pw_toolchain/arm_clang: Reduce binary size

Update size optimization flags for Clang toolchain built artifacts.

Bug: b/254541584
Docs: N/A
Change-Id: Ic4ad573709f687fc4ba052608256247cf2293726
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/169576
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Prabhu Karthikeyan Rajasekaran <prabhukr@google.com>

pw_build/BUILD.gn

@@ -65,9 +65,34 @@ config("optimize_size") {
   ldflags = cflags
 }
 
+config("enable_clang_mlinliner") {
+  cflags = [
+    # Enable ML inliner.
+    "-mllvm",
+    "-enable-ml-inliner=release",
+  ]
+}
+
+config("enable_clang_gvn_sink_hoist") {
+  cflags = [
+    # Enable GVN sink/hoist.
+    "-mllvm",
+    "-enable-gvn-sink=1",
+    "-mllvm",
+    "-enable-gvn-hoist=1",
+  ]
+}
+
 config("optimize_size_clang") {
   cflags = [ "-Oz" ]
-  ldflags = cflags
+  ldflags = [
+    # Identical Code Folding optimization
+    "-Wl,--icf=all",
+
+    # LLD does not recognize `Oz` which is a compiler front-end flag.
+    "-Wl,-O2",
+  ]
+  ldflags += cflags
 }
 
 # Standard compiler flags to reduce output binary size.
@@ -77,6 +102,7 @@ config("reduced_size") {
     "-fno-exceptions",
     "-ffunction-sections",
     "-fdata-sections",
+    "-fomit-frame-pointer",
   ]
   cflags_cc = [ "-fno-rtti" ]
 

pw_toolchain/arm_clang/toolchains.gni

@@ -92,8 +92,11 @@ pw_toolchain_arm_clang = {
     name = "arm_clang_cortex_m0plus_size_optimized"
     forward_variables_from(_arm_clang_toolchain, "*")
     defaults = {
-      default_configs = pigweed_default_configs + _cortex_m0plus +
-                        [ "$dir_pw_build:optimize_size_clang" ]
+      default_configs = pigweed_default_configs + _cortex_m0plus + [
+                          "$dir_pw_build:optimize_size_clang",
+                          "$dir_pw_build:enable_clang_mlinliner",
+                          "$dir_pw_build:enable_clang_gvn_sink_hoist",
+                        ]
     }
   }
   cortex_m3_debug = {