Re: PATCH: PR rtl-optimization/64037: Miscompilation with -Os and enum class : char parameter

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Dec 08, 2014 at 11:16:58PM +0100, Uros Bizjak wrote:
> Hello!
> 
> > On Tue, Nov 25, 2014 at 5:05 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> >> On Tue, Nov 25, 2014 at 7:01 AM, Richard Biener
> >> <richard.guenther@gmail.com> wrote:
> >>> On Tue, Nov 25, 2014 at 1:57 PM, H.J. Lu <hongjiu.lu@intel.com> wrote:
> >>>> Hi,
> >>>>
> >>>> The enclosed testcase fails on x86 when compiled with -Os since we pass
> >>>> a byte parameter with a byte load in caller and read it as an int in
> >>>> callee.  The reason it only shows up with -Os is x86 backend encodes
> >>>> a byte load with an int load if -O isn't used.  When a byte load is
> >>>> used, the upper 24 bits of the register have random value for none
> >>>> WORD_REGISTER_OPERATIONS targets.
> >>>>
> >>>> It happens because setup_incoming_promotions in combine.c has
> >>>>
> >>>>       /* The mode and signedness of the argument before any promotions happen
> >>>>          (equal to the mode of the pseudo holding it at that stage).  */
> >>>>       mode1 = TYPE_MODE (TREE_TYPE (arg));
> >>>>       uns1 = TYPE_UNSIGNED (TREE_TYPE (arg));
> >>>>
> >>>>       /* The mode and signedness of the argument after any source language and
> >>>>          TARGET_PROMOTE_PROTOTYPES-driven promotions.  */
> >>>>       mode2 = TYPE_MODE (DECL_ARG_TYPE (arg));
> >>>>       uns3 = TYPE_UNSIGNED (DECL_ARG_TYPE (arg));
> >>>>
> >>>>       /* The mode and signedness of the argument as it is actually passed,
> >>>>          after any TARGET_PROMOTE_FUNCTION_ARGS-driven ABI promotions.  */
> >>>>       mode3 = promote_function_mode (DECL_ARG_TYPE (arg), mode2, &uns3,
> >>>>                                      TREE_TYPE (cfun->decl), 0);
> >>>>
> >>>> while they are actually passed in register by assign_parm_setup_reg in
> >>>> function.c:
> >>>>
> >>>>   /* Store the parm in a pseudoregister during the function, but we may
> >>>>      need to do it in a wider mode.  Using 2 here makes the result
> >>>>      consistent with promote_decl_mode and thus expand_expr_real_1.  */
> >>>>   promoted_nominal_mode
> >>>>     = promote_function_mode (data->nominal_type, data->nominal_mode, &unsignedp,
> >>>>                              TREE_TYPE (current_function_decl), 2);
> >>>>
> >>>> where nominal_type and nominal_mode are set up with TREE_TYPE (parm)
> >>>> and TYPE_MODE (nominal_type). TREE_TYPE here is
> >>>
> >>> I think the bug is here, not in combine.c.  Can you try going back in history
> >>> for both snippets and see if they matched at some point?
> >>>
> >>
> >> The bug was introduced by
> >>
> >> https://gcc.gnu.org/ml/gcc-cvs/2007-09/msg00613.html
> >>
> >> commit 5d93234932c3d8617ce92b77b7013ef6bede9508
> >> Author: shinwell <shinwell@138bc75d-0d04-0410-961f-82ee72b054a4>
> >> Date:   Thu Sep 20 11:01:18 2007 +0000
> >>
> >>       gcc/
> >>       * combine.c: Include cgraph.h.
> >>       (setup_incoming_promotions): Rework to allow more aggressive
> >>       elimination of sign extensions when all call sites of the
> >>       current function are known to lie within the current unit.
> >>
> >>
> >>     git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@128618
> >> 138bc75d-0d04-0410-961f-82ee72b054a4
> >>
> >> Before this commit, combine.c has
> >>
> >>           enum machine_mode mode = TYPE_MODE (TREE_TYPE (arg));
> >>           int uns = TYPE_UNSIGNED (TREE_TYPE (arg));
> >>
> >>           mode = promote_mode (TREE_TYPE (arg), mode, &uns, 1);
> >>           if (mode == GET_MODE (reg) && mode != DECL_MODE (arg))
> >>             {
> >>               rtx x;
> >>               x = gen_rtx_CLOBBER (DECL_MODE (arg), const0_rtx);
> >>               x = gen_rtx_fmt_e ((uns ? ZERO_EXTEND : SIGN_EXTEND), mode, x);
> >>               record_value_for_reg (reg, first, x);
> >>             }
> >>
> >> It matches function.c:
> >>
> >>   /* This is not really promoting for a call.  However we need to be
> >>      consistent with assign_parm_find_data_types and expand_expr_real_1.  */
> >>   promoted_nominal_mode
> >>     = promote_mode (data->nominal_type, data->nominal_mode, &unsignedp, 1);
> >>
> >> r128618 changed
> >>
> >> mode = promote_mode (TREE_TYPE (arg), mode, &uns, 1);
> >>
> >> to
> >>
> >> mode3 = promote_mode (DECL_ARG_TYPE (arg), mode2, &uns3, 1);
> >>
> >> It breaks none WORD_REGISTER_OPERATIONS targets.
> >
> > Hmm, I think that DECL_ARG_TYPE makes a difference only
> > for non-WORD_REGISTER_OPERATIONS targets.
> >
> > But yeah, isolated the above change looks wrong.
> >
> > Your patch is ok for trunk if nobody objects within 24h and for branches
> > after a week.
> >
> > Thanks,
> > Richard.
> 
> This patch caused PR64213.
> 

Here is the updated patch.  The difference is

      mode3 = promote_function_mode (TREE_TYPE (arg), mode1, &uns3,
				     TREE_TYPE (cfun->decl), 0);

vs

      mode3 = promote_function_mode (TREE_TYPE (arg), mode1, &uns1,
				     TREE_TYPE (cfun->decl), 0);

I made a mistake in my previous patch where I shouldn't have changed
&uns3 to &uns1.  We do want to update mode3 and uns3, not mode3 and
uns1. It generates the same code on PR64213 testcase with a cross
alpha-linux GCC.

Uros, can you test it on Linux/alpha?  OK for master, 4.9 and 4.8
branches if it works on Linux/alpha?

Thanks.


H.J.
---
>From 7b274d517dcaae96f111652283d947c035ab7a22 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Sun, 14 Dec 2014 05:37:41 -0800
Subject: [PATCH] Pass unpromoted argument to promote_function_mode

This patch updates setup_incoming_promotions in combine.c to match what
is actually passed in assign_parm_setup_reg in function.c.

gcc/

	PR rtl-optimization/64037
	* combine.c (setup_incoming_promotions): Pass the argument
	before any promotions happen to promote_function_mode.

gcc/testsuite/

	PR rtl-optimization/64037
	* g++.dg/pr64037.C: New test.
---
diff --git a/gcc/combine.c b/gcc/combine.c
index c95b493..ee7b3f9 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -1579,8 +1579,8 @@ setup_incoming_promotions (rtx_insn *first)
       uns3 = TYPE_UNSIGNED (DECL_ARG_TYPE (arg));
 
       /* The mode and signedness of the argument as it is actually passed,
-         after any TARGET_PROMOTE_FUNCTION_ARGS-driven ABI promotions.  */
-      mode3 = promote_function_mode (DECL_ARG_TYPE (arg), mode2, &uns3,
+         see assign_parm_setup_reg in function.c.  */
+      mode3 = promote_function_mode (TREE_TYPE (arg), mode1, &uns3,
 				     TREE_TYPE (cfun->decl), 0);
 
       /* The mode of the register in which the argument is being passed.  */
diff --git a/gcc/testsuite/g++.dg/pr64037.C b/gcc/testsuite/g++.dg/pr64037.C
new file mode 100644
index 0000000..e5cd0e2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr64037.C
@@ -0,0 +1,27 @@
+// { dg-do run { target i?86-*-* x86_64-*-* } }
+// { dg-options "-std=c++11 -Os" }
+
+enum class X : unsigned char {
+  V = 2,
+};
+
+static void
+__attribute__((noinline,noclone))
+foo(unsigned &out, unsigned a, X b)
+{
+  out = static_cast<unsigned>(b);
+}
+
+int main()
+{
+  unsigned deadbeef = 0xDEADBEEF;
+  asm volatile ("" : "+d" (deadbeef), "+c" (deadbeef));
+
+  unsigned out;
+  foo(out, 2, X::V);
+
+  if (out != 2)
+    __builtin_abort ();
+
+  return 0;
+}
-- 
1.9.3