--- a/make/autoconf/flags-cflags.m4	Wed Nov 26 22:17:19 2025
+++ b/make/autoconf/flags-cflags.m4	Fri Nov 28 12:46:32 2025
@@ -283,7 +283,9 @@
     C_O_FLAG_NONE="-O0"
 
     if test "x$TOOLCHAIN_TYPE" = xgcc; then
-      C_O_FLAG_LTO="-flto=auto -fuse-linker-plugin -fno-strict-aliasing -fno-fat-lto-objects"
+      if test "x$OPENJDK_TARGET_OS" != xsolaris; then
+        C_O_FLAG_LTO="-flto=auto -fuse-linker-plugin -fno-strict-aliasing -fno-fat-lto-objects"
+      fi
     else
       C_O_FLAG_LTO="-flto -fno-strict-aliasing"
     fi
@@ -433,6 +435,9 @@
   if test "x$OPENJDK_TARGET_OS" = xlinux; then
     CFLAGS_OS_DEF_JVM="-DLINUX -D_FILE_OFFSET_BITS=64"
     CFLAGS_OS_DEF_JDK="-D_GNU_SOURCE -D_REENTRANT -D_FILE_OFFSET_BITS=64"
+  elif test "x$OPENJDK_TARGET_OS" = xsolaris; then
+    CFLAGS_OS_DEF_JVM="-DSOLARIS"
+    CFLAGS_OS_DEF_JDK="-D__solaris__ -D_REENTRANT"
   elif test "x$OPENJDK_TARGET_OS" = xmacosx; then
     CFLAGS_OS_DEF_JVM="-D_ALLBSD_SOURCE -D_DARWIN_C_SOURCE -D_XOPEN_SOURCE"
     CFLAGS_OS_DEF_JDK="-D_ALLBSD_SOURCE -D_DARWIN_UNLIMITED_SELECT"
@@ -487,6 +492,7 @@
 
   if test "x$TOOLCHAIN_TYPE" = xgcc; then
     ALWAYS_DEFINES_JVM="-D_GNU_SOURCE -D_REENTRANT"
+    ALWAYS_DEFINES_JDK="-D_GNU_SOURCE -D_REENTRANT -D_LARGEFILE64_SOURCE"
   elif test "x$TOOLCHAIN_TYPE" = xclang; then
     ALWAYS_DEFINES_JVM="-D_GNU_SOURCE"
   elif test "x$TOOLCHAIN_TYPE" = xmicrosoft; then
@@ -509,8 +515,8 @@
   fi
 
   if test "x$TOOLCHAIN_TYPE" = xgcc; then
-    TOOLCHAIN_CFLAGS_JVM="$TOOLCHAIN_CFLAGS_JVM -fstack-protector"
-    TOOLCHAIN_CFLAGS_JDK="-fvisibility=hidden -pipe -fstack-protector"
+    TOOLCHAIN_CFLAGS_JVM="$TOOLCHAIN_CFLAGS_JVM"
+    TOOLCHAIN_CFLAGS_JDK="-fvisibility=hidden -pipe"
     # reduce lib size on linux in link step, this needs also special compile flags
     # do this on s390x also for libjvm (where serviceability agent is not supported)
     if test "x$ENABLE_LINKTIME_GC" = xtrue; then
@@ -575,7 +581,12 @@
 
   # CFLAGS C language level for JDK sources (hotspot only uses C++)
   if test "x$TOOLCHAIN_TYPE" = xgcc || test "x$TOOLCHAIN_TYPE" = xclang; then
-    LANGSTD_CFLAGS="-std=c11"
+    if test "x$OPENJDK_TARGET_OS" = xsolaris; then
+      # illumos headers are confused by c11
+      LANGSTD_CFLAGS="-std=gnu11"
+    else
+      LANGSTD_CFLAGS="-std=c11"
+    fi
   elif test "x$TOOLCHAIN_TYPE" = xmicrosoft; then
     LANGSTD_CFLAGS="-std:c11"
   fi