From 11db9385c39155441ee97d986d5284109a070102 Mon Sep 17 00:00:00 2001
From: Matthew Pickering <matthewtpickering@gmail.com>
Date: Wed, 5 Jan 2022 11:48:52 +0000
Subject: Add support for using mold linker

If mold is on your path when configuring, now GHC will use the
`-fuse-ld=mold` flag which very recent clang and in-future GCC 12 will
support.

Fixes #20871
---
 compiler/GHC/Driver/Session.hs | 1 +
 compiler/GHC/SysTools/Info.hs  | 3 +++
 m4/find_ld.m4                  | 4 +++-
 3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/compiler/GHC/Driver/Session.hs b/compiler/GHC/Driver/Session.hs
index b0b37a822c..3a6813e9a1 100644
--- a/compiler/GHC/Driver/Session.hs
+++ b/compiler/GHC/Driver/Session.hs
@@ -4840,6 +4840,7 @@ sccProfilingEnabled dflags = profileIsProfiling (targetProfile dflags)
 data LinkerInfo
   = GnuLD    [Option]
   | GnuGold  [Option]
+  | Mold     [Option]
   | LlvmLLD  [Option]
   | DarwinLD [Option]
   | SolarisLD [Option]
diff --git a/compiler/GHC/SysTools/Info.hs b/compiler/GHC/SysTools/Info.hs
index 83a76b9efb..8c2f62017d 100644
--- a/compiler/GHC/SysTools/Info.hs
+++ b/compiler/GHC/SysTools/Info.hs
@@ -97,6 +97,7 @@ https://gcc.gnu.org/onlinedocs/gcc-5.2.0/gcc.pdf :
 neededLinkArgs :: LinkerInfo -> [Option]
 neededLinkArgs (GnuLD o)     = o
 neededLinkArgs (GnuGold o)   = o
+neededLinkArgs (Mold o)      = o
 neededLinkArgs (LlvmLLD o)   = o
 neededLinkArgs (DarwinLD o)  = o
 neededLinkArgs (SolarisLD o) = o
@@ -126,6 +127,8 @@ getLinkerInfo' logger dflags = do
 
       -- Try to grab the info from the process output.
       parseLinkerInfo stdo _stde _exitc
+        | any ("mold" `isPrefixOf`) stdo =
+          return (Mold [Option "-Wl,--no-as-needed"])
         | any ("GNU ld" `isPrefixOf`) stdo =
           -- GNU ld specifically needs to use less memory. This especially
           -- hurts on small object files. #5240.
diff --git a/m4/find_ld.m4 b/m4/find_ld.m4
index 3d4443be54..985ec24d4d 100644
--- a/m4/find_ld.m4
+++ b/m4/find_ld.m4
@@ -24,13 +24,15 @@ AC_DEFUN([FIND_LD],[
         # Manually iterate over possible names since we want to ensure that, e.g.,
         # if ld.lld is installed but gcc doesn't support -fuse-ld=lld, that we
         # then still try ld.gold and -fuse-ld=gold.
-        for possible_ld in ld.lld ld.gold ld; do
+        for possible_ld in mold ld.lld ld.gold ld; do
             TmpLd="" # In case the user set LD
             AC_CHECK_TARGET_TOOL([TmpLd], [$possible_ld])
             if test "x$TmpLd" = "x"; then continue; fi
 
             out=`$TmpLd --version`
             case $out in
+              "mold"*)
+                   FP_CC_LINKER_FLAG_TRY(mold, $2) ;;
               "GNU ld"*)
                    FP_CC_LINKER_FLAG_TRY(bfd, $2) ;;
               "GNU gold"*)
-- 
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