> On Aug 11, 2021, at 4:02 AM, Richard Sandiford <richard.sandif...@arm.com>
> wrote:
>
> Qing Zhao via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>>> On Aug 10, 2021, at 3:16 PM, Qing Zhao via Gcc-patches
>>> <gcc-patches@gcc.gnu.org> wrote:
>>>
>>> Hi, Richard,
>>>
>>>> On Aug 10, 2021, at 10:22 AM, Richard Biener <rguent...@suse.de> wrote:
>>>>>>
>>>>>> Especially in the VLA case but likely also in general (though unlikely
>>>>>> since usually the receiver of initializations are simple enough). I'd
>>>>>> expect the VLA case end up as
>>>>>>
>>>>>> *ptr_to_decl = .DEFERRED_INIT (...);
>>>>>>
>>>>>> where *ptr_to_decl is the DECL_VALUE_EXPR of the decl.
>>>>>
>>>>> So, for the following small testing case:
>>>>>
>>>>> ====
>>>>> extern void bar (int);
>>>>>
>>>>> void foo(int n)
>>>>> {
>>>>> int arr[n];
>>>>> bar (arr[2]);
>>>>> return;
>>>>> }
>>>>> =====
>>>>>
>>>>> If I compile it with -ftrivial-auto-var-init=zero -fdump-tree-gimple -S
>>>>> -o auto-init-11.s -fdump-rtl-expand, the *.gimple dump is:
>>>>>
>>>>> =====
>>>>> void foo (int n)
>>>>> {
>>>>> int n.0;
>>>>> sizetype D.1950;
>>>>> bitsizetype D.1951;
>>>>> sizetype D.1952;
>>>>> bitsizetype D.1953;
>>>>> sizetype D.1954;
>>>>> int[0:D.1950] * arr.1;
>>>>> void * saved_stack.2;
>>>>> int arr[0:D.1950] [value-expr: *arr.1];
>>>>>
>>>>> saved_stack.2 = __builtin_stack_save ();
>>>>> try
>>>>> {
>>>>> n.0 = n;
>>>>> _1 = (long int) n.0;
>>>>> _2 = _1 + -1;
>>>>> _3 = (sizetype) _2;
>>>>> D.1950 = _3;
>>>>> _4 = (sizetype) n.0;
>>>>> _5 = (bitsizetype) _4;
>>>>> _6 = _5 * 32;
>>>>> D.1951 = _6;
>>>>> _7 = (sizetype) n.0;
>>>>> _8 = _7 * 4;
>>>>> D.1952 = _8;
>>>>> _9 = (sizetype) n.0;
>>>>> _10 = (bitsizetype) _9;
>>>>> _11 = _10 * 32;
>>>>> D.1953 = _11;
>>>>> _12 = (sizetype) n.0;
>>>>> _13 = _12 * 4;
>>>>> D.1954 = _13;
>>>>> arr.1 = __builtin_alloca_with_align (D.1954, 32);
>>>>> arr = .DEFERRED_INIT (D.1952, 2, 1);
>>>>> _14 = (*arr.1)[2];
>>>>> bar (_14);
>>>>> return;
>>>>> }
>>>>> finally
>>>>> {
>>>>> __builtin_stack_restore (saved_stack.2);
>>>>> }
>>>>> }
>>>>>
>>>>> ====
>>>>>
>>>>> You think that the above .DEFEERED_INIT is not correct?
>>>>> It should be:
>>>>>
>>>>> *arr.1 = .DEFERRED_INIT (D.1952. 2, 1);
>>>>>
>>>>> ?
>>>>
>>>> Yes.
>>>>
>>>
>>> I updated gimplify.c for VLA and now it emits the call to .DEFERRED_INIT as:
>>>
>>> arr.1 = __builtin_alloca_with_align (D.1954, 32);
>>> *arr.1 = .DEFERRED_INIT (D.1952, 2, 1);
>>>
>>> However, this call triggered the assertion failure in verify_gimple_call of
>>> tree-cfg.c because the LHS is not a valid LHS.
>>> Then I modify tree-cfg.c as:
>>>
>>> diff --git a/gcc/tree-cfg.c b/gcc/tree-cfg.c
>>> index 330eb7dd89bf..180d4f1f9e32 100644
>>> --- a/gcc/tree-cfg.c
>>> +++ b/gcc/tree-cfg.c
>>> @@ -3375,7 +3375,11 @@ verify_gimple_call (gcall *stmt)
>>> }
>>>
>>> tree lhs = gimple_call_lhs (stmt);
>>> + /* For .DEFERRED_INIT call, the LHS might be an indirection of
>>> + a pointer for the VLA variable, which is not a valid LHS of
>>> + a gimple call, we ignore the asssertion on this. */
>>> if (lhs
>>> + && (!gimple_call_internal_p (stmt, IFN_DEFERRED_INIT))
>>> && (!is_gimple_reg (lhs)
>>> && (!is_gimple_lvalue (lhs)
>>> || verify_types_in_gimple_reference
>>>
>>> The assertion failure in tree-cfg.c got resolved, but I got another
>>> assertion failure in operands_scanner::get_expr_operands (tree *expr_p, int
>>> flags), line 945:
>>>
>>> 939 /* If we get here, something has gone wrong. */
>>> 940 if (flag_checking)
>>> 941 {
>>> 942 fprintf (stderr, "unhandled expression in
>>> get_expr_operands():\n");
>>> 943 debug_tree (expr);
>>> 944 fputs ("\n", stderr);
>>> 945 gcc_unreachable ();
>>> 946 }
>>>
>>> Looks like that the gimple statement:
>>> *arr.1 = .DEFERRED_INIT (D.1952, 2, 1);
>>>
>>> Is not valid. i.e, the LHS should not be an indirection to a pointer.
>>>
>>> How to resolve this issue?
>>
>> I came up with the following solution:
>>
>> Define the IFN_DEFERRED_INIT function as:
>>
>> LHS = DEFERRED_INIT (SIZE of the DECL, INIT_TYPE, IS_VLA);
>>
>> if IS_VLA is false, the LHS is the DECL itself,
>> if IS_VLA is true, the LHS is the pointer to this DECL that created by
>> gimplify_vla_decl.
>>
>>
>> The benefit of this solution are:
>>
>> 1. Resolved the invalid IR issue;
>> 2. The call stmt carries the address of the VLA natually;
>>
>> The issue with this solution is:
>>
>> For VLA and non-VLA, the LHS will be different,
>>
>> Do you see any other potential issues with this solution?
>
> The idea behind the DECL version of the .DEFERRED_INIT semantics was
> that .DEFERRED_INIT just returns a SIZE-byte value that the caller
> then assigns to a SIZE-byte lhs (with the caller choosing the lhs).
> .DEFEREED_INIT itself doesn't read or write memory and so can be const,
> which in turn allows alias analysis to be more precise.
Yes. That’s right.
>
> If we want to handle the VLA case using pointers instead then I think
> that needs to be a different IFN.
>
> If we did handle the VLA case using pointers (not expressing an opinion
> on that), then it would be the caller's job to allocate the VLA and work
> out the address of the VLA;
the current routine “gimplify_vla_decl” has done this already:
It created a temporary variable for the address of the VLA, and created a call
to “alloca” to allocate the VLA.
My -ftrivial-auto-var-init work just try to use the “address variable of the
VLA” in the new .DEFERRED_INIT call to carry it to RTL expansion phase.
> this isn't something that .DEFERRED_INIT
> would work out on the caller's behalf. The address of the VLA should
> therefore be an argument to the new IFN, rather than something that
> the IFN returns.
Then what’s the LHS of this call? Currently the major issue is the LHS is
invalid gimple.
Qing
>
> Thanks,
> Richard
>
