Fix PR rtl-optimization/81424

[Eric Botcazou]

This is a regression present on the mainline and 7 branch: the compiler aborts 
on the attached testcase for x86 at -O2 -march=i686:

+===========================GNAT BUG DETECTED==============================+
| 8.0.0 20170716 (experimental) [trunk revision 250237] (x86_64-suse-linux) 
GCC error:|
| in emit_move_insn, at expr.c:3704                                        |
| Error detected around opt65.adb:30:4  

because it is trying to generate a move from HImode to SImode.  The problem 
comes from the relatively new code in prepare_cmp_insn:

  /* Don't allow operands to the compare to trap, as that can put the
     compare and branch in different basic blocks.  */
  if (cfun->can_throw_non_call_exceptions)
    {
      if (may_trap_p (x))
        x = force_reg (mode, x);
      if (may_trap_p (y))
        y = force_reg (mode, y);
    }

which assumes that "mode" is the mode of "x" and "y".  While that's true on 
entry to the function, that can be wrong at this point because just above:

      x = result;
      y = const0_rtx;
      mode = TYPE_MODE (integer_type_node);
      methods = OPTAB_LIB_WIDEN;
      unsignedp = false;

can change it to something totally different.  That's why the attached fix 
replaces the call to force_reg with a call to copy_to_reg, which should be OK 
since the mode-less RTXes, i.e. constants, cannot satisfy may_trap_p.

Bootstrapped/regtested on x86_64-suse-linux, applied on mainline and 7 branch.


2017-07-16  Eric Botcazou  <ebotca...@adacore.com>

        PR rtl-optimization/81424
        * optabs.c (prepare_cmp_insn): Use copy_to_reg instead of force_reg
        to remove potential trapping from operands if -fnon-call-exceptions.


2017-07-16  Eric Botcazou  <ebotca...@adacore.com>

        * gnat.dg/opt65.adb: New test.

-- 
Eric Botcazou
Index: optabs.c
===================================================================
--- optabs.c	(revision 250226)
+++ optabs.c	(working copy)
@@ -3844,9 +3844,9 @@ prepare_cmp_insn (rtx x, rtx y, enum rtx
   if (cfun->can_throw_non_call_exceptions)
     {
       if (may_trap_p (x))
-	x = force_reg (mode, x);
+	x = copy_to_reg (x);
       if (may_trap_p (y))
-	y = force_reg (mode, y);
+	y = copy_to_reg (y);
     }
 
   if (GET_MODE_CLASS (mode) == MODE_CC)

-- { dg-do run }
-- { dg-options "-O2" }

with Ada.Command_Line; use Ada.Command_Line;

procedure Opt65 is

   procedure Check_Version_And_Help (Version_String : String) is
      Help_Switch_Present    : Boolean := False;
      Next_Arg               : Natural := 1;
   begin
      while Next_Arg <= Argument_Count loop
         declare
            Next_Argv : constant String := Argument (Next_Arg);
         begin
            if Next_Argv = "--help" then
               Help_Switch_Present := True;
            end if;
            Next_Arg := Next_Arg + 1;
         end;
      end loop;

      if Help_Switch_Present then
         raise Program_Error;
      end if;
   end;

begin
   Check_Version_And_Help ("version");
end;