[glibc] benchtests: Build benchmarks in parallel

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=aafc0024b3935cefffe8b96f3725d0a8fbf778d4

commit aafc0024b3935cefffe8b96f3725d0a8fbf778d4
Author: Adhemerval Zanella <[email protected]>
Date:   Tue Jul 7 15:30:12 2026 -0300

    benchtests: Build benchmarks in parallel
    
    The unconditional '.NOTPARALLEL' in benchtests/Makefile forced the whole
    subdirectory to build serially, even though its only purpose is to keep
    the benchmark *runs* from perturbing each other's timing.
    
    Replace it with ordering that serializes only the benchmark runs, and
    only when more than one benchmark group will actually run.  The combined
    'bench' goal builds every benchmark program in parallel (through
    bench-build) and then runs the bench-set, bench-func and bench-malloc
    groups strictly one after another.
    
    Reviewed-by: Sam James <[email protected]>

Diff:
---
 benchtests/Makefile | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/benchtests/Makefile b/benchtests/Makefile
index 53f78232ff..f407e492cb 100644
--- a/benchtests/Makefile
+++ b/benchtests/Makefile
@@ -409,9 +409,9 @@ $(addprefix $(objpfx)bench-,calloc-thread): $(libm-benchtests)
 # Rules to build and execute the benchmarks.  Do not put any benchmark
 # parameters beyond this point.
 
-# We don't want the benchmark programs to run in parallel since that could
-# affect their performance.
-.NOTPARALLEL:
+# Benchmark programs must not run concurrently, however building them is
+# safe in parallel.  So '.NOTPARALLEL:' is used only when benchmarks run
+# or when more than one group is going to run.
 
 bench-extra-objs = json-lib.o
 
@@ -627,3 +627,24 @@ $(objpfx)bench-%.c: %-inputs $(bench-deps)
 	fi; \
 	$(PYTHON) scripts/bench.py $(patsubst %-inputs,%,$<); } > $@-tmp
 	mv -f $@-tmp $@
+
+# Serialize the benchmark runs so their timing is not perturbed by a
+# concurrent workload, while still building every benchmark program in parallel.
+# Ordering is only needed when more than one benchmark group run: the combined
+# 'bench' goal runs all three groups, and the bench* goals can also be combined
+# on a single command line.  A lone group needs no ordering -- make builds its
+# programs in parallel and then runs its single recipe, which cannot overlap a
+# compile.
+ifneq (,$(filter bench,$(MAKECMDGOALS)))
+bench-run-active := bench-set bench-func bench-malloc
+else
+bench-run-active := $(filter bench-set bench-func bench-malloc,$(MAKECMDGOALS))
+endif
+
+ifneq (,$(word 2,$(bench-run-active)))
+# Hold every active run until all benchmark programs have been built.
+$(bench-run-active): | bench-build
+# And then run the active groups strictly one at a time.
+bench-func: | $(filter bench-set,$(bench-run-active))
+bench-malloc: | $(filter bench-set bench-func,$(bench-run-active))
+endif