Makefile: no LTO and lld by default

.github/workflows/test-build.yml

@@ -56,7 +56,7 @@ jobs:
           mkdir build
           cd build
           make -f ../Makefile config-$CC
-          make -f ../Makefile -j$procs
+          make -f ../Makefile -j$procs ENABLE_LTO=1
 
       - name: Log yosys-config output
         run: |

.github/workflows/test-verific.yml

@@ -41,7 +41,7 @@ jobs:
           echo "ENABLE_VERIFIC_LIBERTY := 1" >> Makefile.conf
           echo "ENABLE_VERIFIC_YOSYSHQ_EXTENSIONS := 1" >> Makefile.conf
           echo "ENABLE_CCACHE := 1" >> Makefile.conf
-          make -j${{ env.procs }}
+          make -j${{ env.procs }} ENABLE_LTO=1
 
       - name: Install Yosys
         run: |

Makefile

@@ -34,7 +34,7 @@ ENABLE_PYOSYS := 0
 ENABLE_GCOV := 0
 ENABLE_GPROF := 0
 ENABLE_DEBUG := 0
-ENABLE_LTO := 1
+ENABLE_LTO := 0
 ENABLE_CCACHE := 0
 # sccache is not always a drop-in replacement for ccache in practice
 ENABLE_SCCACHE := 0
@@ -223,7 +223,9 @@ LTOFLAGS := $(GCC_LTO)
 ifeq ($(CONFIG),clang)
 CXX = clang++
 CXXFLAGS += -std=$(CXXSTD) $(OPT_LEVEL)
+ifeq ($(ENABLE_LTO),1)
 LINKFLAGS += -fuse-ld=lld
+endif
 ABCMKARGS += ARCHFLAGS="-DABC_USE_STDINT_H $(ABC_ARCHFLAGS)"
 LTOFLAGS := $(CLANG_LTO)
 

README.md

@@ -116,6 +116,17 @@ unless `CXX` is assigned in the call to make, e.g.
 
   $ make CXX=$CXX
 
+The Makefile has many variables influencing the build process. These can be
+adjusted by modifying the Makefile.conf file which is created at the
+`make config-...` step (see above), or they can be set by passing an option
+to the make command directly.
+
+For example, if you have clang, and (a compatible version of) `ld.lld`
+available in PATH, it's recommended to speed up incremental builds with
+lld by enabling LTO:
+
+ $ make ENABLE_LTO=1
+
 For other compilers and build configurations it might be
 necessary to make some changes to the config section of the
 Makefile.