Truly disable the effect of -Ofast/-ffast-math

scripts/install.sh

@@ -63,7 +63,8 @@ install () {
         mkdir -p $SNAKEPIT
         mkdir -p $BASE/versions
         update_pyenv $VERSION
-        # -Ofast makes the build take too long and times out Travis.
+        # -Ofast makes the build take too long and times out Travis. It also affects
+        # process-wide floating-point flags - see: scripts/releases/make-manylinux
         CFLAGS="-O1 -pipe -march=native" $BASE/pyenv/plugins/python-build/bin/python-build $VERSION $DESTINATION
     fi
 

scripts/releases/make-manylinux

@@ -42,10 +42,16 @@ if [ "$DOCKER_IMAGE" == "quay.io/pypa/manylinux2014_aarch64" -a -n "$GITHUB_ACTI
     export CFLAGS="-O1 $GEVENT_WARNFLAGS"
     SLOW_ARM=1
 else
-    echo "Compiling with -Ofast"
-    # Note: -Ofast includes -ffast-math which affects process-wide floating-point flags (e.g. can affect numpy).
-    #       We opt out of -ffast-math explicitly. Other libraries can still trigger it.
-    export CFLAGS="-Ofast -fno-fast-math $GEVENT_WARNFLAGS"
+    echo "Compiling with -O3"
+    # Caution against using -Ofast:
+    # -Ofast includes -ffast-math which affects process-wide floating-point flags (e.g. can affect numpy).
+    # -fno-fast-math disables *some* features of -ffast-math, but as long as -Ofast is specified,
+    # it still affects floating-point flags:
+    # https://github.com/gcc-mirror/gcc/blob/releases/gcc-11.2.0/gcc/config/i386/gnu-user-common.h#L50
+    #
+    # We *could* turn on other flags that are included in -Ofast individually, but for gevent's usecase, it's not
+    # a priority to enable aggressive optimizations at the expense of possibly violating language standards.
+    export CFLAGS="-O3 $GEVENT_WARNFLAGS"
 fi
 # -lrt: Needed for clock_gettime libc support on this version.
 # -pthread: Needed for pthread_atfork (cffi).