diff options
| -rw-r--r-- | rules/hs-suffix-way-rules.mk | 82 |
1 files changed, 80 insertions, 2 deletions
diff --git a/rules/hs-suffix-way-rules.mk b/rules/hs-suffix-way-rules.mk index f9ecf6e99a..4735182017 100644 --- a/rules/hs-suffix-way-rules.mk +++ b/rules/hs-suffix-way-rules.mk @@ -29,6 +29,86 @@ $1/$2/build/%.$$(dyn_osuf)-boot: $1/$2/build/%.$$(v_hisuf)-boot fi else +# Note [Implicit rule search algorithm] +# +# The order in which implicit rules are defined can influence a build. +# +# Case study: genprimpos/Lexer.hs +# +# We have two implicit rules for creating .o files, which after instantiating +# with a specific directory ($1=utils/genprimops) and distdir ($2=dist) look +# like this: +# +# utils/genprimops/dist/build/%.o : utils/genprimops/dist/build/%.hs +# <recipe1> +# utils/genprimops/dist/build/%.o : utils/genprimops/./%.hs +# <recipe2> +# +# The first rule is defined in hs-suffix-way-rules.mk (this file), the other +# in hs-suffix-way-rules-srcdir.mk. +# +# Assume for rest of this story that %=Lexer. +# +# In a normal repository checkout, neither Lexer.hs exists, but we have a rule +# to generate the one in the build directory by running alex on Lexer.x (the +# rule for that operation is defined in hs-suffix-rules-srcdir.mk). Since make +# needs to make a choice which of the above two implicit rules to follow (it +# never runs 2 recipes for the same target, unless double colon rules are +# used, which we don't), logically it will choose the first rule: Lexer.o will +# depend on Lexer.hs in the build directory, that file will be build, and then +# Lexer.o can be build. +# +# In an sdist however, Lexer.hs is present in the source directory. It was +# copied there during the creation of the sdist by a rule in +# sdist-ghc-file.mk. And this time we *don't* know how to generate the +# Lexer.hs in the build directory, because 1) alex is not installed when +# building from sdist and 2) the sdist creation process renamed Lexer.x to +# Lexer.x.source. So normally make would now choose the second rule: Lexer.o +# will depend on Lexer.hs in the source directory, for which nothing needs to +# be done, and then Lexer.o can be build. +# +# There is however another actor in play, a rule in sdist-ghc-file.mk, which +# after after instantiating with the same directory ($1=utils/genprimops) and +# distdir ($2=dist) looks like this: +# +# sdist_utils/genprimops_dist_Lexer : utils/genprimops/dist/build/Lexer.hs +# +# Note that this is not an implicit rule, there aren't any %'s. This rule +# *explicitly* depends on Lexer.hs in the build directory. What follows is the +# following: +# +# * make thinks Lexer.hs in the build directory "ought to exist" [1], +# because it is an explicit dependency of /some/ target. +# +# * this puts our two implicit rules on equal footing: one depends on a +# file that exists, the other on a file that ought to exist. Make's +# implicit rule search algorithm doesn't distinguish between these two +# cases [1]. +# +# * to break the tie, make chooses the rule that is defined first. Lexer.o +# will depend on Lexer.hs in the build directory, which doesn't exist, +# and which make doesn't know how to build, causing a build failure. +# +# To prevent this from happening we define rules for haskell source files in +# the source directory before those in the distdir. +# +# Alternative solutions: +# +# * Don't include the explicit rule above when not creating an sdist, as +# that is the only time when it is needed. +# +# * Merge the two implicit rules, with help from $1_$2_HS_SRCS from +# hs-sources.mk, which is sdist aware. +# +# * Require alex and happy to be installed when building from an sdist, +# simplifying all this drastically. +# +# [1] https://www.gnu.org/software/make/manual/make.html#Implicit-Rule-Search + +$$(foreach dir,$$($1_$2_HS_SRC_DIRS),\ + $$(eval $$(call hs-suffix-way-rules-srcdir,$1,$2,$3,$$(dir)))) + + ifneq "$$(BINDIST)" "YES" $1/$2/build/%.$$($3_hcsuf) : $1/$2/build/%.hs $$(LAX_DEPS_FOLLOW) $$$$($1_$2_HC_DEP) @@ -45,8 +125,6 @@ $1/$2/build/%.$$($3_osuf) : $1/$2/build/autogen/%.hs $$(LAX_DEPS_FOLLOW) $$$$($1 endif -$$(foreach dir,$$($1_$2_HS_SRC_DIRS),\ - $$(eval $$(call hs-suffix-way-rules-srcdir,$1,$2,$3,$$(dir)))) endif |