Re: [RFC PATCH, i386]: Allow zero_extended addresses (+ problems with reload and offsetable address, "o" constraint)

[Uros Bizjak <ubizjak at gmail dot com>]

On Fri, Aug 5, 2011 at 8:51 PM, Uros Bizjak <ubizjak@gmail.com> wrote:

> As I read this sentence, the RTX is forced into a temporary register,
> and reload tries to satisfy "o" constraint with plus ((reg ...)
> (const_int ...)), as said at the introduction of "o" constraint a
> couple of pages earlier. Unfortunately, this does not seem to be the
> case.
>
> Is there anything wrong with my approach, or is there something wrong in reload?

To answer my own question, the problem was in *add<dwi>3_doubleword
pattern, defined as:

(define_insn_and_split "*add<dwi>3_doubleword"
  [(set (match_operand:<DWI> 0 "nonimmediate_operand" "=r,o")
	(plus:<DWI>
	  (match_operand:<DWI> 1 "nonimmediate_operand" "%0,0")
	  (match_operand:<DWI> 2 "<general_operand>" "ro<di>,r<di>")))
   (clobber (reg:CC FLAGS_REG))]
  "ix86_binary_operator_ok (PLUS, <DWI>mode, operands)"

When reload tried to satisfy alternative 1 (the "o" and matching "0")
with a non-offsettable (in this particular case, zero-extended)
address, it CSE'd operand 0 and operand 1 to a temporary TImode
register. Unfortunately a Timode move has its own constraints:

(define_insn "*movti_internal_rex64"
  [(set (match_operand:TI 0 "nonimmediate_operand" "=!r,o,x,x,xm")
	(match_operand:TI 1 "general_operand" "riFo,riF,C,xm,x"))]
  "TARGET_64BIT && !(MEM_P (operands[0]) && MEM_P (operands[1]))"

where move from/to a general register to/from non-offsettable memory
is not valid.

Although, it would be nice for reload to subsequently fix CSE'd
non-offsetable memory by copying address to temporary reg (*as said in
the documentation*), we could simply require an XMM temporary for
TImode reloads to/from integer registers, and this fixes ICE for x32.

The testcase to play with (gcc -O2 -mx32):

--cut here--
void test (__int128 *array, int idx, int off)
{
  __int128 *dest = &array [idx];

  dest[0] += 1;
  dest[off] = 0;
}
--cut here--

So, following additional patch saves the day:

Index: i386/i386.c
===================================================================
--- i386/i386.c	(revision 177536)
+++ i386/i386.c	(working copy)
@@ -28233,6 +28248,15 @@ ix86_secondary_reload (bool in_p, rtx x, reg_class
 		       enum machine_mode mode,
 		       secondary_reload_info *sri ATTRIBUTE_UNUSED)
 {
+  /* Double-word spills from general registers to non-offsettable
+     memory references go through XMM register.  Following code
+     handles zero-extended addresses on x32 target.  */
+  if (TARGET_64BIT
+      && GET_MODE_SIZE (mode) > UNITS_PER_WORD
+      && rclass == GENERAL_REGS
+      && !offsettable_memref_p (x))
+    return SSE_REGS;
+
   /* QImode spills from non-QI registers require
      intermediate register on 32bit targets.  */
   if (!TARGET_64BIT

Uros.