Re: PATCH: PR rtl-optimization/54157: [x32] -maddress-mode=long failures

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

On Wed, Aug 8, 2012 at 1:08 AM, Richard Sandiford
<rdsandiford@googlemail.com> wrote:
> "H.J. Lu" <hjl.tools@gmail.com> writes:
>> diff --git a/gcc/combine.c b/gcc/combine.c
>> index a07c046..b9a3589 100644
>> --- a/gcc/combine.c
>> +++ b/gcc/combine.c
>> @@ -10784,12 +10784,30 @@ gen_lowpart_for_combine (enum machine_mode omode, rtx
>> x)
>>    if (omode == imode)
>>      return x;
>>
>> -  /* Return identity if this is a CONST or symbolic reference.  */
>> -  if (omode == Pmode
>> -      && (GET_CODE (x) == CONST
>> -       || GET_CODE (x) == SYMBOL_REF
>> -       || GET_CODE (x) == LABEL_REF))
>> -    return x;
>> +  if (omode == Pmode)
>> +    {
>> +      /* Return identity if this is a symbolic reference.  */
>> +      if (GET_CODE (x) == SYMBOL_REF || GET_CODE (x) == LABEL_REF)
>> +     return x;
>> +
>> +      /* Return identity for CONST unless this is a PLUS of 2 constant
>> +      operands.  */
>> +      if (GET_CODE (x) == CONST)
>> +     {
>> +       rtx y = XEXP (x, 0);
>> +       if (GET_CODE (y) == PLUS
>> +           && ((CONST_INT_P (XEXP (y, 1))
>> +                && (GET_CODE (XEXP (y, 0)) == CONST
>> +                    || GET_CODE (XEXP (y, 0)) == SYMBOL_REF
>> +                    || GET_CODE (XEXP (y, 0)) == LABEL_REF))
>> +               || (CONST_INT_P (XEXP (y, 1))
>> +                   && (GET_CODE (XEXP (y, 0)) == CONST
>> +                       || GET_CODE (XEXP (y, 0)) == SYMBOL_REF
>> +                       || GET_CODE (XEXP (y, 0)) == LABEL_REF))))
>> +         goto fail;
>> +       return x;
>> +     }
>> +    }
>>
>>    /* We can only support MODE being wider than a word if X is a
>>       constant integer or has a mode the same size.  */
>>
>> works for the testcase.
>
> My point was that the "return x" is always wrong.  Whenever we return x
> here, we know we're returning something in a different mode from the one
> that the caller wanted.  Returning a Pmode LABEL_REF might not trigger
> that plus_constant assert, but it's still wrong.
>
> It looks like this came from the mips-rewrite branch:
>
> 2003-03-13  Richard Sandiford  <rsandifo@redhat.com>
>
>         * combine.c (gen_lowpart_for_combine): Treat the lowpart Pmode of
>         a CONST as identity.  Check the return value of gen_lowpart_common.
>
> so I can categorically confirm that the person who wrote it didn't
> know what they were doing.  It also means that this case was motivated
> by an experiment to make Pmode == DImode for n32, which we ended up
> discarding because it produced worse code.
>
> If this case really is important, we might consider using
> convert_memory_address (Pmode, x) instead.  I'm not sure whether
> that would be right for targets with address spaces though, because
> we don't know which address space is associated with the address.
> Hopefully someone who knows address spaces can comment.
>
> If it is correct, then it should probably go in gen_lowpart_common
> rather than gen_lowpart_for_combine.
>
> But given how few people have hit this, it doesn't look like a
> particularly important attempted optimisation.  I'll pre-approve
> a patch that undoes my mistake and simply removes:
>
>   /* Return identity if this is a CONST or symbolic reference.  */
>   if (omode == Pmode
>       && (GET_CODE (x) == CONST
>           || GET_CODE (x) == SYMBOL_REF
>           || GET_CODE (x) == LABEL_REF))
>     return x;
>
> Richard

This is the patch I checked in.

Thanks.

-- 
H.J.
---
gcc/

2012-08-08  Richard Sandiford  <rdsandiford@googlemail.com>
            H.J. Lu  <hongjiu.lu@intel.com>

        PR rtl-optimization/54157
        * combine.c (gen_lowpart_for_combine): Don't return identity
        for CONST or symbolic reference.

gcc/testsuite/

2012-08-08  H.J. Lu  <hongjiu.lu@intel.com>

        PR rtl-optimization/54157
        * gcc.target/i386/pr54157.c: New file.

diff --git a/gcc/combine.c b/gcc/combine.c
index 495e129..2b91eb9 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -10634,13 +10634,6 @@ gen_lowpart_for_combine (enum machine_mode
omode, rtx x)
   if (omode == imode)
     return x;

-  /* Return identity if this is a CONST or symbolic reference.  */
-  if (omode == Pmode
-      && (GET_CODE (x) == CONST
-         || GET_CODE (x) == SYMBOL_REF
-         || GET_CODE (x) == LABEL_REF))
-    return x;
-
   /* We can only support MODE being wider than a word if X is a
      constant integer or has a mode the same size.  */
   if (GET_MODE_SIZE (omode) > UNITS_PER_WORD
diff --git a/gcc/testsuite/gcc.target/i386/pr54157.c
b/gcc/testsuite/gcc.target/i386/pr54157.c
new file mode 100644
index 0000000..b5c4528
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr54157.c
@@ -0,0 +1,21 @@
+/* { dg-do compile { target { ! { ia32 } } } } */
+/* { dg-options "-O2 -mx32 -maddress-mode=long -ftree-vectorize" } */
+
+struct s2{
+  int n[24 -1][24 -1][24 -1];
+};
+
+struct test2{
+  struct s2 e;
+};
+
+struct test2 tmp2[4];
+
+void main1 ()
+{
+  int i,j;
+
+  for (i = 0; i < 24 -4; i++)
+      for (j = 0; j < 24 -4; j++)
+          tmp2[2].e.n[1][i][j] = 8;
+}