[PATCH] Makefile: Avoid repeated evaluation of shell commands

[Samuel Holland]

Recursively expanded variables (defined with '=') are expanded at
evaluation time. These version information variables are evaluated
inside a recipe as part of GENFLAGS. As a result, the shell commands
are executed separately for each compiler invocation. Convert the
version information variables to be simply expanded, so the shell
commands are executed only once, at Makefile evaluation time. This
speeds up the build by as much as 75%.

A separate check is needed to maintain the behavior of preferring the
value of OPENSBI_BUILD_TIME_STAMP from the environment.

Signed-off-by: Samuel Holland <samuel.holland at sifive.com>
---

 Makefile | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/Makefile b/Makefile
index 419ce66b..c1a13037 100644
--- a/Makefile
+++ b/Makefile
@@ -94,11 +94,11 @@ OPENSBI_VERSION_MINOR=`grep "define OPENSBI_VERSION_MINOR" $(include_dir)/sbi/sb
 OPENSBI_VERSION_GIT=
 
 # Detect 'git' presence before issuing 'git' commands
-GIT_AVAIL=$(shell command -v git 2> /dev/null)
+GIT_AVAIL := $(shell command -v git 2> /dev/null)
 ifneq ($(GIT_AVAIL),)
-GIT_DIR=$(shell git rev-parse --git-dir 2> /dev/null)
+GIT_DIR := $(shell git rev-parse --git-dir 2> /dev/null)
 ifneq ($(GIT_DIR),)
-OPENSBI_VERSION_GIT=$(shell if [ -d $(GIT_DIR) ]; then git describe 2> /dev/null; fi)
+OPENSBI_VERSION_GIT := $(shell if [ -d $(GIT_DIR) ]; then git describe 2> /dev/null; fi)
 endif
 endif
 
@@ -202,16 +202,18 @@ endif
 BUILD_INFO ?= n
 ifeq ($(BUILD_INFO),y)
 OPENSBI_BUILD_DATE_FMT = +%Y-%m-%d %H:%M:%S %z
+ifndef OPENSBI_BUILD_TIME_STAMP
 ifdef SOURCE_DATE_EPOCH
-	OPENSBI_BUILD_TIME_STAMP ?= $(shell date -u -d "@$(SOURCE_DATE_EPOCH)" \
+	OPENSBI_BUILD_TIME_STAMP := $(shell date -u -d "@$(SOURCE_DATE_EPOCH)" \
 		"$(OPENSBI_BUILD_DATE_FMT)" 2>/dev/null || \
 		date -u -r "$(SOURCE_DATE_EPOCH)" \
 		"$(OPENSBI_BUILD_DATE_FMT)" 2>/dev/null || \
 		date -u "$(OPENSBI_BUILD_DATE_FMT)")
 else
-	OPENSBI_BUILD_TIME_STAMP ?= $(shell date "$(OPENSBI_BUILD_DATE_FMT)")
+	OPENSBI_BUILD_TIME_STAMP := $(shell date "$(OPENSBI_BUILD_DATE_FMT)")
 endif
-OPENSBI_BUILD_COMPILER_VERSION=$(shell $(CC) -v 2>&1 | grep ' version ' | \
+endif
+OPENSBI_BUILD_COMPILER_VERSION := $(shell $(CC) -v 2>&1 | grep ' version ' | \
 	sed 's/[[:space:]]*$$//')
 endif
 
-- 
2.47.2