From 4de4221ab4b645ff77503c777c9b195a962e9fa1 Mon Sep 17 00:00:00 2001
From: Andrew Chow <achow101-github@achow101.com>
Date: Mon, 11 Jul 2022 20:26:25 -0400
Subject: [PATCH] build: Check for std::atomic::exchange rather than
 std::atomic_exchange

Our usage of std::atomic is with it's own exchange function, not
std::atomic_exchange. So we should be looking specifically for that
function.

Additionally, -pthread and -lpthread have an effect on whether -latomic
will be needed, so the atomics check needs to use these flags as well.
This will make the flags in use better match what is actually used when
linking.

This removes the need for -latomic for riscv builds, which resolves a
guix cross architecture reproducibility issue.
---
 build-aux/m4/l_atomic.m4 | 5 ++++-
 configure.ac             | 6 +++---
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/build-aux/m4/l_atomic.m4 b/build-aux/m4/l_atomic.m4
index 40639dfe618..602b57fe432 100644
--- a/build-aux/m4/l_atomic.m4
+++ b/build-aux/m4/l_atomic.m4
@@ -18,7 +18,7 @@ m4_define([_CHECK_ATOMIC_testbody], [[
 
   int main() {
     std::atomic<bool> lock{true};
-    std::atomic_exchange(&lock, false);
+    lock.exchange(false);
 
     std::atomic<std::chrono::seconds> t{0s};
     t.store(2s);
@@ -34,6 +34,8 @@ m4_define([_CHECK_ATOMIC_testbody], [[
 AC_DEFUN([CHECK_ATOMIC], [
 
   AC_LANG_PUSH(C++)
+  TEMP_CXXFLAGS="$CXXFLAGS"
+  CXXFLAGS="$CXXFLAGS $PTHREAD_CFLAGS"
 
   AC_MSG_CHECKING([whether std::atomic can be used without link library])
 
@@ -51,5 +53,6 @@ AC_DEFUN([CHECK_ATOMIC], [
         ])
     ])
 
+  CXXFLAGS="$TEMP_CXXFLAGS"
   AC_LANG_POP
 ])
diff --git a/configure.ac b/configure.ac
index 0e1968f5c47..db7ac620b9d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -87,9 +87,6 @@ else
 AX_CXX_COMPILE_STDCXX([20], [noext], [mandatory])
 fi
 
-dnl Check if -latomic is required for <std::atomic>
-CHECK_ATOMIC
-
 dnl check if additional link flags are required for std::filesystem
 CHECK_FILESYSTEM
 
@@ -887,6 +884,9 @@ AC_C_BIGENDIAN
 dnl Check for pthread compile/link requirements
 AX_PTHREAD
 
+dnl Check if -latomic is required for <std::atomic>
+CHECK_ATOMIC
+
 dnl The following macro will add the necessary defines to bitcoin-config.h, but
 dnl they also need to be passed down to any subprojects. Pull the results out of
 dnl the cache and add them to CPPFLAGS.