commit 834cfe16d36f94d3c6f5cad30a9b696c5faed073
author Chris Kay <chris.kay@arm.com> Tue Aug 06 15:32:57 2024 +0000
committer Chris Kay <chris.kay@arm.com> Mon Oct 28 12:10:36 2024 +0000
tree 9143163d7bcdcbc533caa5714a06e428fa6d5d00
parent 2ae833e1f11e240991bea5ab8173ac4736ca4796

build: fix grouped targets on Make <= 4.2

Grouped targets are a feature introduced with GNU Make 4.3 which enable
rules with multiple targets to communicate that all of the targets of
that rule are built simultaneously, rather than independently.

For example, without grouped targets the following rule may be executed
twice:

    a.txt b.txt:
    	touch a.txt b.txt

    # $ remake -j2 a.txt b.txt
    # touch a.txt b.txt
    # touch a.txt b.txt

In this example, both `a.txt` and `b.txt` are touched twice, when the
rule should only be executed once. Instead, this rule can use a grouped
target:

    a.txt b.txt &:
    	touch a.txt b.txt

    # $ remake -j2 a.txt b.txt
    # touch a.txt b.txt
    # remake: 'b.txt' is up to date.

In this case, both `a.txt` and `b.txt` are created once only, as Make
now knows that the recipe will create both files.

Note that pattern rules with multiple targets always behave this way.

Previously, we assumed that the grouped target feature was available,
but on systems still packaging Make 4.2, most prominently Ubuntu 20.04,
this is not the case. This change adds a check to ensure that we do not
use grouped targets if they are unavailable.

Change-Id: Ifd9da35421ae11468d7a25d3cbc76f6036921749
Signed-off-by: Chris Kay <chris.kay@arm.com>

lib/romlib/Makefile

diff --git a/lib/romlib/Makefile b/lib/romlib/Makefile
index 3d2b850..4cac75b 100644
--- a/lib/romlib/Makefile
+++ b/lib/romlib/Makefile
@@ -73,7 +73,7 @@
 	$(s)echo "  PRE     $@"
 	$(q)$(ROMLIB_GEN) pre --output $@ --deps $(BUILD_DIR)/jmptbl.d $<
 
-$(WRAPPER_SOURCES) &: $(BUILD_DIR)/jmptbl.i | $$(@D)/
+$(WRAPPER_SOURCES) $&: $(BUILD_DIR)/jmptbl.i | $$(@D)/
 	$(s)echo "  WRP     $<"
 	$(q)$(ROMLIB_GEN) genwrappers --bti=$(ENABLE_BTI) -b $(WRAPPER_DIR) $<
 

make_helpers/utilities.mk

diff --git a/make_helpers/utilities.mk b/make_helpers/utilities.mk
index efa0ab9..fcccd24 100644
--- a/make_helpers/utilities.mk
+++ b/make_helpers/utilities.mk
@@ -22,6 +22,13 @@
 escape-shell = '$(subst ','\'',$(1))'
 
 #
+# The grouped-target symbol. Grouped targets are not supported on versions of
+# GNU Make <= 4.2, which was most recently packaged with Ubuntu 20.04.
+#
+
+& := $(if $(filter grouped-target,$(.FEATURES)),&)
+
+#
 # Upper-case a string value.
 #
 # Parameters: