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makefiles/utils: function to memoize a variable evaluation

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This allow deferring a variable evaluation to its usage but still
benefit from only evaluating it once on multiple uses.
This commit is contained in:
Gaëtan Harter 2019-06-07 18:10:24 +02:00
parent fcf8c4782d
commit 2c5eeca47a
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GPG Key ID: 76DF6BCF1B1F883B
3 changed files with 37 additions and 0 deletions
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19
makefiles/utils/test-variables.mk
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@ -15,5 +15,24 @@ test-exported-variables:
$(Q)bash -c 'test "$(MY_VARIABLE)" = "$${MY_VARIABLE}" || { echo ERROR: "$(MY_VARIABLE)" != "$${MY_VARIABLE}"; exit 1; }'
$(Q)bash -c 'test $(PARSE_TIME) -lt $${CURRENT_TIME} || { echo ERROR: $(PARSE_TIME) \>= $${CURRENT_TIME} >&2; exit 1; }'
MEMOIZED_CURRENT_TIME = $(call memoized,MEMOIZED_CURRENT_TIME,$(call date_nanoseconds))
MEMOIZED_CURRENT_TIME_2 = $(call memoized,MEMOIZED_CURRENT_TIME_2,$(call date_nanoseconds))
PRE_MEMOIZED_TIME := $(call date_nanoseconds)
# Two separate evaluations
REF_CURRENT_TIME_1 := $(MEMOIZED_CURRENT_TIME)
# Strip to detect added whitespaces by function
REF_CURRENT_TIME_2 := $(strip $(MEMOIZED_CURRENT_TIME))
test-memoized-variables:
@# The value was only evaluated on first use
$(Q)test $(PRE_MEMOIZED_TIME) -lt $(REF_CURRENT_TIME_1) || { echo ERROR: $(PRE_MEMOIZED_TIME) \>= $(REF_CURRENT_TIME_1) >&2; exit 1; }
@# Both evaluation return the same time and without added whitespace
$(Q)test "$(REF_CURRENT_TIME_1)" = "$(REF_CURRENT_TIME_2)" || { echo ERROR: "$(REF_CURRENT_TIME_1)" != "$(REF_CURRENT_TIME_2)" >&2; exit 1; }
@# The second memoized value was only evaluated when calling the target
$(Q)test $(PARSE_TIME) -lt $(MEMOIZED_CURRENT_TIME_2) || { echo ERROR: $(PARSE_TIME) \>= $(MEMOIZED_CURRENT_TIME_2) >&2; exit 1; }
# Immediate evaluation for comparing
PARSE_TIME := $(call date_nanoseconds)

14
makefiles/utils/variables.mk
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@ -2,6 +2,20 @@
# These functions should help replacing immediate evaluation and global 'export'
# Evaluate a deferred variable only once on its first usage
# Uses after that will be as if it was an immediate evaluation
# This can replace using `:=` by default
#
# The goal is to use it for `shell` commands
#
# variable = $(call memoized,<variable>,<value>)
#
# Parameters
# variable: name of the variable you set
# value: value that should be set when evaluated
memoized = $2$(eval $1:=$2)
# Target specific export the variables for that target
#
# target-export-variables <target> <variables>

4
tests/build_system_utils/Makefile
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@ -18,6 +18,7 @@ MAKEFILES_UTILS = $(RIOTMAKE)/utils
COMPILE_TESTS += test-ensure_value test-ensure_value-negative
COMPILE_TESTS += test-exported-variables
COMPILE_TESTS += test-memoized-variables
# Tests will be run both in the host machine and in `docker`
all: build-system-utils-tests
@ -35,3 +36,6 @@ test-ensure_value-negative:
test-exported-variables:
$(Q)$(call command_should_succeed,"$(MAKE)" -C $(MAKEFILES_UTILS) -f test-variables.mk test-exported-variables)
test-memoized-variables:
$(Q)$(call command_should_succeed,"$(MAKE)" -C $(MAKEFILES_UTILS) -f test-variables.mk test-memoized-variables)