RISC-V: Remove user-level interrupts

There was once a RISC-V extension draft ("N"), which introduced
user-level interrupts.  However, it was never ratified and the
specification draft has been removed from the RISC-V ISA manual
in commit `b6cade07034` with the comment "it'll likely need to
be redesigned".

Support for a N extension never made it to GCC, but we support
fuction attributes for user-level interrupt handlers that use
the URET instruction.

The "user" interrupt attribute was documented in the RISC-V C API,
but has been removed in PR #106 in May 2025 (driven by LLVM devs/
maintainers and ack'ed by at least one GCC maintainer).

Let's drop URET support from GCC as well.

gcc/ChangeLog:

	* config/riscv/riscv.cc (enum riscv_privilege_levels): Remove USER_MODE.
	(riscv_handle_type_attribute): Remove "user" interrupts.
	(riscv_expand_epilogue): Likewise.
	(riscv_get_interrupt_type): Likewise.
	* config/riscv/riscv.md (riscv_uret): Remove URET pattern.
	* doc/extend.texi: Remove documentation of user interrupts.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/interrupt-conflict-mode.c: Remove "user"
	interrupts.
	* gcc.target/riscv/xtheadint-push-pop.c: Likewise.
	* gcc.target/riscv/interrupt-umode.c: Removed.

Reported-by: Sam Elliott <[email protected]>
Signed-off-by: Christoph Müllner <[email protected]>

Author: cmuellner

gcc/config/riscv/riscv.cc

@@ -170,7 +170,7 @@ struct GTY(())  riscv_frame_info {
 };
 
 enum riscv_privilege_levels {
-  UNKNOWN_MODE, USER_MODE, SUPERVISOR_MODE, MACHINE_MODE, RNMI_MODE
+  UNKNOWN_MODE, SUPERVISOR_MODE, MACHINE_MODE, RNMI_MODE
 };
 
 struct GTY(()) mode_switching_info {
@@ -6929,11 +6929,12 @@ riscv_handle_type_attribute (tree *node ATTRIBUTE_UNUSED, tree name, tree args,
 	      error ("attribute 'rnmi' requires the Smrnmi ISA extension");
 	      *no_add_attrs = true;
 	    }
-	  else if (strcmp (string, "user") && strcmp (string, "supervisor")
-	      && strcmp (string, "machine") && strcmp (string, "rnmi"))
+	  else if (strcmp (string, "supervisor")
+		   && strcmp (string, "machine")
+		   && strcmp (string, "rnmi"))
 	    {
 	      warning (OPT_Wattributes,
-		       "argument to %qE attribute is not %<\"user\"%>, %<\"supervisor\"%>, "
+		       "argument to %qE attribute is not %<\"supervisor\"%>, "
 		       "%<\"machine\"%>, or %<\"rnmi\"%>", name);
 	      *no_add_attrs = true;
 	    }
@@ -9715,14 +9716,12 @@ riscv_expand_epilogue (int style)
 
       if (th_int_mask && TH_INT_INTERRUPT (cfun))
 	emit_jump_insn (gen_th_int_pop ());
-      else if (mode == MACHINE_MODE)
-	emit_jump_insn (gen_riscv_mret ());
       else if (mode == SUPERVISOR_MODE)
 	emit_jump_insn (gen_riscv_sret ());
       else if (mode == RNMI_MODE)
 	emit_jump_insn (gen_riscv_mnret ());
-      else
-	emit_jump_insn (gen_riscv_uret ());
+      else /* Must be MACHINE_MODE.  */
+	emit_jump_insn (gen_riscv_mret ());
     }
   else if (style != SIBCALL_RETURN)
     {
@@ -12064,9 +12063,7 @@ riscv_get_interrupt_type (tree decl)
     {
       const char *string = TREE_STRING_POINTER (TREE_VALUE (attr_args));
 
-      if (!strcmp (string, "user"))
-	return USER_MODE;
-      else if (!strcmp (string, "supervisor"))
+      if (!strcmp (string, "supervisor"))
 	return SUPERVISOR_MODE;
       else if (!strcmp (string, "rnmi"))
 	return RNMI_MODE;

gcc/config/riscv/riscv.md

@@ -120,7 +120,6 @@
   ;; Interrupt handler instructions.
   UNSPECV_MRET
   UNSPECV_SRET
-  UNSPECV_URET
   UNSPECV_MNRET
 
   ;; Blockage and synchronization.
@@ -4167,13 +4166,6 @@
   "sret"
   [(set_attr "type" "ret")])
 
-(define_insn "riscv_uret"
-  [(return)
-   (unspec_volatile [(const_int 0)] UNSPECV_URET)]
-  ""
-  "uret"
-  [(set_attr "type" "ret")])
-
 (define_insn "riscv_mnret"
   [(return)
    (unspec_volatile [(const_int 0)] UNSPECV_MNRET)]

gcc/doc/extend.texi

@@ -5654,10 +5654,10 @@ You can specify the kind of interrupt to be handled by adding an optional
 parameter to the interrupt attribute like this:
 
 @smallexample
-void f (void) __attribute__ ((interrupt ("user")));
+void f (void) __attribute__ ((interrupt ("supervisor")));
 @end smallexample
 
-Permissible values for this parameter are @code{user}, @code{supervisor},
+Permissible values for this parameter are @code{supervisor},
 @code{machine}, and @code{rnmi}.  If there is no parameter, then it
 defaults to @code{machine}.
 

gcc/testsuite/gcc.target/riscv/interrupt-conflict-mode.c

@@ -1,7 +1,7 @@
 /* Verify proper errors are generated for conflicted interrupt type.  */
 /* { dg-do compile } */
 /* { dg-options "" } */
-void __attribute__ ((interrupt ("user")))
+void __attribute__ ((interrupt ("supervisor")))
 foo(void);
 
 void __attribute__ ((interrupt ("machine")))

gcc/testsuite/gcc.target/riscv/interrupt-umode.c

@@ -20,12 +20,6 @@ void func_machine (void)
 /* { dg-final { scan-assembler-times {\mth\.ipop\M} 2 { target { rv32 } } } } */
 
 
-__attribute__ ((interrupt ("user")))
-void func_usr (void)
-{
-  f ();
-}
-
 __attribute__ ((interrupt ("supervisor")))
 void func_supervisor (void)
 {