[PATCH v2 18/18] perf build: Convert llvm-config shell queries to simply expanded variables

[Ian Rogers <irogers@google.com>]

In Makefile.config, CFLAGS, CXXFLAGS, LIBLLVM, and EXTLIBS were assigned using
recursive expansion or appended with raw $(shell $(LLVM_CONFIG) ...) calls.
Because these variables were expanded during dependency evaluation across
every single object file compilation rule, Kbuild continuously re-executed
llvm-config forks nearly 200 times during incremental builds.

Convert llvm-config shell queries to simply expanded variables (:=) to ensure
Make evaluates LLVM compiler flags and library paths exactly once when
Makefile.config is parsed, eliminating ~185 redundant sub-processes during
build startup.

Assisted-by: Gemini:gemini-3.1-pro-preview
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/Makefile.config | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
index fbe7fa83b0bb..7a61da082c0b 100644
--- a/tools/perf/Makefile.config
+++ b/tools/perf/Makefile.config
@@ -951,11 +951,14 @@ ifndef NO_LIBLLVM
   $(call feature_check,llvm-perf)
   ifeq ($(feature-llvm-perf), 1)
     CFLAGS += -DHAVE_LIBLLVM_SUPPORT
-    CFLAGS += $(shell $(LLVM_CONFIG) --cflags)
-    CXXFLAGS += -DHAVE_LIBLLVM_SUPPORT
-    CXXFLAGS += $(shell $(LLVM_CONFIG) --cxxflags)
-    LIBLLVM = $(shell $(LLVM_CONFIG) --libs all) $(shell $(LLVM_CONFIG) --system-libs)
-    EXTLIBS += -L$(shell $(LLVM_CONFIG) --libdir) $(LIBLLVM)
+    LLVM_CFLAGS := $(shell $(LLVM_CONFIG) --cflags 2>/dev/null)
+    LLVM_CXXFLAGS := $(shell $(LLVM_CONFIG) --cxxflags 2>/dev/null)
+    LLVM_LIBLLVM := $(shell $(LLVM_CONFIG) --libs all 2>/dev/null) $(shell $(LLVM_CONFIG) --system-libs 2>/dev/null)
+    LLVM_LIBDIR := $(shell $(LLVM_CONFIG) --libdir 2>/dev/null)
+    CFLAGS += $(LLVM_CFLAGS)
+    CXXFLAGS += -DHAVE_LIBLLVM_SUPPORT $(LLVM_CXXFLAGS)
+    LIBLLVM := $(LLVM_LIBLLVM)
+    EXTLIBS += -L$(LLVM_LIBDIR) $(LIBLLVM)
     EXTLIBS += -lstdc++
     $(call detected,CONFIG_LIBLLVM)
   else
-- 
2.54.0.563.g4f69b47b94-goog