tree 9143163d7bcdcbc533caa5714a06e428fa6d5d00
parent 2ae833e1f11e240991bea5ab8173ac4736ca4796
author Chris Kay <chris.kay@arm.com> 1722958377 +0000
committer Chris Kay <chris.kay@arm.com> 1730117436 +0000

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>