1: On many systems pkgsrc supports, gcc is the standard compiler. In
2: general, different versions of each OS have different gcc versions,
3: and some packages require newer gcc versions, in order to support
4: newer language standards (e.g. c++11, written in the style of
5: USE_LANGUAGES), or because older versions don't work (infrequently).
6:
7: This page discusses issues related to version selection, and intends
8: to be a design document for how pkgsrc should address this problem, to
9: be converted into historical design rationale once implemented. It
10: freely takes content from extensive mailinglist discussions, and
11: attempts to follow the rough consensus that has emerged.
12:
13: ## Base system gcc vs pkgsrc gcc
14:
15: Systems using gcc (e.g. NetBSD) have a compiler as /usr/bin/gcc, and
16: this is usable by pkgsrc without any bootstrapping activity. One can
17: build gcc versions (typically newer versions) from pkgsrc, resulting
18: in a compiler within ${PREFIX}, e.g. /usr/pkg/gcc6/bin/gcc. This
19: compiler can then be used to compile other packages.
20:
21: The Issue with using base system gcc is typically that it is too old,
22: such as gcc 4.5 with NetBSD 6, which cannot compile c++11. Another
23: example is gcc 4.8 with NetBSD 7. While this can compile most c++11
24: programs, it cannot be used for firefox or glibmm (and therefore any
25: package that links against glibmm).
26:
27: Issues when using pkgsrc gcc are that
28:
29: - it must be bootstrapped, requiring compiling a number of packages
30: with the system compiler
31: - C++ packages that are linked together should be built with the
32: same compiler, because the standard library ABI is not necessarily
33: the same for each compiler version
34: - While C packages can be built with mixed versions, the binary
35: should be linked with the higher version because the support
36: library is backwards compatible but not forward compatible.
37:
38: ## Specific constraints and requirements
39:
40: This section attempts to gather all the requirements.
41:
42: - By default, pkgsrc should be able to build working packages, even
43: for packages that need a newer compiler than that provided in the
44: base system.
45:
46: - The set of packages that are needed when building a bootstrap
47: compiler should be minimized.
48:
49: - All packages that use C should have final linking with the highest
50: version used in any included library.
51:
52: - All packages that use C++ should be built with the same compiler
53: version. Because these in the general case may include C, the
54: version used for C++ must be at least as new as the version used
55: for any used C package.
56:
57: - pkgsrc should avoid building gcc unless it is more or less
58: necessary to build packges. (As an example, if the base system
59: gcc can build c99 but not c++11, building a c99-only program
60: should not trigger building a gcc version adequate for c++11.)
61:
62: - The compiler selection logic should work on NetBSD 6 and newer,
63: and other systems currently supported by pkgsrc, including in-use
64: LTS GNU/Linux systems. It should work on systems that default to
65: clang, when set to use GCC, at least as well as the current
66: scheme. It is desirable for this logic to work on NetBSD 5.
67:
68: - The compiler selection logic should be understandable and not brittle.
69:
70: ## Design
71:
72: The above requirements could in theory be satisfied in many ways, but
73: most of them are too complicated. We present a design that aims to be
74: sound while mimimizing complexity.
75:
76: - Packages declare what languages they need, with c++, c++11, and
77: c++14 being expressed differently. (This is exactly current
78: practice and just noted for completeness.)
79:
80: - The package-settable variable GCC_REQD will be used only when a
81: compiler that generally can compile the declared language version
82: is insufficient. These cases are expected to be relatively rare;
83: an example is firefox that is in c++ (but not c+11) and needs gcc
84: 4.9.
85:
86: - A user-settable variable PKGSRC_GCC_VERSION will declare the
87: version of gcc to be used for C programs, with an OS- and
88: version--specific default.
89:
90: - A user-settable variable PKGSRC_GXX_VERSION will declare the
91: version of gcc to be used for all C++ programs, again with an OS-
92: and version-specific default. It must be at least
93: PKGSRC_GCC_VERSION.
94:
95: - Each of c99, c++, c++11, and c++14 will be associated with a
96: minimum gcc version, such that almost all programs declaring that
97: language can be built with that version. (This avoids issues of
98: strict compliance with c++11, which requires a far higher version
99: of gcc than the version required to compile almost all actual
100: programs in c++11.)
101:
102: - The minimum version inferred from the language tag will be
103: combined with any GCC_REQD declarations to find a minimum version
104: for a specific package. If that is greater than
105: PKGSRC_GCC_VERSION (programs using only C) or PKGSRC_GXX_VERSION,
106: package building will fail. We call the resulting
107: PKGSRC_GCC_VERSION or PKGSRC_GXX_VERSION the chosen version.
108:
109: - When building a program using C or C++, if the chosen version is
110: not provided by the base system, and the chosen version is not
111: installed via pkgsrc, then it (and its dependencies) will be built
112: from pkgsrc in a special bootstrap mode. When building in
113: bootstrap mode, the version selection logic is ignored and the
114: base system compiler is used. Consistency and reproducible builds
115: require that a package built with the normal prefix must be the
116: same whether built because of compiler bootstrapping or normal
117: use.
118:
119: There are thus two choices for dealing with bootstrapping. One is
120: to use a distinct prefix, which will ensure that all packages that
121: are part of the compiler bootstrap will not be linked into normal
122: pkgsrc programs. This implies that any dependencies of gcc may
123: exist twice, once in bootstrap mode and once if built normally. A
124: gcc version itself will be built twice, if it is desired for
125: regular use. This double building and the complexity of a second
126: prefix are the negatives of this approach.
127:
128: The other choice is to mark gcc and all depending packages as used
129: for compiler bootstrapping, and to always build those with the
130: base compiler. We use the package-settable variable
131: PKGSRC_GCC_BOOTSTRAP=yes to denote this. The negative with this
132: approach is possible inconsistency with gcc's dependencies being
133: built with the base compiler and used later.
134:
135: - We hope that the chosen version can be built using the base system
136: version, and hope to avoid multi-stage bootstrapping.
137:
138: - We expect that any program containing C++ will undergo final
139: linking with a C++ compiler. This is not a change from the
140: current situation.
141:
142: ## Remaining issues
143:
144: ### gcc dependencies introduction
145:
146: Because gcc can have dependencies, there could be packages built with
147: the system compiler that are then later used with the chosen version.
148: For now, we defer worrying about these problems (judging that they
149: will be less serious than the current situation where all c++11
150: programs fail to build on NetBSD 6).
151:
152: \todo: Change gcc 4.8 and 4.9 to enable gcc-inplace-math by default.
153:
154: \todo: Analyze what build-time and install-time dependencies actually
155: exist. Include old GNU/Linux in this analysis.
156:
157: \todo: Consider if dropping nls would help. (On NetBSD, it seems that
158: base system libraries are used, so it would not help.)
159:
160: ### managing gcc dependencies
161:
162: There are multiple paths forward.
163:
164: \todo Choose one. Straw proposal is "Don't worry" and recursive
165: variable for the initial implementation.
166:
167: #### Separate prefix
168:
169: Build compilers in a separate prefix, or a subprefix, so that the
170: compiler and the packages needed to build it will not be used by any
171: normal packages. This completely avoids the issue of building a
172: package one way in bootstrap and another not in bootstrap, at the cost
173: of two builds and writing the separate-prefix code.
174:
175: #### Don't worry
176:
177: Don't worry that packages used to bootstrap the needed compiler are
178: compiled with an older compiler. Don't worry that they might be
179: different depending on build order. If we have an actual problem,
180: deal with it. This requires choosing an approach to omit compiler
181: selection logic when building the compiler:
182:
183: ##### Mark bootstrap packages
184:
185: Mark packages used to build gcc as PKGSRC_GCC_BOOTSTRAP=yes.
186: Conditionalize this on OPSYS if necessary. Don't force the compiler
187: if this is set.
188:
189: ##### Pass a recursive variable
190:
191: As above, but set PKGSRC_GCC_BOOTSTRAP=yes in the evniroment of the
192: call to build the compiler, so that all dependencies inherit
193: permission to skip compiler selection logic. (Alternatively, use some
194: other mechanism such as passing a make variable explicitly.)
195:
196: ### Differing GCC and GXX versions
197:
198: Perhaps it is a mistake to allow the chosen GCC and GXX versions to
199: differ. If we require them to be the same, then essentially all
200: systems with a base system compiler older than gcc 5 will have to
201: bootstrap the compiler. For now, we allow them to differ and will
202: permit the defaults to differ.
203:
204: ### gcc versions and number of buildable packages
205:
206: A gcc version that is too old will not build a number of packages.
207: Anything older than 4.8 fails for c++11. 4.8 fails on some c++11
208: packages, such as firefox and glibmm.
209:
210: A version that is too new also fails to build packages. Analyses
211: posted to tech-pkg indicate that 5 is close to 4.9 in the number of
212: packages built, and that moving to 6 causes hundreds of additional
213: failures.
214:
215: Therefore, the current answer to "What is the best version to use" is
216: 5.
217:
218: \todo Check this with Jason Bacon.
219:
220: ### Default versions for various systems
221:
222: Note that if for any particular system's set of installed packages (or
223: bulk build), a newer gcc has to be built, it does not hurt to have
224: built it earlier.
225:
226: When the base system is old (e.g., gcc 4.5 in NetBSD 6, or 4.1, in
227: NetBSD 5), then it is clear that a newer version must be built. For
228: these, PKGSRC_GXX_VERSION should default to a newish gcc, avoiding
229: being so new as to cause building issues. Currently, gcc5 is probably
230: a good choice, with gcc6 compiling significantly but not vastly fewer
231: packages. PKGSRC_GCC_VERSION should probably default to the system
232: version if it can build all C99 programs, or match PKGSRC_GXX_VERSION,
233: if the system version is too old. Perhaps gcc 4.5 would be used, but
234: 4.1 not used. \todo Discuss.
235:
236: When the base system is almost new enough, the decision about the
237: default is more complicated. A key example is gcc 4.8, found in
238: NetBSD 7. Firefox requires gcc 4.9, and all programs using c++14 also
239: need a newer version. One options is to choose 4.8, resulting in
240: firefox failing, as well as all c++14 programs. Another is to choose
241: 4.9, but this makes little sense because c++14 programs will still
242: fail, and the general rule of moving to the most recent
243: generally-acceptable version applies, which currently leads to gcc6.
244: This is in effect a declaration that "almost new enough" does not
245: count as new enough. Thus the plan for NetBSD 7 is to set
246: PKGSRC_GCC_VERSION to 4.8 and PKGSRC_GXX_VERSION to 5.
247:
248: When the base system is new enough, e.g. gcc 5, 6 or 7 it should
249: simply be used. By "new enough", we mean that almost no programs in
250: pkgsrc fail to build with it (because it is too old), which implies
251: that it supports (almost all) C++14 programs. Our current definiton
252: of new enough is gcc 5.
253:
254: ### Limited mixed versions
255:
256: One approach would be to allow limited mixed versions, where
257: individual programs could force a specific version to be bootstrapped
258: and used, so that e.g. firefox could use 4.9 even though most programs
259: use 4.8, which is what happens now on NetBSD 7. This would rely on
260: being able to link c++ with 4.9 including some things built with 4.8
261: (which is done presently). However, this approach would become
262: unsound with a library rather than an end program. We reject this as
263: too much complexity for avoiding building a newer compiler in limited
264: situations.
265:
266: ### Fortran
267:
268: Fortran support is currently somewhat troubled.. It seems obvious to
269: extend to PGKSRC_GFORTRAN_VERSION, and have that match
270: PKGSRC_GCC_VERSION or PKGSRC_GXX_VERSION, but the Fortran situation is
271: not worsened by the above design.
272:
273: When building a gcc version, we get gfortran. Perhaps, because of
274: fortran, we should require a single version, vs a C and a C++ version.
275:
276: \todo Discuss.
277:
278: ### C++ programs used by C programs
279:
280: The choice of one version for C++ and one for C (e.g. 5, 4.8 on
281: netbsd-7) breaks down if a C program links against a library that is
282: written in C++ but provides a C API, because we still need the C++
283: version's stdlib.
284:
285: \todo Define a variable for such packages to have in their buildlink3,
286: which will not add c++ to USE_LANGUAGES but will force
287: PKGSRC_GXX_VERSION to be used. Or decide that this is a good reason
288: to really just have one compiler version.
289:
290: ## Path forward
291:
292: - Modify all gcc packages to have minimal dependencies, and to add
293: PKGSRC_GCC_BOOTSTRAP.
294:
295: - Modify the compiler selection logic to do nothing if
296: PKGSRC_GCC_BOOTSTRAP is set.
297:
298: - Modify the compiler selection logic for LANGUAGES= to fail if
299: PKGSRC_GCC_VERSION/PKGSRC_GXX_VERSION is not new enough.
300:
301: - Modify the compiler selection logic for GCC_REQD to fail if the
302: version of GCC/GXX is not new enough.
303:
304: - Decide on defaults. The straw proposal is that PKGSRC_GCC_VERSION
305: is the base system version if >= 4.5 (or 4.4?), and otherwise 5,
306: and that PKGSRC_GXX_VERSION is the base system version if >= 5, and
307: otherwise 5.
308:
309: ### Later steps
310:
311: - Address fortran.
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