Merge #17929: build: add linker optimisation flags to gitian & guix (Linux)

f2b5b0a3b48f9241971c14238040048f8b630811 build: add linker optimization flags to guix (fanquake)
b8b050a8d642e38c81d1e451750c2c8db92fee5e build: add linker optimization flags to gitian descriptors (fanquake)

Pull request description:

  This PR adds `-Wl,O2` to our gitian and guix LDFLAGS. This makes the linker perform certain optimisations (and is different from LTO).

  Any -O argument will enable optimizations in GNU ld. We can use -O2 here, as this matches our compile flags. Note that this would also enable additional optimizations if using the lld or gold linkers, when compared to -O0.

  A nice writeup + diagrams of some of these optimizations is  available here: http://lwn.net/Articles/192624/.

  #### master
  ```bash
  # bitcoind
  Histogram for `.gnu.hash' bucket list length (total of 3 buckets)
   Length  Number     % of total  Coverage
        0  1          ( 33.3%)       0.0%
        1  0          (  0.0%)       0.0%
        2  1          ( 33.3%)      40.0%
        3  1          ( 33.3%)     100.0%
  ```
  ```bash
  # bitcoin-qt
  Histogram for `.gnu.hash' bucket list length (total of 3 buckets)
   Length  Number     % of total  Coverage
        0  0          (  0.0%)       0.0%
        1  1          ( 33.3%)      10.0%
        2  0          (  0.0%)      10.0%
        3  0          (  0.0%)      10.0%
        4  1          ( 33.3%)      50.0%
        5  1          ( 33.3%)     100.0%
  ```

  #### this PR:
  ```bash
  # bitcoind
  Histogram for `.gnu.hash' bucket list length (total of 8 buckets)
   Length  Number     % of total  Coverage
        0  3          ( 37.5%)       0.0%
        1  5          ( 62.5%)     100.0%
  ```
  ```bash
  # bitcoin-qt
  Histogram for `.gnu.hash' bucket list length (total of 19 buckets)
   Length  Number     % of total  Coverage
        0  9          ( 47.4%)       0.0%
        1  10         ( 52.6%)     100.0%
  ```

  #### GNU ld -O

  > If level is a numeric values greater than zero ld optimizes the output. This might take significantly longer and therefore probably should only be enabled for the final binary. At the moment this option only affects ELF shared library generation. Future releases of the linker may make more use of this option. Also currently there is no difference in the linker’s behaviour for different non-zero values of this option. Again this may change with future releases.

  #### lld -O

  > Optimize output file size

ACKs for top commit:
  dongcarl:
    ACK f2b5b0a3b48f9241971c14238040048f8b630811
  laanwj:
    ACK f2b5b0a3b48f9241971c14238040048f8b630811

Tree-SHA512: e53f3a4338317dbec65d3a93b57b5a6204aabdf9ac82d99447847a3c8627facc53c58c2cf947376f13edd979fc8129a80f18d9ebeccd191a576c83f1dad5c513

2 files changed, 2 insertions, 2 deletions

diff --git a/contrib/gitian-descriptors/gitian-linux.yml b/contrib/gitian-descriptors/gitian-linux.yml
index 5d190de54..6f79d10e6 100644
--- a/contrib/gitian-descriptors/gitian-linux.yml
+++ b/contrib/gitian-descriptors/gitian-linux.yml
@@ -46,7 +46,7 @@ script: |
   FAKETIME_PROGS="date ar ranlib nm"
   HOST_CFLAGS="-O2 -g"
   HOST_CXXFLAGS="-O2 -g"
-  HOST_LDFLAGS_BASE="-static-libstdc++"
+  HOST_LDFLAGS_BASE="-static-libstdc++ -Wl,-O2"
 
   export QT_RCC_TEST=1
   export QT_RCC_SOURCE_DATE_OVERRIDE=1
diff --git a/contrib/guix/libexec/build.sh b/contrib/guix/libexec/build.sh
index 6ef803340..550b1b8f4 100644
--- a/contrib/guix/libexec/build.sh
+++ b/contrib/guix/libexec/build.sh
@@ -177,7 +177,7 @@ HOST_CXXFLAGS="$HOST_CFLAGS"
 
 # LDFLAGS
 case "$HOST" in
-    *linux*)  HOST_LDFLAGS="-Wl,--as-needed -Wl,--dynamic-linker=$glibc_dynamic_linker -static-libstdc++" ;;
+    *linux*)  HOST_LDFLAGS="-Wl,--as-needed -Wl,--dynamic-linker=$glibc_dynamic_linker -static-libstdc++ -Wl,-O2" ;;
     *mingw*)  HOST_LDFLAGS="-Wl,--no-insert-timestamp" ;;
 esac