Re: [PATCH 2/2] Enable elimination of zext/sext

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Jul 14, 2014 at 4:57 AM, Kugan
<kugan.vivekanandarajah@linaro.org> wrote:
> On 11/07/14 22:47, Richard Biener wrote:
>> On Fri, Jul 11, 2014 at 1:52 PM, Kugan
>> <kugan.vivekanandarajah@linaro.org> wrote:
>>> Thanks foe the review and suggestions.
>>>
>>> On 10/07/14 22:15, Richard Biener wrote:
>>>> On Mon, Jul 7, 2014 at 8:55 AM, Kugan <kugan.vivekanandarajah@linaro.org> wrote:
>>>
>>> [...]
>>>
>>>>>
>>>>> For -fwrapv, it is due to how PROMOTE_MODE is defined in arm back-end.
>>>>> In the test-case, a function (which has signed char return type) returns
>>>>> -1 in one of the paths. ARM PROMOTE_MODE changes that to 255 and relies
>>>>> on zero/sign extension generated by RTL again for the correct value. I
>>>>> saw some other targets also defining similar think. I am therefore
>>>>> skipping removing zero/sign extension if the ssa variable can be set to
>>>>> negative integer constants.
>>>>
>>>> Hm?  I think you should rather check that you are removing a
>>>> sign-/zero-extension - PROMOTE_MODE tells you if it will sign- or
>>>> zero-extend.  Definitely
>>>>
>>>> +  /* In some architectures, negative integer constants are truncated and
>>>> +     sign changed with target defined PROMOTE_MODE macro. This will impact
>>>> +     the value range seen here and produce wrong code if zero/sign extensions
>>>> +     are eliminated. Therefore, return false if this SSA can have negative
>>>> +     integers.  */
>>>> +  if (is_gimple_assign (stmt)
>>>> +      && (TREE_CODE_CLASS (gimple_assign_rhs_code (stmt)) == tcc_unary))
>>>> +    {
>>>> +      tree rhs1 = gimple_assign_rhs1 (stmt);
>>>> +      if (TREE_CODE (rhs1) == INTEGER_CST
>>>> +         && !TYPE_UNSIGNED (TREE_TYPE (ssa))
>>>> +         && tree_int_cst_compare (rhs1, integer_zero_node) == -1)
>>>> +       return false;
>>>>
>>>> looks completely bogus ... (an unary op with a constant operand?)
>>>> instead you want to do sth like
>>>
>>> I see that unary op with a constant operand is not possible in gimple.
>>> What I wanted to check here is any sort of constant loads; but seems
>>> that will not happen in gimple. Is PHI statements the only possible
>>> statements where we will end up with such constants.
>>
>> No, in theory you can have
>>
>>   ssa_1 = -1;
>>
>> but that's not unary but a GIMPLE_SINGLE_RHS and thus
>> gimple_assign_rhs_code (stmt) == INTEGER_CST.
>>
>>>>   mode = TYPE_MODE (TREE_TYPE (ssa));
>>>>   rhs_uns = TYPE_UNSIGNED (TREE_TYPE (ssa));
>>>>   PROMOTE_MODE (mode, rhs_uns, TREE_TYPE (ssa));
>>>>
>>>> instead of initializing rhs_uns from ssas type.  That is, if
>>>> PROMOTE_MODE tells you to promote _not_ according to ssas sign then
>>>> honor that.
>>>
>>> This is triggered in pr43017.c in function foo for arm-none-linux-gnueabi.
>>>
>>> where, the gimple statement that cause this looks like:
>>> .....
>>>   # _3 = PHI <_17(7), -1(2)>
>>> bb43:
>>>   return _3;
>>>
>>> ARM PROMOTE_MODE changes the sign for integer constants only and hence
>>> looking at the variable with PROMOTE_MODE is not changing the sign in
>>> this case.
>>>
>>> #define PROMOTE_MODE(MODE, UNSIGNEDP, TYPE)     \
>>>   if (GET_MODE_CLASS (MODE) == MODE_INT         \
>>>       && GET_MODE_SIZE (MODE) < 4)              \
>>>     {                                           \
>>>       if (MODE == QImode)                       \
>>>         UNSIGNEDP = 1;                          \
>>>       else if (MODE == HImode)                  \
>>>         UNSIGNEDP = 1;                          \
>>>       (MODE) = SImode;                          \
>>>     }
>>
>> Where does it only apply for "constants"?  It applies to all QImode and
>> HImode entities.
>
> oops, sorry. I don’t know what I was thinking or looking at when I wrote
> that :( It indeed fixes my problems. Thanks for that.
>
> Here is the modified patch. Bootstrapped and regression tested for
> 86_64-unknown-linux-gnu and arm-none-linux-gnueabi with no new regressions.
>
>
> Is this OK?

+  lhs_type = lang_hooks.types.type_for_mode (lhs_mode, lhs_uns);
...
+      && ((!lhs_uns && !wi::neg_p (min, TYPE_SIGN (lhs_type)))
...
+  type_min = wide_int::from (TYPE_MIN_VALUE (lhs_type), prec,
+                            TYPE_SIGN (TREE_TYPE (ssa)));
+  type_max = wide_int::from (TYPE_MAX_VALUE (lhs_type), prec,
+                            TYPE_SIGN (TREE_TYPE (ssa)));

you shouldn't try getting at lhs_type.  Btw, do you want to constrain
lhs_mode to MODE_INTs somewhere?

For TYPE_SIGN use lhs_uns instead, for the min/max value you
should use wi::min_value () and wi::max_value () instead.

You are still using TYPE_SIGN (TREE_TYPE (ssa)) here and later,
but we computed rhs_uns "properly" using PROMOTE_MODE.
I think  the code with re-setting lhs_uns if rhs_uns != lhs_uns
and later using TYPE_SIGN again is pretty hard to follow.

Btw, it seems you need to conditionalize the call to PROMOTE_MODE
on its availability.

Isn't it simply about choosing a proper range we need to restrict
ssa to?  That is, dependent on rhs_uns computed by PROMOTE_MODE,
simply:

+  mode = TYPE_MODE (TREE_TYPE (ssa));
+  rhs_uns = TYPE_UNSIGNED (TREE_TYPE (ssa));
#ifdef PROMOTE_MODE
+  PROMOTE_MODE (mode, rhs_uns, TREE_TYPE (ssa));
#endif

 if (rhs_uns)
   return wi::ge_p (min, 0);  // if min >= 0 then range contains positive values
 else
   return wi::le_p (max, wi::max_value (TYPE_PRECISION (TREE_TYPE
(ssa)), SIGNED);  // if max <= signed-max-of-type then range doesn't
need sign-extension

?

Thanks,
Richard.

> Thanks,
> Kugan
>
>
> gcc/
>
> 2014-07-14  Kugan Vivekanandarajah  <kuganv@linaro.org>
>
>         * calls.c (precompute_arguments): Check is_promoted_for_type
>         and set the promoted mode.
>         (is_promoted_for_type): New function.
>         (expand_expr_real_1): Check is_promoted_for_type
>         and set the promoted mode.
>         * expr.h (is_promoted_for_type): New function definition.
>         * cfgexpand.c (expand_gimple_stmt_1): Call emit_move_insn if
>         SUBREG is promoted with SRP_SIGNED_AND_UNSIGNED.
>
>
> gcc/testsuite
> 2014-07-14  Kugan Vivekanandarajah  <kuganv@linaro.org>
>
>         * gcc.dg/zero_sign_ext_test.c: New test.