[PATCH] Fix bug in simplify_ternary_operation

Jeff Law law@redhat.com
Mon Nov 20 03:12:00 GMT 2017 

Sorry, it's taken so long to get back to this patch...


On 09/01/2017 02:51 AM, Tom de Vries wrote:
> On 08/31/2017 11:44 PM, Jeff Law wrote:
>> On 08/28/2017 12:26 PM, Tom de Vries wrote:
>>> Hi,
>>>
>>> I think I found a bug in r17465:
>>> ...
>>>>         * cse.c (simplify_ternary_operation): Handle more IF_THEN_ELSE
>>>>         simplifications.
>>>>
>>>> diff --git a/gcc/cse.c b/gcc/cse.c
>>>> index e001597..3c27387 100644
>>>> --- a/gcc/cse.c
>>>> +++ b/gcc/cse.c
>>>> @@ -4713,6 +4713,17 @@ simplify_ternary_operation (code, mode,
>>>> op0_mode, op0, op1, op2)
>>>
>>> Note: the parameters of simplify_ternary_operation have the following
>>> meaning:
>>> ...
>>> /* Simplify CODE, an operation with result mode MODE and three operands,
>>>     OP0, OP1, and OP2.  OP0_MODE was the mode of OP0 before it became
>>>     a constant.  Return 0 if no simplifications is possible.  */
>>>
>>> rtx
>>> simplify_ternary_operation (code, mode, op0_mode, op0, op1, op2)
>>>       enum rtx_code code;
>>>       enum machine_mode mode, op0_mode;
>>>       rtx op0, op1, op2;
>>> ...
>>>
>>>>            && rtx_equal_p (XEXP (op0, 1), op1)
>>>>            && rtx_equal_p (XEXP (op0, 0), op2))
>>>>          return op2;
>>>> +      else if (! side_effects_p (op0))
>>>> +       {
>>>> +         rtx temp;
>>>> +         temp = simplify_relational_operation (GET_CODE (op0),
>>>> op0_mode,
>>>> +                                               XEXP (op0, 0), XEXP
>>>> (op0, 1));
>>>
>>> We're handling code == IF_THEN_ELSE here, so op0 is the condition, op1
>>> is the 'then expr' and op2 is the 'else expr'.
>>>
>>> The parameters of simplify_relational_operation have the following
>>> meaning:
>>> ...
>>> /* Like simplify_binary_operation except used for relational operators.
>>>     MODE is the mode of the operands, not that of the result.  If MODE
>>>     is VOIDmode, both operands must also be VOIDmode and we compare the
>>>     operands in "infinite precision".
>>>
>>>     If no simplification is possible, this function returns zero.
>>>     Otherwise, it returns either const_true_rtx or const0_rtx.  */
>>>
>>> rtx
>>> simplify_relational_operation (code, mode, op0, op1)
>>>       enum rtx_code code;
>>>       enum machine_mode mode;
>>>       rtx op0, op1;
>>> ...
>>>
>>> The problem in the patch is that we use op0_mode argument for the mode
>>> parameter. The mode parameter of simplify_relational_operation needs to
>>> be the mode of the operands of the condition, while op0_mode is the mode
>>> of the condition.
>>>
>>> Patch below fixes this on current trunk.
>>>
>>> [ I found this by running into an ICE in
>>> gcc.c-torture/compile/pr28776-2.c for gcn target. I haven't been able to
>>> reproduce this with an upstream branch yet. ]
>>>
>>> OK for trunk if bootstrap and reg-test for x86_64 succeeds?
>> So clearly setting cmp_mode to op0_mode is wrong.   But we also have to
>> make sure that if cmp_mode is VOIDmode that either XEXP (op0, 0) has a
>> non-void mode or that XEXP (op0, 1) has a non-void mode, otherwise we're
>> likely to abort down in simplify_const_relational_operation.
>>
> 
> You're referring to this assert:
> ...
> /* Check if the given comparison (done in the given MODE) is actually
>    a tautology or a contradiction.  If the mode is VOID_mode, the
>    comparison is done in "infinite precision".  If no simplification
>    is possible, this function returns zero.  Otherwise, it returns
>    either const_true_rtx or const0_rtx.  */
> 
> rtx
> simplify_const_relational_operation (enum rtx_code code,
>                                      machine_mode mode,
>                                      rtx op0, rtx op1)
> {
>   ...
> 
>   gcc_assert (mode != VOIDmode
>               || (GET_MODE (op0) == VOIDmode
>                   && GET_MODE (op1) == VOIDmode));
> ...
> 
> added by Honza:
> ...
>     * simplify-rtx.c (simplify_relational_operation): Verify that
>         mode == VOIDmode implies both operands to be VOIDmode.
> ...
> 
> In other words, rewriting the assert in more readable form:
> ...
> #define BOOL_IMPLIES(a, b) (!(a) || (b))
>   gcc_assert (BOOL_IMPLIES (mode == VOIDmode,
>                             (GET_MODE (op0) == VOIDmode
>                              && GET_MODE (op1) == VOIDmode)));
> ...
> [ I'd be in favor of rewriting imply relations using a macro or some
> such, I find it easier to understand. ]
> 
> Now, simplify_relational_operation starts like this:
> ...
> rtx
> simplify_relational_operation (enum rtx_code code, machine_mode mode,
>                                machine_mode cmp_mode, rtx op0, rtx op1)
> {
>   rtx tem, trueop0, trueop1;
> 
>   if (cmp_mode == VOIDmode)
>     cmp_mode = GET_MODE (op0);
>   if (cmp_mode == VOIDmode)
>     cmp_mode = GET_MODE (op1);
> 
>   tem = simplify_const_relational_operation (code, cmp_mode, op0, op1);
> ...
> 
> AFAIU, the cmp_mode ifs ensure that the assert in
> simplify_const_relational_operation doesn't trigger.
> 
>> ISTM a better fix is to return NULL_RTX if cmp_mode is VOIDmode and both
>> the sub-operations are VOIDmode as well.
>>
> 
> I don't think we need that. simplify_const_relational_operation can
> handle the situation that mode == VOIDmode && GET_MODE (op0) == VOIDmode
> && GET_MODE (op1) == VOIDmode.
I think you're right -- looking back at it again I think I mis-read the
assert.

Go ahead and commit your change.

Thanks again for your patience.

jeff

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