It appears libjsig is being compiled without optimization. The reason as to why that is seems rather unclear. It might have been an oversight as with JDK-8210647 (libsaproc). We should compile it with some reasonable opt level.

From a JDK 12 build today:
/usr/bin/gcc -I/disk/openjdk/upstream-sources/openjdk-head-2/build/linux-x86_64-normal-server-release/support/modules_include/java.base -I/disk/openjdk/upstream-sources/openjdk-head-2/build/linux-x86_64-normal-server-release/support/modules_include/java.base/linux -I/disk/openjdk/upstream-sources/openjdk-head-2/src/java.base/share/native/libjava -I/disk/openjdk/upstream-sources/openjdk-head-2/src/java.base/unix/native/libjava -I/disk/openjdk/upstream-sources/openjdk-head-2/src/hotspot/share/include -I/disk/openjdk/upstream-sources/openjdk-head-2/src/hotspot/os/posix/include -pipe -D_GNU_SOURCE -D_REENTRANT -D_LARGEFILE64_SOURCE -DLINUX -DNDEBUG -Wall -Wextra -Wno-unused -Wno-unused-parameter -Wformat=2 -fno-strict-aliasing -m64 -D_LITTLE_ENDIAN -DARCH='"amd64"' -Damd64 -D_LP64=1 -fno-omit-frame-pointer -fno-delete-null-pointer-checks -fno-lifetime-dse -fPIC -fvisibility=hidden -I/disk/openjdk/upstream-sources/openjdk-head-2/src/java.base/unix/native/libjsig -I/disk/openjdk/upstream-sources/openjdk-head-2/build/linux-x86_64-normal-server-release/support/headers/java.base -g -DTHIS_FILE='""' -c -MMD -MF /disk/openjdk/upstream-sources/openjdk-head-2/build/linux-x86_64-normal-server-release/support/native/java.base/libjsig/jsig.d -o /disk/openjdk/upstream-sources/openjdk-head-2/build/linux-x86_64-normal-server-release/support/native/java.base/libjsig/jsig.o /disk/openjdk/upstream-sources/openjdk-head-2/src/java.base/unix/native/libjsig/jsig.c