On 11/23/20 12:04 PM, Richard Biener wrote:
> On Mon, Nov 23, 2020 at 10:53 AM Marius Hillenbrand via Gcc
> <gcc@gcc.gnu.org> wrote:
>>
>> Hi,
>>
>> Digging into a test case failure with section anchors, I found
>> dependence analysis return false negatives, leading to bad optimization
>> by cse1. Two variables are synthetically constructed aliases. One is
>> addressed relative to the section anchor and the other using a symbol
>> ref, yet write_dependence_p() claims that they could not alias. I can
>> reproduce that miscompile on aarch64, ppc64, and s390x. How is
>> write_dependence_p() supposed to handle such cases with section anchors?
>>
>>
>> Example: (simplified version of gcc.c-torture/execute/alias-2.c)
>> int off; // causing nonzero offset of a to the section anchor
>> int a;
>> extern int b __attribute__ ((alias("a")));
>> /* ... */
>> int main()
>> b=1; <-- uses symbol_ref to b
>> a=2; <-- uses the section anchor + offset 4
>> if (b!=2)
>> __builtin_abort ();
>> return 0;
>> }
>>
>> so write_dependence gets to compare the canonicalized addresses
>> (const:DI (plus:DI (symbol_ref:DI ("*.LANCHOR0") [flags 0x182])
>> (const_int 4 [0x4])))
>> and
>> (symbol_ref:DI ("b") [flags 0x602] <var_decl b>)
>> and
>>
[...]
>
> If the above is all we get to see then we have to give up when either
> of the address is based on a section anchor and we know the other
> one could possibly be refered through a section anchor.
>
> In those cases we have to rely on MEM_EXPRs for disambiguation
> which, for the anchored address, is unfortunately lost?
Actually, both mem rtx have their MEM_EXPRs attached, in the cases I
looked at.
for example
(mem/c:SI (const:DI (plus:DI
(symbol_ref:DI ("*.LANCHOR0") [flags 0x182])
(const_int 4 [0x4]))) [1 a+0 S4 A32])
We could add a check for cases where MEM_EXPRs are available and clearly
indicate aliasing or overlap. That check would happen before the other
methods that only look at partial information (e.g., base_alias_check,
memrefs_conflict_p) -- to catch such clear cases without the risk of
making the other checks more pessimistic.
>
>> (see also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92294 for the
>> initial report on aarch64 and my full example)
>>
>> Marius
--
Marius Hillenbrand
Linux on Z development
IBM Deutschland Research & Development GmbH
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