diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
index 1dfff33..b992dc3 100644
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -5435,10 +5435,6 @@ aarch64_libgcc_cmp_return_mode (void)
 #error Cannot use simple address calculation for stack probing
 #endif
 
-/* The pair of scratch registers used for stack probing.  */
-#define PROBE_STACK_FIRST_REG  R9_REGNUM
-#define PROBE_STACK_SECOND_REG R10_REGNUM
-
 /* Emit code to probe a range of stack addresses from FIRST to FIRST+POLY_SIZE,
    inclusive.  These are offsets from the current stack pointer.  */
 
@@ -5452,7 +5448,7 @@ aarch64_emit_probe_stack_range (HOST_WIDE_INT first, poly_int64 poly_size)
       return;
     }
 
-  rtx reg1 = gen_rtx_REG (Pmode, PROBE_STACK_FIRST_REG);
+  rtx reg1 = gen_rtx_REG (Pmode, PROBE_STACK_FIRST_REGNUM);
 
   /* See the same assertion on PROBE_INTERVAL above.  */
   gcc_assert ((first % ARITH_FACTOR) == 0);
@@ -5510,7 +5506,7 @@ aarch64_emit_probe_stack_range (HOST_WIDE_INT first, poly_int64 poly_size)
      equality test for the loop condition.  */
   else
     {
-      rtx reg2 = gen_rtx_REG (Pmode, PROBE_STACK_SECOND_REG);
+      rtx reg2 = gen_rtx_REG (Pmode, PROBE_STACK_SECOND_REGNUM);
 
       /* Step 1: round SIZE to the previous multiple of the interval.  */
 
@@ -15527,6 +15523,14 @@ aarch64_conditional_register_usage (void)
       fixed_regs[SPECULATION_SCRATCH_REGNUM] = 1;
       call_used_regs[SPECULATION_SCRATCH_REGNUM] = 1;
     }
+
+  /* If the target OS has a private use of R18, preserve it.  */
+  if (TARGET_OS_USES_R18)
+    {
+      fixed_regs[R18_REGNUM] = 1;
+      call_used_regs[R18_REGNUM] = 1;
+      gcc_assert(STATIC_CHAIN_REGNUM != R18_REGNUM);
+    }
 }
 
 /* Walk down the type tree of TYPE counting consecutive base elements.
diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h
index 425a363..904b347 100644
--- a/gcc/config/aarch64/aarch64.h
+++ b/gcc/config/aarch64/aarch64.h
@@ -472,7 +472,12 @@ extern unsigned aarch64_architecture_version;
    uses alloca.  */
 #define EXIT_IGNORE_STACK	(cfun->calls_alloca)
 
-#define STATIC_CHAIN_REGNUM		R18_REGNUM
+/* Whether the target OS has a private use of R18, as allowed by the ABI.
+   OS specific configuration files may redefine this.  */
+#define TARGET_OS_USES_R18 0
+
+#define STATIC_CHAIN_REGNUM		(TARGET_OS_USES_R18 \
+					 ? R9_REGNUM : R18_REGNUM)
 #define HARD_FRAME_POINTER_REGNUM	R29_REGNUM
 #define FRAME_POINTER_REGNUM		SFP_REGNUM
 #define STACK_POINTER_REGNUM		SP_REGNUM
diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
index f19e227..b8cf4ab 100644
--- a/gcc/config/aarch64/aarch64.md
+++ b/gcc/config/aarch64/aarch64.md
@@ -117,6 +117,9 @@
     ;; Scratch registers for prologue/epilogue use.
     (EP0_REGNUM         12)
     (EP1_REGNUM         13)
+    ;; Scratch registers used for stack probing during prologue.
+    (PROBE_STACK_FIRST_REGNUM 10)
+    (PROBE_STACK_SECOND_REGNUM 11)
     ;; A couple of call-clobbered registers that we need to reserve when
     ;; tracking speculation this is not ABI, so is subject to change.
     (SPECULATION_SCRATCH_REGNUM 14)