[XS] Change in coreboot[main]: util/crossgcc: Link libLLVM dynamically reducing build time with ccache

[Felix Singer (Code Review)]

Felix Singer has uploaded this change for review. ( https://review.coreboot.org/c/coreboot/+/81673?usp=email )

Change subject: util/crossgcc: Link libLLVM dynamically reducing build time with ccache
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util/crossgcc: Link libLLVM dynamically reducing build time with ccache

When coreboot is built with Clang and ccache enabled, the build time is
incredibly long and it feels like that an existing cache is not used for
the build. This is not the case though since the statistics created by
ccache proof that the cache is used.

While tracking down the issue, it led to our configuration of the build
system, which configures the ccache option `CCACHE_COMPILERCHECK` and
sets it to `content`. Enabling this option makes ccache including the
state of the compiler binary in the hash in some way. While the default
configuration (`mtime`) only includes the modification time and file
size, the mode `content` hashes the compiler binary instead. Depending
on its file size, the time to calculate the hash might vary a lot.
Looking at the current sizes of the compiler binaries, it's 1.4 MB for
GCC and 155 MB for Clang.

However, the huge file size for the Clang binary originates from that
libLLVM is linked statically. Linking libLLVM dynamically reduces the
file size of the Clang binary to 195 KB.

The build times compared without and with this patch using Clang as
compiler with ccache enabled and QEMU Q35 with payload disabled:

Static, no cache (first build)

real    1m45.128s
    user    0m42.828s
    sys     1m2.428s

Static, with cache (second build)

real    1m17.927s
    user    0m22.290s
    sys     0m55.842s

Dynamic, no cache (first build)

real    0m32.648s
    user    0m24.244s
    sys     0m8.609s

Dynamic, with cache (second build)

real    0m3.635s
    user    0m2.250s
    sys     0m1.630s

Change-Id: I62d4e3fd8ce7f54bd12b1e62eca4beb1259c3da8
Signed-off-by: Felix Singer 
---
M util/crossgcc/buildgcc
1 file changed, 1 insertion(+), 0 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/73/81673/1

diff --git a/util/crossgcc/buildgcc b/util/crossgcc/buildgcc
index b1cb02c..3dd8d63 100755
--- a/util/crossgcc/buildgcc
+++ b/util/crossgcc/buildgcc
@@ -890,6 +890,7 @@
    	-DLLVM_INCLUDE_EXAMPLES="OFF" \
    	-DCMAKE_BUILD_TYPE=Release \
    	-DLLVM_TARGETS_TO_BUILD="AArch64;ARM;PowerPC;RISCV;X86" \
+		-DLLVM_LINK_LLVM_DYLIB="ON" \
    	../llvm || touch .failed
    # shellcheck disable=SC2086
    $MAKE $JOBS || touch .failed