Annotation of wikisrc/pkgsrc/gcc.mdwn, revision 1.7
1.1 gdt 1: On many systems pkgsrc supports, gcc is the standard compiler. In
2: general, different versions of each OS have different gcc versions,
1.2 gdt 3: and some packages require newer gcc versions, in order to support
1.1 gdt 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:
1.3 gdt 21: The Issue with using base system gcc is typically that it is too old,
1.6 gdt 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).
1.1 gdt 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:
1.2 gdt 52: - All packages that use C++ should be built with the same compiler
1.3 gdt 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.
1.2 gdt 56:
1.1 gdt 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:
1.2 gdt 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.
1.1 gdt 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
1.2 gdt 73: most of them are too complicated. We present a design that aims to be
74: sound while mimimizing complexity.
1.1 gdt 75:
76: - Packages declare what languages they need, with c++, c++11, and
1.2 gdt 77: c++14 being expressed differently. (This is exactly current
78: practice and just noted for completeness.)
1.1 gdt 79:
80: - The package-settable variable GCC_REQD will be used only when a
81: compiler that generally can compile the declared language version
1.2 gdt 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.
1.1 gdt 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:
1.6 gdt 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.
1.1 gdt 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:
1.3 gdt 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
1.1 gdt 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:
1.2 gdt 135: - We hope that the chosen version can be built using the base system
136: version, and hope to avoid multi-stage bootstrapping.
137:
1.1 gdt 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:
1.6 gdt 144: ### gcc dependencies introduction
1.1 gdt 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:
1.6 gdt 152: \todo: Change gcc 4.8 and 4.9 to enable gcc-inplace-math by default.
153:
1.1 gdt 154: \todo: Analyze what build-time and install-time dependencies actually
1.6 gdt 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:
1.7 ! gdt 160: \todo: Consider failing if optins that we want one way are another,
! 161: when bootstrapping.
! 162:
1.6 gdt 163: ### managing gcc dependencies
164:
165: There are multiple paths forward.
166:
167: \todo Choose one. Straw proposal is "Don't worry" and recursive
168: variable for the initial implementation.
169:
170: #### Separate prefix
171:
172: Build compilers in a separate prefix, or a subprefix, so that the
173: compiler and the packages needed to build it will not be used by any
174: normal packages. This completely avoids the issue of building a
175: package one way in bootstrap and another not in bootstrap, at the cost
176: of two builds and writing the separate-prefix code.
177:
178: #### Don't worry
179:
180: Don't worry that packages used to bootstrap the needed compiler are
181: compiled with an older compiler. Don't worry that they might be
182: different depending on build order. If we have an actual problem,
183: deal with it. This requires choosing an approach to omit compiler
184: selection logic when building the compiler:
185:
186: ##### Mark bootstrap packages
187:
188: Mark packages used to build gcc as PKGSRC_GCC_BOOTSTRAP=yes.
189: Conditionalize this on OPSYS if necessary. Don't force the compiler
190: if this is set.
1.1 gdt 191:
1.6 gdt 192: ##### Pass a recursive variable
193:
194: As above, but set PKGSRC_GCC_BOOTSTRAP=yes in the evniroment of the
195: call to build the compiler, so that all dependencies inherit
196: permission to skip compiler selection logic. (Alternatively, use some
197: other mechanism such as passing a make variable explicitly.)
1.1 gdt 198:
1.3 gdt 199: ### Differing GCC and GXX versions
200:
201: Perhaps it is a mistake to allow the chosen GCC and GXX versions to
202: differ. If we require them to be the same, then essentially all
203: systems with a base system compiler older than gcc 5 will have to
204: bootstrap the compiler. For now, we allow them to differ and will
205: permit the defaults to differ.
206:
1.6 gdt 207: ### gcc versions and number of buildable packages
208:
209: A gcc version that is too old will not build a number of packages.
210: Anything older than 4.8 fails for c++11. 4.8 fails on some c++11
211: packages, such as firefox and glibmm.
212:
213: A version that is too new also fails to build packages. Analyses
214: posted to tech-pkg indicate that 5 is close to 4.9 in the number of
215: packages built, and that moving to 6 causes hundreds of additional
216: failures.
217:
218: Therefore, the current answer to "What is the best version to use" is
219: 5.
220:
221: \todo Check this with Jason Bacon.
222:
1.1 gdt 223: ### Default versions for various systems
224:
1.3 gdt 225: Note that if for any particular system's set of installed packages (or
226: bulk build), a newer gcc has to be built, it does not hurt to have
227: built it earlier.
1.1 gdt 228:
229: When the base system is old (e.g., gcc 4.5 in NetBSD 6, or 4.1, in
230: NetBSD 5), then it is clear that a newer version must be built. For
231: these, PKGSRC_GXX_VERSION should default to a newish gcc, avoiding
1.4 gdt 232: being so new as to cause building issues. Currently, gcc5 is probably
233: a good choice, with gcc6 compiling significantly but not vastly fewer
234: packages. PKGSRC_GCC_VERSION should probably default to the system
235: version if it can build all C99 programs, or match PKGSRC_GXX_VERSION,
236: if the system version is too old. Perhaps gcc 4.5 would be used, but
237: 4.1 not used. \todo Discuss.
1.1 gdt 238:
239: When the base system is almost new enough, the decision about the
240: default is more complicated. A key example is gcc 4.8, found in
1.2 gdt 241: NetBSD 7. Firefox requires gcc 4.9, and all programs using c++14 also
1.1 gdt 242: need a newer version. One options is to choose 4.8, resulting in
243: firefox failing, as well as all c++14 programs. Another is to choose
244: 4.9, but this makes little sense because c++14 programs will still
245: fail, and the general rule of moving to the most recent
246: generally-acceptable version applies, which currently leads to gcc6.
247: This is in effect a declaration that "almost new enough" does not
248: count as new enough. Thus the plan for NetBSD 7 is to set
1.4 gdt 249: PKGSRC_GCC_VERSION to 4.8 and PKGSRC_GXX_VERSION to 5.
1.1 gdt 250:
1.4 gdt 251: When the base system is new enough, e.g. gcc 5, 6 or 7 it should
252: simply be used. By "new enough", we mean that almost no programs in
1.6 gdt 253: pkgsrc fail to build with it (because it is too old), which implies
254: that it supports (almost all) C++14 programs. Our current definiton
255: of new enough is gcc 5.
1.1 gdt 256:
1.2 gdt 257: ### Limited mixed versions
258:
259: One approach would be to allow limited mixed versions, where
260: individual programs could force a specific version to be bootstrapped
261: and used, so that e.g. firefox could use 4.9 even though most programs
262: use 4.8, which is what happens now on NetBSD 7. This would rely on
263: being able to link c++ with 4.9 including some things built with 4.8
264: (which is done presently). However, this approach would become
265: unsound with a library rather than an end program. We reject this as
266: too much complexity for avoiding building a newer compiler in limited
267: situations.
268:
1.1 gdt 269: ### Fortran
270:
271: Fortran support is currently somewhat troubled.. It seems obvious to
272: extend to PGKSRC_GFORTRAN_VERSION, and have that match
273: PKGSRC_GCC_VERSION or PKGSRC_GXX_VERSION, but the Fortran situation is
1.6 gdt 274: not worsened by the above design.
275:
276: When building a gcc version, we get gfortran. Perhaps, because of
277: fortran, we should require a single version, vs a C and a C++ version.
278:
279: \todo Discuss.
280:
281: ### C++ programs used by C programs
282:
283: The choice of one version for C++ and one for C (e.g. 5, 4.8 on
284: netbsd-7) breaks down if a C program links against a library that is
285: written in C++ but provides a C API, because we still need the C++
286: version's stdlib.
287:
288: \todo Define a variable for such packages to have in their buildlink3,
289: which will not add c++ to USE_LANGUAGES but will force
290: PKGSRC_GXX_VERSION to be used. Or decide that this is a good reason
291: to really just have one compiler version.
1.1 gdt 292:
293: ## Path forward
294:
295: - Modify all gcc packages to have minimal dependencies, and to add
296: PKGSRC_GCC_BOOTSTRAP.
297:
298: - Modify the compiler selection logic to do nothing if
299: PKGSRC_GCC_BOOTSTRAP is set.
300:
301: - Modify the compiler selection logic for LANGUAGES= to fail if
302: PKGSRC_GCC_VERSION/PKGSRC_GXX_VERSION is not new enough.
303:
304: - Modify the compiler selection logic for GCC_REQD to fail if the
305: version of GCC/GXX is not new enough.
306:
307: - Decide on defaults. The straw proposal is that PKGSRC_GCC_VERSION
1.5 gdt 308: is the base system version if >= 4.5 (or 4.4?), and otherwise 5,
1.1 gdt 309: and that PKGSRC_GXX_VERSION is the base system version if >= 5, and
1.5 gdt 310: otherwise 5.
1.3 gdt 311:
312: ### Later steps
313:
314: - Address fortran.
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