<div dir="ltr">I'm curious how much of this is due to not using LLD? Last I checked, using ld.bfd instead of LLD (or gold) was one of my major build time issues, and somehow our default configuration would use ld.bfd.<div><br></div><div>-- Sean Silva</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Nov 17, 2020 at 10:25 AM Luke Drummond via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi all<br>
<br>
I wanted to do a quick build of a clean branch yesterday and noticed<br>
something surprising in the configure log:<br>
<br>
> -- No build type selected, default to Debug<br>
<br>
It appears that llvm's configuration forces Debug builds if the user<br>
does not specify the build type.<br>
<br>
<a href="https://github.com/llvm/llvm-project/blob/9218ff50f93085d0a16a974db28ca8f14bc66f64/llvm/CMakeLists.txt#L57-L60" rel="noreferrer" target="_blank">https://github.com/llvm/llvm-project/blob/9218ff50f93085d0a16a974db28ca8f14bc66f64/llvm/CMakeLists.txt#L57-L60</a><br>
<br>
I've just done a build of llvm and clang 10 in debug mode for X86 and<br>
ARM targets and it weighs in at a whopping 75GiB. I feel that forcing<br>
Debug builds in the absence of an option to be a wasteful default. It is<br>
a valid and useful thing to call cmake without specifying a build type;<br>
absence of a command line switch does not always imply absence of a<br>
choice.<br>
<br>
I took some measurements. The machine under test is an otherwise<br>
unloaded Ryzen 2 server with 6 cores / 12 threads and fast NVME SSDs<br>
running Debian sid with gcc-10.2 as the host toolchain.<br>
<br>
$ cmake ../llvm -DCMAKE_BUILD_TYPE=Debug -GNinja \<br>
-DLLVM_TARGETS_TO_BUILD=X86 -DCMAKE_EXPORT_COMPILE_COMMANDS=ON<br>
...<br>
$ time ninja llc<br>
...<br>
[1199/1199] Linking CXX executable bin/llc<br>
ninja llc 6858.95s user 218.53s system 1095% cpu 10:45.78 total<br>
<br>
and then:<br>
<br>
$ git apply <<'EOF'<br>
diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt<br>
index 21563e15118..3f5821351a1 100644<br>
--- a/llvm/CMakeLists.txt<br>
+++ b/llvm/CMakeLists.txt<br>
@@ -55,8 +55,8 @@ set(CMAKE_CXX_STANDARD_REQUIRED YES)<br>
set(CMAKE_CXX_EXTENSIONS NO)<br>
<br>
if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)<br>
- message(STATUS "No build type selected, default to Debug")<br>
- set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)" FORCE)<br>
+ # message(STATUS "No build type selected, default to Debug")<br>
+ # set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)" FORCE)<br>
endif()<br>
<br>
# Side-by-side subprojects layout: automatically set the<br>
EOF<br>
<br>
$ cmake ../llvm -DCMAKE_BUILD_TYPE= -GNinja -DLLVM_TARGETS_TO_BUILD=X86 \<br>
-DCMAKE_EXPORT_COMPILE_COMMANDS=ON<br>
...<br>
$ time ninja llc<br>
[1199/1199] Linking CXX executable bin/llc<br>
ninja llc 5714.52s user 159.31s system 1125% cpu 8:41.99 total<br>
<br>
There's two minutes difference just for an X86 capable `llc` target on<br>
this machine. I imagine there are plenty of configurations in which the<br>
ratio is *much* larger e.g. machines with slow disks and lots of cores.<br>
<br>
I think this speedup will be especially useful in CI systems where<br>
getting a compiler build quickly is more important than getting a quick<br>
compiler. I don't imagine we ever use the debuginfo generated by the<br>
host toolchain in the buildbots, either.<br>
<br>
Is there anything I'm missing, or can this override be safely deleted?<br>
Would a patch to Zorg be needed / appropriate for the fast buildbots?<br>
<br>
All the Best<br>
<br>
Luke<br>
<br>
-- <br>
Codeplay Software Ltd.<br>
Company registered in England and Wales, number: 04567874<br>
Registered office: Regent House, 316 Beulah Hill, London, SE19 3HF<br>
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</blockquote></div>