Makefile: avoid recursively expanding expensive variables

Specifically, do not recursively expand $(CFLAGS) because this leads to
repeatedly performing compile tests (e. g. cc-disable-warning) on every
recipe execution.

Without (nproc=32):
```
$ time env -i PATH=/usr/bin BCACHEFS_FUSE=1 NO_RUST=1 make -j$(nproc)
<...>
    [LD]     bcachefs
  72,48s user 11,29s system 190% cpu 44,036 total
```

With:
```
$ time env -i PATH=/usr/bin BCACHEFS_FUSE=1 NO_RUST=1 make -j$(nproc)
<...>
    [LD]     bcachefs
  66,79s user 5,17s system 1955% cpu 3,679 total
```

diff --git a/Makefile b/Makefile
index 418ffffc7e84feccddbe76cf6d6931fc211b1b49..581e8b41d1c75e7c2840c15b52ae8b43a98ea680 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -21,6 +21,10 @@ else
   CARGO_CLEAN_ARGS = --quiet
 endif
 
+# Prevent recursive expansions of $(CFLAGS) to avoid repeatedly performing
+# compile tests
+CFLAGS:=$(CFLAGS)
+
 CFLAGS+=-std=gnu11 -O2 -g -MMD -Wall -fPIC                     \
        -Wno-pointer-sign                                       \
        -Wno-deprecated-declarations                            \
@@ -38,6 +42,9 @@ CFLAGS+=-std=gnu11 -O2 -g -MMD -Wall -fPIC                    \
        -DNO_BCACHEFS_SYSFS                                     \
        -DVERSION_STRING='"$(VERSION)"'                         \
        $(EXTRA_CFLAGS)
+
+# Intenionally not doing the above to $(LDFLAGS) because we rely on
+# recursive expansion here (CFLAGS is not yet completely built by this line)
 LDFLAGS+=$(CFLAGS) $(EXTRA_LDFLAGS)
 
 ifdef CARGO_TOOLCHAIN_VERSION
@@ -154,11 +161,11 @@ TAGS:
 tags:
        ctags -R .
 
-SRCS=$(sort $(shell find . -type f ! -path '*/.*/*' -iname '*.c'))
-DEPS=$(SRCS:.c=.d)
+SRCS:=$(sort $(shell find . -type f ! -path '*/.*/*' -iname '*.c'))
+DEPS:=$(SRCS:.c=.d)
 -include $(DEPS)
 
-OBJS=$(SRCS:.c=.o)
+OBJS:=$(SRCS:.c=.o)
 
 %.o: %.c
        @echo "    [CC]     $@"
@@ -180,7 +187,7 @@ libbcachefs.a: $(filter-out ./tests/%.o, $(OBJS))
        @echo "    [AR]     $@"
        $(Q)ar -rc $@ $+
 
-RUST_SRCS=$(shell find rust-src/src rust-src/bch_bindgen/src -type f -iname '*.rs')
+RUST_SRCS:=$(shell find rust-src/src rust-src/bch_bindgen/src -type f -iname '*.rs')
 rust-src/target/release/libbcachefs_rust.a: $(RUST_SRCS)
        $(CARGO_BUILD)