From 166a147ee472c9f2374df67511f782f9723726f6 Mon Sep 17 00:00:00 2001
From: Travis Geiselbrecht <travisg@gmail.com>
Date: Sun, 12 Jul 2020 13:17:49 -0700
Subject: [PATCH] WIP [LTO] try to build and link with LTO

Doesn't work, it strips all of the magic sections.
---
 engine.mk      |  5 +++++
 make/build.mk  |  6 ++++++
 make/module.mk | 18 ++++++++++++------
 3 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/engine.mk b/engine.mk
index 8b3051ee..4b80f5cf 100644
--- a/engine.mk
+++ b/engine.mk
@@ -64,6 +64,11 @@ GLOBAL_CPPFLAGS := --std=c++11 -fno-exceptions -fno-rtti -fno-threadsafe-statics
 GLOBAL_ASMFLAGS := -DASSEMBLY
 GLOBAL_LDFLAGS :=
 
+WITH_LTO ?= true
+ifeq (true,$(call TOBOOL,$(WITH_LTO)))
+GLOBAL_COMPILEFLAGS += -flto -fno-fat-lto-objects
+endif
+
 # if WERROR is set, add to the compile args
 ifeq (true,$(call TOBOOL,$(WERROR)))
 GLOBAL_COMPILEFLAGS += -Werror
diff --git a/make/build.mk b/make/build.mk
index 49d26b3a..b017cb9b 100644
--- a/make/build.mk
+++ b/make/build.mk
@@ -21,10 +21,16 @@ $(OUTELF).hex: $(OUTELF)
 
 $(OUTELF): $(ALLMODULE_OBJS) $(EXTRA_OBJS) $(LINKER_SCRIPT) $(EXTRA_LINKER_SCRIPTS)
 	$(info linking $@)
+ifeq (true,$(call TOBOOL,$(WITH_LTO)))
+	$(NOECHO)$(CC) -nostdlib -flto $(GLOBAL_LDFLAGS) $(ARCH_LDFLAGS) -T $(LINKER_SCRIPT) \
+		$(addprefix -T,$(EXTRA_LINKER_SCRIPTS)) -march=rv64imac -mabi=lp64 -mcmodel=medany \
+		$(ALLMODULE_OBJS) $(EXTRA_OBJS) $(LIBGCC) -o $@
+else
 	$(NOECHO)$(SIZE) -t --common $(sort $(ALLMODULE_OBJS)) $(EXTRA_OBJS)
 	$(NOECHO)$(LD) $(GLOBAL_LDFLAGS) $(ARCH_LDFLAGS) -dT $(LINKER_SCRIPT) \
 		$(addprefix -T,$(EXTRA_LINKER_SCRIPTS)) \
 		$(ALLMODULE_OBJS) $(EXTRA_OBJS) $(LIBGCC) -Map=$(OUTELF).map -o $@
+endif
 
 $(OUTELF).sym: $(OUTELF)
 	$(info generating symbols: $@)
diff --git a/make/module.mk b/make/module.mk
index dcd97936..386fc51c 100644
--- a/make/module.mk
+++ b/make/module.mk
@@ -83,6 +83,13 @@ include make/compile.mk
 # MODULE_OBJS is passed back from compile.mk
 #$(info MODULE_OBJS = $(MODULE_OBJS))
 
+# track all of the source files compiled
+ALLSRCS += $(MODULE_SRCS)
+
+# track all the objects built
+ALLOBJS += $(MODULE_OBJS)
+
+ifeq (false,$(call TOBOOL,$(WITH_LTO)))
 # build a ld -r style combined object
 MODULE_OBJECT := $(call TOBUILDDIR,$(MODULE_SRCDIR).mod.o)
 $(MODULE_OBJECT): $(MODULE_OBJS) $(MODULE_EXTRA_OBJS)
@@ -90,17 +97,16 @@ $(MODULE_OBJECT): $(MODULE_OBJS) $(MODULE_EXTRA_OBJS)
 	$(info linking $@)
 	$(NOECHO)$(LD) $(GLOBAL_MODULE_LDFLAGS) -r $^ -o $@
 
-# track all of the source files compiled
-ALLSRCS += $(MODULE_SRCS)
-
-# track all the objects built
-ALLOBJS += $(MODULE_OBJS)
-
 # track the module object for make clean
 GENERATED += $(MODULE_OBJECT)
 
 # make the rest of the build depend on our output
 ALLMODULE_OBJS := $(ALLMODULE_OBJS) $(MODULE_OBJECT)
+else
+# for LTO builds just add all of the .o files from this module to the global
+# link line. ld -r seems to foul up LTO
+ALLMODULE_OBJS := $(ALLMODULE_OBJS) $(MODULE_OBJS) $(MODULE_EXTRA_OBJS)
+endif
 
 # empty out any vars set here
 MODULE :=