diff --git a/erts/emulator/Makefile.in b/erts/emulator/Makefile.in
index 0eaadfd18736..c446fc3815a9 100644
--- a/erts/emulator/Makefile.in
+++ b/erts/emulator/Makefile.in
@@ -209,8 +209,8 @@ LIBS += $(TYPE_LIBS)
 ORIG_LIBS:= $(LIBS)
 
 comma:=,
-space:=
-space+=
+empty:=
+space:=$(empty) $(empty)
 
 STATIC_NIFS=@STATIC_NIFS@
 ifneq ($(STATIC_NIFS),no)
@@ -351,7 +351,7 @@ endif
 
 CS_CFLAGS_ = $(TYPE_FLAGS) $(DEFS) $(WFLAGS)
 ifeq ($(GCC),yes)
-CS_CFLAGS = $(subst -O2, $(GEN_OPT_FLGS) $(UNROLL_FLG), $(CS_CFLAGS_))
+CS_CFLAGS = $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) $(UNROLL_FLG) , $(CS_CFLAGS_))
 else
 CS_CFLAGS = $(CS_CFLAGS_)
 endif
@@ -856,28 +856,28 @@ endif
 # Usually the same as the default rule, but certain platforms (e.g. win32) mix
 # different compilers
 $(OBJDIR)/beam_emu.o: beam/emu/beam_emu.c
-	$(V_EMU_CC) $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_EMU_CC) $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/beam_emu.S: beam/emu/beam_emu.c
-	$(V_EMU_CC) -S -fverbose-asm $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_EMU_CC) -S -fverbose-asm $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/%_pg.o: beam/%.c
-	$(V_CC) $(PROFILE_GENERATE) $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) $(PROFILE_GENERATE) $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/%_pg.o: beam/emu/%.c
-	$(V_CC) $(PROFILE_GENERATE) $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) $(PROFILE_GENERATE) $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/%_pu.o: beam/%.c $(PROFILE_USE_DEPS)
-	$(V_CC) $(PROFILE_USE) $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) $(PROFILE_USE) $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/%_pu.o: beam/emu/%.c $(PROFILE_USE_DEPS)
-	$(V_CC) $(PROFILE_USE) $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) $(PROFILE_USE) $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/%_pu.S: beam/%.c $(PROFILE_USE_DEPS)
-	$(V_CC) -S -fverbose-asm $(PROFILE_USE) $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) -S -fverbose-asm $(PROFILE_USE) $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/%_pu.S: beam/emu/%.c $(PROFILE_USE_DEPS)
-	$(V_CC) -S -fverbose-asm $(PROFILE_USE) $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) -S -fverbose-asm $(PROFILE_USE) $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/PROFILE: $(BINDIR)/$(PROFILE_EXECUTABLE)
 	$(V_at)echo " PROFILE ${PROFILE_EXECUTABLE}"
@@ -903,7 +903,7 @@ ifeq ($(ERTS_BUILD_FALLBACK_POLL),yes)
 # Have to treat erl_poll differently as the same .c file is used
 # twice for kernel poll builds.
 $(OBJDIR)/erl_poll.o: sys/common/erl_poll.c
-	$(V_CC) -DERTS_KERNEL_POLL_VERSION $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) -DERTS_KERNEL_POLL_VERSION $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 # Do a copy in order to make debuggers less confused
 $(TTF_DIR)/erl_poll.flbk.c: sys/common/erl_poll.c
@@ -911,7 +911,7 @@ $(TTF_DIR)/erl_poll.flbk.c: sys/common/erl_poll.c
 	@touch $@
 
 $(OBJDIR)/erl_poll.flbk.o: $(TTF_DIR)/erl_poll.flbk.c
-	$(V_CC) -DERTS_NO_KERNEL_POLL_VERSION $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) -DERTS_NO_KERNEL_POLL_VERSION $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 endif
 
 
@@ -919,13 +919,13 @@ endif
 # General targets
 #
 $(OBJDIR)/%.o: beam/%.c
-	$(V_CC) $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/%.o: beam/emu/%.c
-	$(V_CC) $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/%.o: beam/jit/%.c
-	$(V_CC) $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/%.o: $(TARGET)/%.c
 	$(V_CC) $(CFLAGS) $(INCLUDES) -Idrivers/common -c $< -o $@
@@ -937,7 +937,7 @@ $(OBJDIR)/%.o: sys/$(ERLANG_OSTYPE)/%.c
 	$(V_CC) $(CFLAGS) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/%.o: sys/common/%.c
-	$(V_CC) $(subst -O2, $(GEN_OPT_FLGS), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 
 $(OBJDIR)/%.o: drivers/common/%.c
 	$(V_CC) $(CFLAGS) -DLIBSCTP=$(LIBSCTP) $(INCLUDES) -Idrivers/common -Idrivers/$(ERLANG_OSTYPE) -c $< -o $@
@@ -968,24 +968,24 @@ ASMJIT_PCH_SRC=$(TTF_DIR)/asmjit/asmjit.hpp
 
 $(OBJDIR)/%.o: beam/jit/%.cpp $(ASMJIT_PCH_OBJ)
 	$(V_CXX) $(ASMJIT_FLAGS) $(INCLUDES)                        \
-          $(subst -O2, $(GEN_OPT_FLGS), $(CXXFLAGS))                \
+          $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CXXFLAGS))                \
           -include $(ASMJIT_PCH_SRC) -c $< -o $@
 
 $(OBJDIR)/%.o: beam/jit/$(JIT_ARCH)/%.cpp beam/jit/$(JIT_ARCH)/beam_asm.hpp $(ASMJIT_PCH_OBJ)
 	$(V_CXX) $(ASMJIT_FLAGS) $(INCLUDES)                        \
-          $(subst -O2, $(GEN_OPT_FLGS), $(CXXFLAGS))                \
+          $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CXXFLAGS))                \
           -include $(ASMJIT_PCH_SRC) -c $< -o $@
 
 $(OBJDIR)/asmjit/%.o: asmjit/%.cpp $(ASMJIT_PCH_OBJ) $(dir $@)
 	$(V_CXX) $(ASMJIT_FLAGS) $(INCLUDES)                        \
-          $(subst -O2, $(GEN_OPT_FLGS), $(ASMJIT_CXXFLAGS))         \
+          $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(ASMJIT_CXXFLAGS))         \
           -include $(ASMJIT_PCH_SRC) -c $< -o $@
 
 ## The dependency on erl_bif_info.c is in order to trigger a rebuild when
 ## the git sha define on the command line argument changes
 $(ASMJIT_PCH_OBJ): $(ASMJIT_PCH_SRC) $(ASMJIT_TTF_H) beam/erl_bif_info.c
 	$(V_CXX) $(ASMJIT_FLAGS) $(INCLUDES)                        \
-          $(subst -O2, $(GEN_OPT_FLGS), $(CXXFLAGS))                \
+          $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) , $(CXXFLAGS))                \
           -I$(TTF_DIR) -c $< -o $@
 
 ## We copy all asmjit headers into TTF_DIR in order for cross-compilation
@@ -1017,7 +1017,7 @@ $(BINDIR)/$(CS_EXECUTABLE): $(TTF_DIR)/GENERATED $(PRELOAD_SRC) $(CS_OBJ) $(ERTS
 ifeq ($(GCC),yes)
 
 $(OBJDIR)/erl_goodfit_alloc.o: beam/erl_goodfit_alloc.c
-	$(V_CC) $(subst -O2, $(GEN_OPT_FLGS) $(UNROLL_FLG), $(CFLAGS)) $(INCLUDES) -c $< -o $@
+	$(V_CC) $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) $(UNROLL_FLG) , $(CFLAGS)) $(INCLUDES) -c $< -o $@
 endif
 
 # ----------------------------------------------------------------------
@@ -1384,8 +1384,8 @@ DEP_CXX=$(CXX)
 DEP_INCLUDES=$(INCLUDES) -I../etc/win32 \
 	-Idrivers/common -Idrivers/$(ERLANG_OSTYPE) \
 	-Inifs/common -Inifs/$(ERLANG_OSTYPE)
-DEP_FLAGS=-MM  $(subst -O2,,$(CFLAGS)) $(INCLUDES) 
-DEP_CXXFLAGS=-MM $(subst -O2,,$(CXXFLAGS)) $(DEP_INCLUDES) $(ASMJIT_FLAGS)
+DEP_FLAGS=-MM  $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) ,$(CFLAGS)) $(INCLUDES)	
+DEP_CXXFLAGS=-MM $(subst $(space)-O2$(space), $(GEN_OPT_FLGS) ,$(CXXFLAGS)) $(DEP_INCLUDES) $(ASMJIT_FLAGS)
 
 # ifeq ($(USING_VC),yes)
 # VC++ used for compiling. If __GNUC__ is defined we will include
diff --git a/erts/emulator/zstd/zstd.mk b/erts/emulator/zstd/zstd.mk
index 2362be95e21f..5c1886df97cd 100644
--- a/erts/emulator/zstd/zstd.mk
+++ b/erts/emulator/zstd/zstd.mk
@@ -51,7 +51,8 @@ ifeq ($(TYPE),debug)
 ZSTD_CFLAGS = -DDEBUGLEVEL=1 $(DEBUG_CFLAGS) $(DEFS) $(THR_DEFS)
 else # !debug && !gcov
 
-ZSTD_CFLAGS = $(subst -O2, -O3, $(CONFIGURE_CFLAGS) $(DEFS) $(THR_DEFS))
+space:=$(subst ,, )
+ZSTD_CFLAGS = $(subst $(space)-O2$(space), -O3 , $(CONFIGURE_CFLAGS) $(DEFS) $(THR_DEFS))
 ifeq ($(TYPE), asan)
 ZSTD_CFLAGS += -DZSTD_ADDRESS_SANITIZER
 endif # asan
diff --git a/erts/lib_src/Makefile.in b/erts/lib_src/Makefile.in
index b92210ba74e5..28bb60c079a8 100644
--- a/erts/lib_src/Makefile.in
+++ b/erts/lib_src/Makefile.in
@@ -43,7 +43,9 @@ INSTALL_PROGRAM=@INSTALL_PROGRAM@
 ERLANG_OSTYPE=@ERLANG_OSTYPE@
 
 OMIT_FP=false
-CFLAGS=$(subst O2,O3, @CFLAGS@)
+empty:=
+space:=$(empty) $(empty)
+CFLAGS:=$(subst $(space)-O2$(space), -O3 , @CFLAGS@)
 LDFLAGS=@LDFLAGS@
 
 ifeq ($(TYPE),debug)