Hi all
I wanted to do a quick build of a clean branch yesterday and noticed
something surprising in the configure log:
> -- No build type selected, default to Debug
It appears that llvm's configuration forces Debug builds if the user
does not specify the build type.
https://github.com/llvm/llvm-project/blob/9218ff50f93085d0a16a974db28ca8f14bc66f64/llvm/CMakeLists.txt#L57-L60
I've just done a build of llvm and clang 10 in debug mode for X86 and
ARM targets and it weighs in at a whopping 75GiB. I feel that forcing
Debug builds in the absence of an option to be a wasteful default. It is
a valid and useful thing to call cmake without specifying a build type;
absence of a command line switch does not always imply absence of a
choice.
I took some measurements. The machine under test is an otherwise
unloaded Ryzen 2 server with 6 cores / 12 threads and fast NVME SSDs
running Debian sid with gcc-10.2 as the host toolchain.
$ cmake ../llvm -DCMAKE_BUILD_TYPE=Debug -GNinja \
-DLLVM_TARGETS_TO_BUILD=X86 -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
...
$ time ninja llc
...
[1199/1199] Linking CXX executable bin/llc
ninja llc 6858.95s user 218.53s system 1095% cpu 10:45.78 total
and then:
$ git apply <<'EOF'
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 21563e15118..3f5821351a1 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -55,8 +55,8 @@ set(CMAKE_CXX_STANDARD_REQUIRED YES)
set(CMAKE_CXX_EXTENSIONS NO)
if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
- message(STATUS "No build type selected, default to Debug")
- set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)" FORCE)
+ # message(STATUS "No build type selected, default to Debug")
+ # set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)" FORCE)
endif()
# Side-by-side subprojects layout: automatically set the
EOF
$ cmake ../llvm -DCMAKE_BUILD_TYPE= -GNinja -DLLVM_TARGETS_TO_BUILD=X86 \
-DCMAKE_EXPORT_COMPILE_COMMANDS=ON
...
$ time ninja llc
[1199/1199] Linking CXX executable bin/llc
ninja llc 5714.52s user 159.31s system 1125% cpu 8:41.99 total
There's two minutes difference just for an X86 capable `llc` target on
this machine. I imagine there are plenty of configurations in which the
ratio is *much* larger e.g. machines with slow disks and lots of cores.
I think this speedup will be especially useful in CI systems where
getting a compiler build quickly is more important than getting a quick
compiler. I don't imagine we ever use the debuginfo generated by the
host toolchain in the buildbots, either.
Is there anything I'm missing, or can this override be safely deleted?
Would a patch to Zorg be needed / appropriate for the fast buildbots?
All the Best
Luke
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