Values of WIDE_INT_MAX_ELTS in gcc11 and gcc12 are different

[Qing Zhao]

Hi, I tried both the following patches:

Patch1:

[opc@qinzhao-ol8u3-x86 gcc]$ git diff 
diff --git a/gcc/internal-fn.c b/gcc/internal-fn.c
index 0cba95411a6..ca49d2b4514 100644
--- a/gcc/internal-fn.c
+++ b/gcc/internal-fn.c
@@ -3073,12 +3073,14 @@ expand_DEFERRED_INIT (internal_fn, gcall *stmt)
       /* If this variable is in a register use expand_assignment.
         For boolean scalars force zero-init.  */
       tree init;
+      scalar_int_mode var_mode;
       if (TREE_CODE (TREE_TYPE (lhs)) != BOOLEAN_TYPE
          && tree_fits_uhwi_p (var_size)
          && (init_type == AUTO_INIT_PATTERN
              || !is_gimple_reg_type (var_type))
          && int_mode_for_size (tree_to_uhwi (var_size) * BITS_PER_UNIT,
-                               0).exists ())
+                               0).exists (&var_mode)
+         && targetm.scalar_mode_supported_p (var_mode))
        {
          unsigned HOST_WIDE_INT total_bytes = tree_to_uhwi (var_size);
          unsigned char *buf = (unsigned char *) xmalloc (total_bytes);

AND

Patch2:
diff --git a/gcc/internal-fn.c b/gcc/internal-fn.c
index 0cba95411a6..7f129655926 100644
--- a/gcc/internal-fn.c
+++ b/gcc/internal-fn.c
@@ -3073,12 +3073,14 @@ expand_DEFERRED_INIT (internal_fn, gcall *stmt)
       /* If this variable is in a register use expand_assignment.
         For boolean scalars force zero-init.  */
       tree init;
+      scalar_int_mode var_mode;
       if (TREE_CODE (TREE_TYPE (lhs)) != BOOLEAN_TYPE
          && tree_fits_uhwi_p (var_size)
          && (init_type == AUTO_INIT_PATTERN
              || !is_gimple_reg_type (var_type))
          && int_mode_for_size (tree_to_uhwi (var_size) * BITS_PER_UNIT,
-                               0).exists ())
+                               0).exists (&var_mode)
+         && have_insn_for (SET, var_mode))
        {
          unsigned HOST_WIDE_INT total_bytes = tree_to_uhwi (var_size);
          unsigned char *buf = (unsigned char *) xmalloc (total_bytes);

Have the same effect:

1. Resolved the ICE in gcc11;
2. For _Complex long double variables, both return FALSE, as a result, for PATTERN initialization of _Complex long double variables, now they are initialization with ZEROs instead of FEs.

Let me know you opinion on this, If the above 2 is okay, then I might pick the above Patch 1 for the final patch to this issue.

Thanks.

Qing

> On Nov 8, 2021, at 2:41 AM, Richard Biener <richard.guenther@gmail.com> wrote:
> 
> On Sat, Nov 6, 2021 at 10:56 AM Jakub Jelinek <jakub@redhat.com> wrote:
>> 
>> On Fri, Nov 05, 2021 at 05:37:25PM +0000, Qing Zhao wrote:
>>>> On Nov 5, 2021, at 11:17 AM, Jakub Jelinek <jakub@redhat.com> wrote:
>>>> 
>>>> On Fri, Nov 05, 2021 at 04:11:36PM +0000, Qing Zhao wrote:
>>>>> 3076       if (TREE_CODE (TREE_TYPE (lhs)) != BOOLEAN_TYPE
>>>>> 3077           && tree_fits_uhwi_p (var_size)
>>>>> 3078           && (init_type == AUTO_INIT_PATTERN
>>>>> 3079               || !is_gimple_reg_type (var_type))
>>>>> 3080           && int_mode_for_size (tree_to_uhwi (var_size) * BITS_PER_UNIT,
>>>>> 3081                                 0).exists ())
>>>>> 3082         {
>>>>> 3083           unsigned HOST_WIDE_INT total_bytes = tree_to_uhwi (var_size);
>>>>> 3084           unsigned char *buf = (unsigned char *) xmalloc (total_bytes);
>>>>> 3085           memset (buf, (init_type == AUTO_INIT_PATTERN
>>>>> 3086                         ? INIT_PATTERN_VALUE : 0), total_bytes);
>>>>> 3087           tree itype = build_nonstandard_integer_type
>>>>> 3088                          (total_bytes * BITS_PER_UNIT, 1);
>>>>> 
>>>>> The exact failing point is at function “set_min_and_max_values_for_integral_type”:
>>>>> 
>>>>> 2851   gcc_assert (precision <= WIDE_INT_MAX_PRECISION);
>>>>> 
>>>>> For _Complex long double,  “precision” is 256.
>>>>> In GCC11, “WIDE_INT_MAX_PRECISION” is 192,  in GCC12, it’s 512.
>>>>> As a result, the above assertion failed on GCC11.
>>>>> 
>>>>> I am wondering what’s the best fix for this issue in gcc11?
>>>> 
>>>> Even for gcc 12 the above is wrong, you can't blindly assume that
>>>> build_nonstandard_integer_type will work for arbitrary precisions,
>>>> and even if it works that it will actually work.
>>>> The fact that such a mode exist is one thing, but
>>>> targetm.scalar_mode_supported_p should be tested for whether the mode
>>>> is actually supported.
>>> 
>>> You mean “int_mode_for_size().exists()” is not enough to make sure
>>> “build_nonstandard_integer_type” to be valid?  We should add
>>> “targetm.scalar_mode_supported_p” too ?
>> 
>> Yeah.  The former says whether the backend has that mode at all.
>> But some modes may be there only in some specific patterns but
>> without support for mov, add, etc.  Only for
>> targetm.scalar_mode_supported_p modes the backend guarantees that
>> one can use them e.g. in mode attribute and can expect expansion
>> to expand everything with that mode that is needed in some way.
>> E.g. only if targetm.scalar_mode_supported_p (TImode) the FEs
>> support __int128_t type, etc.
> 
> The memcpy folding code now checks
> 
>              scalar_int_mode mode;
>              if (int_mode_for_size (ilen * 8, 0).exists (&mode)
>                  && GET_MODE_SIZE (mode) * BITS_PER_UNIT == ilen * 8
>                  && have_insn_for (SET, mode)
> 
> thus specifically only have_insn_for (SET, mode), which I guess is
> good enough for this case as well?
> 
>>        Jakub