activity. One can
! 19: build gcc versions (typically newer versions) from pkgsrc, resulting
! 20: in a compiler within ${PREFIX}, e.g. /usr/pkg/gcc6/bin/gcc. This
! 21: compiler can then be used to compile other packages.
! 22:
! 23: Issues with using base system gcc are typically that it is too old,
! 24: such as gcc 4.5 with NetBSD 6, which cannot compile c++11.
! 25:
! 26: Issues when using pkgsrc gcc are that
! 27:
! 28: - it must be bootstrapped, requiring compiling a number of packages
! 29: with the system compiler
! 30: - C++ packages that are linked together should be built with the
! 31: same compiler, because the standard library ABI is not necessarily
! 32: the same for each compiler version
! 33: - While C packages can be built with mixed versions, the binary
! 34: should be linked with the higher version because the support
! 35: library is backwards compatible but not forward compatible.
! 36:
! 37: ## Specific constraints and requirements
! 38:
! 39: This section attempts to gather all the requirements.
! 40:
! 41: - By default, pkgsrc should be able to build working packages, even
! 42: for packages that need a newer compiler than that provided in the
! 43: base system.
! 44:
! 45: - The set of packages that are needed when building a bootstrap
! 46: compiler should be minimized.
! 47:
! 48: - All packages that use C++ should be built with the same compiler version.
! 49:
! 50: - All packages that use C should have final linking with the highest
! 51: version used in any included library.
! 52:
! 53: - pkgsrc should avoid building gcc unless it is more or less
! 54: necessary to build packges. (As an example, if the base system
! 55: gcc can build c99 but not c++11, building a c99-only program
! 56: should not trigger building a gcc version adequate for c++11.)
! 57:
! 58: - The compiler selection logic should work on NetBSD 6, and in-use
! 59: (including LTS) GNU/Linux systems. It is desirable for this logic
! 60: to work on NetBSD 5.
! 61:
! 62: - The compiler selection logic should be understandable and not brittle.
! 63:
! 64: ## Design
! 65:
! 66: The above requirements could in theory be satisfied in many ways, but
! 67: most of them are too complicated.
! 68:
! 69: - Packages declare what languages they need, with c++, c++11, and
! 70: c++14 being expressed differently.
! 71:
! 72: - The package-settable variable GCC_REQD will be used only when a
! 73: compiler that generally can compile the declared language version
! 74: is insufficient. These cases are expected to be relatively rare.
! 75:
! 76: - A user-settable variable PKGSRC_GCC_VERSION will declare the
! 77: version of gcc to be used for C programs, with an OS- and
! 78: version--specific default.
! 79:
! 80: - A user-settable variable PKGSRC_GXX_VERSION will declare the version of gcc to
! 81: be used for all C++ programs, again with an OS- and
! 82: version-specific default. It must be at least PKGSRC_GCC_VERSION.
! 83:
! 84: - Each of c99, c++, c++11, and c++14 will be associated with a
! 85: minimum gcc version, such that almost all programs declaring that
! 86: language can be built with that version. (This avoids issues of
! 87: strict compliance with c++11, which requires a far higher version
! 88: of gcc than the version required to compile almost all actual
! 89: programs in c++11.)
! 90:
! 91: - The minimum version inferred from the language tag will be
! 92: combined with any GCC_REQD declarations to find a minimum version
! 93: for a specific package. If that is greater than
! 94: PKGSRC_GCC_VERSION (programs using only C) or PKGSRC_GXX_VERSION,
! 95: package building will fail. We call the resulting
! 96: PKGSRC_GCC_VERSION or PKGSRC_GXX_VERSION the chosen version.
! 97:
! 98: - When building a program using C or C++, the chosen version is not
! 99: provided by the base system, and the chosen version is not
! 100: installed via pkgsrc, then it (and its dependencies) will be built
! 101: from pkgsrc in a special bootstrap mode. When building in
! 102: bootstrap mode, the version selection logic is ignored and the
! 103: base system compiler is used. Consistency and reproducible builds
! 104: require that a package built with the normal prefix must be the
! 105: same whether built because of compiler bootstrapping or normal
! 106: use.
! 107:
! 108: There are thus two choices for dealing with bootstrapping. One is
! 109: to use a distinct prefix, which will ensure that all packages that
! 110: are part of the compiler bootstrap will not be linked into normal
! 111: pkgsrc programs. This implies that any dependencies of gcc may
! 112: exist twice, once in bootstrap mode and once if built normally. A
! 113: gcc version itself will be built twice, if it is desired for
! 114: regular use. This double building and the complexity of a second
! 115: prefix are the negatives of this approach.
! 116:
! 117: The other choice is to mark gcc and all depending packages as used
! 118: for compiler bootstrapping, and to always build those with the
! 119: base compiler. We use the package-settable variable
! 120: PKGSRC_GCC_BOOTSTRAP=yes to denote this. The negative with this
! 121: approach is possible inconsistency with gcc's dependencies being
! 122: built with the base compiler and used later.
! 123:
! 124: - We expect that any program containing C++ will undergo final
! 125: linking with a C++ compiler. This is not a change from the
! 126: current situation.
! 127:
! 128: ## Remaining issues
! 129:
! 130: ### gcc dependencies
! 131:
! 132: Because gcc can have dependencies, there could be packages built with
! 133: the system compiler that are then later used with the chosen version.
! 134: For now, we defer worrying about these problems (judging that they
! 135: will be less serious than the current situation where all c++11
! 136: programs fail to build on NetBSD 6).
! 137:
! 138: \todo: Analyze what build-time and install-time dependencies actually
! 139: exist.
! 140:
! 141: \todo: Discuss adjusting options to minimize dependencies, including
! 142: gcc-inplace-math and nls.
! 143:
! 144: ### Default versions for various systems
! 145:
! 146: Note that if any particular system (or bulk build), a newer gcc has to
! 147: be built, it does not hurt incrementally to have built it earlier.
! 148:
! 149: When the base system is old (e.g., gcc 4.5 in NetBSD 6, or 4.1, in
! 150: NetBSD 5), then it is clear that a newer version must be built. For
! 151: these, PKGSRC_GXX_VERSION should default to a newish gcc, avoiding
! 152: being so new as to cause building issues. Currently, gcc6 is probably
! 153: a good choice. PKGSRC_GCC_VERSION should probably default to the
! 154: system version if it can build C99, or match PKGSRC_GXX_VERSION, if
! 155: the system version is too old. Perhaps gcc 4.5 would be used, but 4.1
! 156: not used. \todo Discuss.
! 157:
! 158: When the base system is almost new enough, the decision about the
! 159: default is more complicated. A key example is gcc 4.8, found in
! 160: NetBSD 7. Firefox requires gcc 4.9 (\todo because the c++11 support
! 161: in 4.8 is not quite good enough), and all programs using c++14 also
! 162: need a newer version. One options is to choose 4.8, resulting in
! 163: firefox failing, as well as all c++14 programs. Another is to choose
! 164: 4.9, but this makes little sense because c++14 programs will still
! 165: fail, and the general rule of moving to the most recent
! 166: generally-acceptable version applies, which currently leads to gcc6.
! 167: This is in effect a declaration that "almost new enough" does not
! 168: count as new enough. Thus the plan for NetBSD 7 is to set
! 169: PKGSRC_GCC_VERSION to 4.8 and PKGSRC_GXX_VERSION to 6.
! 170:
! 171: When the base system is new, e.g. gcc 5 or gcc 6 it should simply be
! 172: used. By "new enough", we mean that almost no programs in pkgsrc fail
! 173: to build with it, which implies that it supports (almost all) C++14
! 174: programs. Our current definiton of new enough is gcc 5.
! 175:
! 176: ### Fortran
! 177:
! 178: Fortran support is currently somewhat troubled.. It seems obvious to
! 179: extend to PGKSRC_GFORTRAN_VERSION, and have that match
! 180: PKGSRC_GCC_VERSION or PKGSRC_GXX_VERSION, but the Fortran situation is
! 181: not worsened by the above design. \todo Discuss.
! 182:
! 183: ## Path forward
! 184:
! 185: - Modify all gcc packages to have minimal dependencies, and to add
! 186: PKGSRC_GCC_BOOTSTRAP.
! 187:
! 188: - Modify the compiler selection logic to do nothing if
! 189: PKGSRC_GCC_BOOTSTRAP is set.
! 190:
! 191: - Modify the compiler selection logic for LANGUAGES= to fail if
! 192: PKGSRC_GCC_VERSION/PKGSRC_GXX_VERSION is not new enough.
! 193:
! 194: - Modify the compiler selection logic for GCC_REQD to fail if the
! 195: version of GCC/GXX is not new enough.
! 196:
! 197: - Decide on defaults. The straw proposal is that PKGSRC_GCC_VERSION
! 198: is the base system version if >= 4.5 (or 4.4?), and otherwise 6,
! 199: and that PKGSRC_GXX_VERSION is the base system version if >= 5, and
! 200: otherwise 6.
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