Makefile support for link-time optimization (#7181)

Summary:
`USE_LTO=1` in `make` commands now enables LTO. The archiver (`ar`) needed
to change in this PR to use a wrapper that enables the LTO plugin.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7181

Test Plan:
build a few ways
```
$ make clean && USE_LTO=1 make -j48 db_bench
$ make clean && USE_CLANG=1 USE_LTO=1 make -j48 db_bench
$ make clean && ROCKSDB_NO_FBCODE=1 USE_LTO=1 make -j48 db_bench
```

Reviewed By: cheng-chang

Differential Revision: D22784994

Pulled By: ajkr

fbshipit-source-id: 9c45333bd49bf4615aa04c85b7c6fd3925421152

Makefile

@@ -195,6 +195,17 @@ endif
 $(warning Warning: Compiling in debug mode. Don't use the resulting binary in production)
 endif
 
+# `USE_LTO=1` enables link-time optimizations. Among other things, this enables
+# more devirtualization opportunities and inlining across translation units.
+# This can save significant overhead introduced by RocksDB's pluggable
+# interfaces/internal abstractions, like in the iterator hierarchy. It works
+# better when combined with profile-guided optimizations (not currently
+# supported natively in Makefile).
+ifeq ($(USE_LTO), 1)
+	CXXFLAGS += -flto
+	LDFLAGS += -flto -fuse-linker-plugin
+endif
+
 #-----------------------------------------------
 include src.mk
 

build_tools/build_detect_platform

@@ -89,6 +89,16 @@ if test -z "$CXX"; then
     fi
 fi
 
+if test -z "$AR"; then
+    if [ -x "$(command -v gcc-ar)" ]; then
+        AR=gcc-ar
+    elif [ -x "$(command -v llvm-ar)" ]; then
+        AR=llvm-ar
+    else
+        AR=ar
+    fi
+fi
+
 # Detect OS
 if test -z "$TARGET_OS"; then
     TARGET_OS=`uname -s`
@@ -756,6 +766,7 @@ ROCKSDB_PATCH=`build_tools/version.sh patch`
 
 echo "CC=$CC" >> "$OUTPUT"
 echo "CXX=$CXX" >> "$OUTPUT"
+echo "AR=$AR" >> "$OUTPUT"
 echo "PLATFORM=$PLATFORM" >> "$OUTPUT"
 echo "PLATFORM_LDFLAGS=$PLATFORM_LDFLAGS" >> "$OUTPUT"
 echo "JAVA_LDFLAGS=$JAVA_LDFLAGS" >> "$OUTPUT"

build_tools/fbcode_config.sh

@@ -109,6 +109,7 @@ if [ -z "$USE_CLANG" ]; then
   # gcc
   CC="$GCC_BASE/bin/gcc"
   CXX="$GCC_BASE/bin/g++"
+  AR="$GCC_BASE/bin/gcc-ar"
 
   CFLAGS+=" -B$BINUTILS/gold"
   CFLAGS+=" -isystem $GLIBC_INCLUDE"
@@ -119,6 +120,7 @@ else
   CLANG_INCLUDE="$CLANG_LIB/clang/stable/include"
   CC="$CLANG_BIN/clang"
   CXX="$CLANG_BIN/clang++"
+  AR="$CLANG_BIN/llvm-ar"
 
   KERNEL_HEADERS_INCLUDE="$KERNEL_HEADERS_BASE/include"
 

build_tools/fbcode_config4.8.1.sh

@@ -69,6 +69,7 @@ if [ -z "$USE_CLANG" ]; then
   # gcc
   CC="$GCC_BASE/bin/gcc"
   CXX="$GCC_BASE/bin/g++"
+  CXX="$GCC_BASE/bin/gcc-ar"
 
   CFLAGS="-B$BINUTILS/gold -m64 -mtune=generic"
   CFLAGS+=" -isystem $GLIBC_INCLUDE"
@@ -81,6 +82,7 @@ else
   CLANG_INCLUDE="$CLANG_LIB/clang/*/include"
   CC="$CLANG_BIN/clang"
   CXX="$CLANG_BIN/clang++"
+  AR="$CLANG_BIN/llvm-ar"
 
   KERNEL_HEADERS_INCLUDE="$KERNEL_HEADERS_BASE/include/"
 

build_tools/fbcode_config_platform007.sh

@@ -118,6 +118,7 @@ if [ -z "$USE_CLANG" ]; then
   # gcc
   CC="$GCC_BASE/bin/gcc"
   CXX="$GCC_BASE/bin/g++"
+  AR="$GCC_BASE/bin/gcc-ar"
 
   CFLAGS+=" -B$BINUTILS/gold"
   CFLAGS+=" -isystem $LIBGCC_INCLUDE"
@@ -128,6 +129,7 @@ else
   CLANG_INCLUDE="$CLANG_LIB/clang/stable/include"
   CC="$CLANG_BIN/clang"
   CXX="$CLANG_BIN/clang++"
+  AR="$CLANG_BIN/llvm-ar"
 
   KERNEL_HEADERS_INCLUDE="$KERNEL_HEADERS_BASE/include"