diff --git a/.github/actions/setup-build-env/action.yml b/.github/actions/setup-build-env/action.yml
index 345b6db8ada..c5a0d14abdf 100644
--- a/.github/actions/setup-build-env/action.yml
+++ b/.github/actions/setup-build-env/action.yml
@@ -14,7 +14,7 @@ runs:
       if: runner.os == 'macOS'
       shell: bash
       run: |
-        HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK=1 brew install bison flex gawk libffi pkg-config bash autoconf
+        HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK=1 brew install bison flex gawk libffi pkg-config bash autoconf llvm
 
     - name: Linux runtime environment
       if: runner.os == 'Linux'
@@ -28,6 +28,7 @@ runs:
       shell: bash
       run: |
         echo "${{ github.workspace }}/.local/bin" >> $GITHUB_PATH
+        echo "$(brew --prefix llvm)/bin" >> $GITHUB_PATH
         echo "$(brew --prefix bison)/bin" >> $GITHUB_PATH
         echo "$(brew --prefix flex)/bin" >> $GITHUB_PATH
         echo "procs=$(sysctl -n hw.ncpu)" >> $GITHUB_ENV
diff --git a/Brewfile b/Brewfile
index b50c70a4b0e..18e4e2917e9 100644
--- a/Brewfile
+++ b/Brewfile
@@ -9,4 +9,5 @@ brew "python3"
 brew "tcl-tk"
 brew "xdot"
 brew "bash"
-brew 'boost-python3'
+brew "boost-python3"
+brew "llvm"
diff --git a/Dockerfile b/Dockerfile
index 549c73c9712..9806696e051 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -8,6 +8,7 @@ RUN apt-get update -qq \
  && DEBIAN_FRONTEND=noninteractive apt-get -y install --no-install-recommends \
     ca-certificates \
     clang \
+    lld \
     curl \
     libffi-dev \
     libreadline-dev \
diff --git a/Makefile b/Makefile
index 456aa2a40ae..bc6577714ba 100644
--- a/Makefile
+++ b/Makefile
@@ -34,7 +34,7 @@ ENABLE_PYOSYS := 0
 ENABLE_GCOV := 0
 ENABLE_GPROF := 0
 ENABLE_DEBUG := 0
-ENABLE_NDEBUG := 0
+ENABLE_LTO := 1
 ENABLE_CCACHE := 0
 # sccache is not always a drop-in replacement for ccache in practice
 ENABLE_SCCACHE := 0
@@ -53,6 +53,11 @@ SANITIZER =
 # SANITIZER = undefined
 # SANITIZER = cfi
 
+# Prefer using ENABLE_DEBUG over setting these
+OPT_LEVEL := -O3
+GCC_LTO :=
+CLANG_LTO := -flto=thin
+
 PROGRAM_PREFIX :=
 
 OS := $(shell uname -s)
@@ -212,10 +217,15 @@ ifeq ($(OS), OpenBSD)
 ABC_ARCHFLAGS += "-DABC_NO_RLIMIT"
 endif
 
+# This gets overridden later.
+LTOFLAGS := $(GCC_LTO)
+
 ifeq ($(CONFIG),clang)
 CXX = clang++
-CXXFLAGS += -std=$(CXXSTD) -Os
+CXXFLAGS += -std=$(CXXSTD) $(OPT_LEVEL)
+LINKFLAGS += -fuse-ld=lld
 ABCMKARGS += ARCHFLAGS="-DABC_USE_STDINT_H $(ABC_ARCHFLAGS)"
+LTOFLAGS := $(CLANG_LTO)
 
 ifneq ($(SANITIZER),)
 $(info [Clang Sanitizer] $(SANITIZER))
@@ -231,19 +241,20 @@ endif
 ifneq ($(findstring cfi,$(SANITIZER)),)
 CXXFLAGS += -flto
 LINKFLAGS += -flto
+LTOFLAGS =
 endif
 endif
 
 else ifeq ($(CONFIG),gcc)
 CXX = g++
-CXXFLAGS += -std=$(CXXSTD) -Os
+CXXFLAGS += -std=$(CXXSTD) $(OPT_LEVEL)
 ABCMKARGS += ARCHFLAGS="-DABC_USE_STDINT_H $(ABC_ARCHFLAGS)"
 
 else ifeq ($(CONFIG),gcc-static)
 LINKFLAGS := $(filter-out -rdynamic,$(LINKFLAGS)) -static
 LIBS := $(filter-out -lrt,$(LIBS))
 CXXFLAGS := $(filter-out -fPIC,$(CXXFLAGS))
-CXXFLAGS += -std=$(CXXSTD) -Os
+CXXFLAGS += -std=$(CXXSTD) $(OPT_LEVEL)
 ABCMKARGS = CC="$(CC)" CXX="$(CXX)" LD="$(CXX)" ABC_USE_LIBSTDCXX=1 LIBS="-lm -lpthread -static" OPTFLAGS="-O" \
                        ARCHFLAGS="-DABC_USE_STDINT_H -DABC_NO_DYNAMIC_LINKING=1 -Wno-unused-but-set-variable $(ARCHFLAGS)" ABC_USE_NO_READLINE=1
 ifeq ($(DISABLE_ABC_THREADS),1)
@@ -252,12 +263,12 @@ endif
 
 else ifeq ($(CONFIG),afl-gcc)
 CXX = AFL_QUIET=1 AFL_HARDEN=1 afl-gcc
-CXXFLAGS += -std=$(CXXSTD) -Os
+CXXFLAGS += -std=$(CXXSTD) $(OPT_LEVEL)
 ABCMKARGS += ARCHFLAGS="-DABC_USE_STDINT_H"
 
 else ifeq ($(CONFIG),cygwin)
 CXX = g++
-CXXFLAGS += -std=gnu++11 -Os
+CXXFLAGS += -std=gnu++11 $(OPT_LEVEL)
 ABCMKARGS += ARCHFLAGS="-DABC_USE_STDINT_H"
 
 else ifeq ($(CONFIG),wasi)
@@ -272,7 +283,7 @@ AR = $(WASI_SDK)/bin/ar
 RANLIB = $(WASI_SDK)/bin/ranlib
 WASIFLAGS := --sysroot $(WASI_SDK)/share/wasi-sysroot $(WASIFLAGS)
 endif
-CXXFLAGS := $(WASIFLAGS) -std=$(CXXSTD) -Os -D_WASI_EMULATED_PROCESS_CLOCKS $(filter-out -fPIC,$(CXXFLAGS))
+CXXFLAGS := $(WASIFLAGS) -std=$(CXXSTD) $(OPT_LEVEL) -D_WASI_EMULATED_PROCESS_CLOCKS $(filter-out -fPIC,$(CXXFLAGS))
 LINKFLAGS := $(WASIFLAGS) -Wl,-z,stack-size=1048576 $(filter-out -rdynamic,$(LINKFLAGS))
 LIBS := -lwasi-emulated-process-clocks $(filter-out -lrt,$(LIBS))
 ABCMKARGS += AR="$(AR)" RANLIB="$(RANLIB)"
@@ -290,7 +301,7 @@ endif
 else ifeq ($(CONFIG),mxe)
 PKG_CONFIG = /usr/local/src/mxe/usr/bin/i686-w64-mingw32.static-pkg-config
 CXX = /usr/local/src/mxe/usr/bin/i686-w64-mingw32.static-g++
-CXXFLAGS += -std=$(CXXSTD) -Os -D_POSIX_SOURCE -DYOSYS_MXE_HACKS -Wno-attributes
+CXXFLAGS += -std=$(CXXSTD) $(OPT_LEVEL) -D_POSIX_SOURCE -DYOSYS_MXE_HACKS -Wno-attributes
 CXXFLAGS := $(filter-out -fPIC,$(CXXFLAGS))
 LINKFLAGS := $(filter-out -rdynamic,$(LINKFLAGS)) -s
 LIBS := $(filter-out -lrt,$(LIBS))
@@ -301,7 +312,7 @@ EXE = .exe
 
 else ifeq ($(CONFIG),msys2-32)
 CXX = i686-w64-mingw32-g++
-CXXFLAGS += -std=$(CXXSTD) -Os -D_POSIX_SOURCE -DYOSYS_WIN32_UNIX_DIR
+CXXFLAGS += -std=$(CXXSTD) $(OPT_LEVEL) -D_POSIX_SOURCE -DYOSYS_WIN32_UNIX_DIR
 CXXFLAGS := $(filter-out -fPIC,$(CXXFLAGS))
 LINKFLAGS := $(filter-out -rdynamic,$(LINKFLAGS)) -s
 LIBS := $(filter-out -lrt,$(LIBS))
@@ -311,7 +322,7 @@ EXE = .exe
 
 else ifeq ($(CONFIG),msys2-64)
 CXX = x86_64-w64-mingw32-g++
-CXXFLAGS += -std=$(CXXSTD) -Os -D_POSIX_SOURCE -DYOSYS_WIN32_UNIX_DIR
+CXXFLAGS += -std=$(CXXSTD) $(OPT_LEVEL) -D_POSIX_SOURCE -DYOSYS_WIN32_UNIX_DIR
 CXXFLAGS := $(filter-out -fPIC,$(CXXFLAGS))
 LINKFLAGS := $(filter-out -rdynamic,$(LINKFLAGS)) -s
 LIBS := $(filter-out -lrt,$(LIBS))
@@ -320,13 +331,20 @@ ABCMKARGS += LIBS="-lpthread -lshlwapi -s" ABC_USE_NO_READLINE=0 CC="x86_64-w64-
 EXE = .exe
 
 else ifeq ($(CONFIG),none)
-CXXFLAGS += -std=$(CXXSTD) -Os
+CXXFLAGS += -std=$(CXXSTD) $(OPT_LEVEL)
 ABCMKARGS += ARCHFLAGS="-DABC_USE_STDINT_H $(ABC_ARCHFLAGS)"
+LTOFLAGS =
 
 else
 $(error Invalid CONFIG setting '$(CONFIG)'. Valid values: clang, gcc, mxe, msys2-32, msys2-64, none)
 endif
 
+
+ifeq ($(ENABLE_LTO),1)
+CXXFLAGS += $(LTOFLAGS)
+LINKFLAGS += $(LTOFLAGS)
+endif
+
 ifeq ($(ENABLE_LIBYOSYS),1)
 TARGETS += libyosys.so
 endif
@@ -441,16 +459,8 @@ CXXFLAGS += -pg
 LINKFLAGS += -pg
 endif
 
-ifeq ($(ENABLE_NDEBUG),1)
-CXXFLAGS := -O3 -DNDEBUG $(filter-out -Os -ggdb,$(CXXFLAGS))
-endif
-
 ifeq ($(ENABLE_DEBUG),1)
-ifeq ($(CONFIG),clang)
-CXXFLAGS := -O0 -DDEBUG $(filter-out -Os,$(CXXFLAGS))
-else
-CXXFLAGS := -Og -DDEBUG $(filter-out -Os,$(CXXFLAGS))
-endif
+CXXFLAGS := -Og -DDEBUG $(filter-out $(OPT_LEVEL),$(CXXFLAGS))
 endif
 
 ifeq ($(ENABLE_ABC),1)
diff --git a/README.md b/README.md
index bb1c4d443e4..59637b9bbb1 100644
--- a/README.md
+++ b/README.md
@@ -79,7 +79,7 @@ Xdot (graphviz) is used by the ``show`` command in yosys to display schematics.
 For example on Ubuntu Linux 16.04 LTS the following commands will install all
 prerequisites for building yosys:
 
-	$ sudo apt-get install build-essential clang bison flex \
+	$ sudo apt-get install build-essential clang lld bison flex \
 		libreadline-dev gawk tcl-dev libffi-dev git \
 		graphviz xdot pkg-config python3 libboost-system-dev \
 		libboost-python-dev libboost-filesystem-dev zlib1g-dev
diff --git a/docs/source/getting_started/installation.rst b/docs/source/getting_started/installation.rst
index 1a89886089b..aea8b6a2dd2 100644
--- a/docs/source/getting_started/installation.rst
+++ b/docs/source/getting_started/installation.rst
@@ -94,7 +94,7 @@ Installing all prerequisites for Ubuntu 20.04:
 
 .. code:: console
 
-   sudo sudo apt-get install build-essential clang bison flex \
+   sudo sudo apt-get install build-essential clang lld bison flex \
       libreadline-dev gawk tcl-dev libffi-dev git make \
       graphviz xdot pkg-config python3 libboost-system-dev \
       libboost-python-dev libboost-filesystem-dev zlib1g-dev
diff --git a/flake.nix b/flake.nix
index 1e202324647..90fa5328c09 100644
--- a/flake.nix
+++ b/flake.nix
@@ -18,7 +18,7 @@
         yosys = pkgs.clangStdenv.mkDerivation {
           name = "yosys";
           src = ./. ;
-          buildInputs = with pkgs; [ clang bison flex libffi tcl readline python3 llvmPackages.libcxxClang zlib git pkg-configUpstream ];
+          buildInputs = with pkgs; [ clang bison flex libffi tcl readline python3 llvmPackages.libcxxClang zlib git pkg-configUpstream llvmPackages.bintools ];
           checkInputs = with pkgs; [ gtest ];
           propagatedBuildInputs = [ abc-verifier ];
           preConfigure = "make config-clang";
@@ -41,7 +41,7 @@
         packages.default = yosys;
         defaultPackage = yosys;
         devShell = pkgs.mkShell {
-          buildInputs = with pkgs; [ clang bison flex libffi tcl readline python3 llvmPackages.libcxxClang zlib git gtest abc-verifier ];
+          buildInputs = with pkgs; [ clang llvmPackages.bintools bison flex libffi tcl readline python3 llvmPackages.libcxxClang zlib git gtest abc-verifier ];
         };
       }
     );