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Improve the incremental rebuilding support of "coccicheck" by piggy-backing on the computed dependency information of the corresponding *.o file, rather than rebuilding all <RULE>/<FILE> pairs if either their corresponding file changes, or if any header changes. This in effect uses the same method that the "sparse" target was made to use inc234e8a0ec(Makefile: make the "sparse" target non-.PHONY, 2021-09-23), except that the dependency on the *.o file isn't a hard one, we check with $(wildcard) if the *.o file exists, and if so we'll depend on it. This means that the common case of: make make coccicheck Will benefit from incremental rebuilding, now changing e.g. a header will only re-run "spatch" on those those *.c files that make use of it: By depending on the *.o we piggy-back on COMPUTE_HEADER_DEPENDENCIES. Seec234e8a0ec(Makefile: make the "sparse" target non-.PHONY, 2021-09-23) for prior art of doing that for the *.sp files. E.g.: make contrib/coccinelle/free.cocci.patch make -W column.h contrib/coccinelle/free.cocci.patch Will take around 15 seconds for the second command on my 8 core box if I didn't run "make" beforehand to create the *.o files. But around 2 seconds if I did and we have those "*.o" files. Notes about the approach of piggy-backing on *.o for dependencies: * It *is* a trade-off since we'll pay the extra cost of running the C compiler, but we're probably doing that anyway. The compiler is much faster than "spatch", so even though we need to re-compile the *.o to create the dependency info for the *.c for "spatch" it's faster (especially if using "ccache"). * There *are* use-cases where some would like to have *.o files around, but to have the "make coccicheck" ignore them. See: https://lore.kernel.org/git/20220826104312.GJ1735@szeder.dev/ For those users a: make make coccicheck SPATCH_USE_O_DEPENDENCIES= Will avoid considering the *.o files. * If that *.o file doesn't exist we'll depend on an intermediate file of ours which in turn depends on $(FOUND_H_SOURCES). This covers both an initial build, or where "coccicheck" is run without running "all" beforehand, and because we run "coccicheck" on e.g. files in compat/* that we don't know how to build unless the requisite flag was provided to the Makefile. Most of the runtime of "incremental" runs is now spent on various compat/* files, i.e. we conditionally add files to COMPAT_OBJS, and therefore conflate whether we *can* compile an object and generate dependency information for it with whether we'd like to link it into our binary. Before this change the distinction didn't matter, but now one way to make this even faster on incremental builds would be to peel those concerns apart so that we can see that e.g. compat/mmap.c doesn't depend on column.h. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Taylor Blau <me@ttaylorr.com>
59 lines
2.8 KiB
Text
59 lines
2.8 KiB
Text
This directory provides examples of Coccinelle (http://coccinelle.lip6.fr/)
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semantic patches that might be useful to developers.
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There are two types of semantic patches:
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* Using the semantic transformation to check for bad patterns in the code;
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The target 'make coccicheck' is designed to check for these patterns and
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it is expected that any resulting patch indicates a regression.
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The patches resulting from 'make coccicheck' are small and infrequent,
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so once they are found, they can be sent to the mailing list as per usual.
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Example for introducing new patterns:
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67947c34ae (convert "hashcmp() != 0" to "!hasheq()", 2018-08-28)
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b84c783882 (fsck: s/++i > 1/i++/, 2018-10-24)
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Example of fixes using this approach:
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248f66ed8e (run-command: use strbuf_addstr() for adding a string to
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a strbuf, 2018-03-25)
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f919ffebed (Use MOVE_ARRAY, 2018-01-22)
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These types of semantic patches are usually part of testing, c.f.
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0860a7641b (travis-ci: fail if Coccinelle static analysis found something
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to transform, 2018-07-23)
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* Using semantic transformations in large scale refactorings throughout
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the code base.
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When applying the semantic patch into a real patch, sending it to the
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mailing list in the usual way, such a patch would be expected to have a
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lot of textual and semantic conflicts as such large scale refactorings
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change function signatures that are used widely in the code base.
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A textual conflict would arise if surrounding code near any call of such
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function changes. A semantic conflict arises when other patch series in
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flight introduce calls to such functions.
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So to aid these large scale refactorings, semantic patches can be used.
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However we do not want to store them in the same place as the checks for
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bad patterns, as then automated builds would fail.
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That is why semantic patches 'contrib/coccinelle/*.pending.cocci'
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are ignored for checks, and can be applied using 'make coccicheck-pending'.
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This allows to expose plans of pending large scale refactorings without
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impacting the bad pattern checks.
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Git-specific tips & things to know about how we run "spatch":
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* The "make coccicheck" will piggy-back on
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"COMPUTE_HEADER_DEPENDENCIES". If you've built a given object file
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the "coccicheck" target will consider its depednency to decide if
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it needs to re-run on the corresponding source file.
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This means that a "make coccicheck" will re-compile object files
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before running. This might be unexpected, but speeds up the run in
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the common case, as e.g. a change to "column.h" won't require all
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coccinelle rules to be re-run against "grep.c" (or another file
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that happens not to use "column.h").
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To disable this behavior use the "SPATCH_USE_O_DEPENDENCIES=NoThanks"
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flag.
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