Thanks a lot for the reply.
> On Jan 12, 2024, at 11:28 AM, Richard Biener <[email protected]>
> wrote:
>
>
>
>> Am 12.01.2024 um 16:55 schrieb Qing Zhao <[email protected]>:
>>
>> Hi,
>>
>> I have some questions on using the utility routine “unshare_expr”:
>>
>> From my understanding, there should be NO shared nodes in a GENERIC function.
>> Otherwise, gimplication might fail.
>
> There is sharing and this is why we unshare everything before gimplification.
Okay, so, the "unsharing everything” is done automatically by the compiler
before gimplification?
I don’t need to worry about this?
I see many places in FE where “unshare_expr” is used, for example,
“ubsan_instrument_division”,
“ubsan_instrument_shift”, etc.
So, usually, when should “unshare_expr” be used?
>> Therefore, when we insert new tree nodes manually into the GENERIC function,
>> we should
>> Make sure there is no shared nodes introduced.
>>
>> 1. Is the above understanding correct?
>
> No
>
>> 2. Is there any tool to check there is no shared nodes in the GENERIC
>> function?
>> 3. Are there any tree nodes that are allowed to be shared in a GENERIC
>> function? If so, what are they?
>
> There’s some allowed sharing on GIMPLE and a verifier.
What’s the name of the verifier that I can search and check?
>
>> 4. For the following:
>>
>> If both “op1” and “op2” are existing tree nodes in the current GENERIC
>> function,
>> and we will insert a new tree node:
>>
>> tree new_tree = build2 (CODE, TYPE, op1, op2)
>>
>>
>> Should we add “unshare_expr” on both “op1” and “op2” as:
>>
>> Tree new_tree = build2 (CODE, TYPE, unshare_expr (op1), unshare_expr (op2))
>> ?
>
> Not necessarily but instead you have to watch for evaluating side-effects
> only once. See save_expr.
Okay. I see.
>
>>
>> If op2 is a node that is allowed to be shared, whether the additional
>> “unshare_expr” on it trigger any potential problem?
>
> If you unshare side-effects that’s generating wrong-code. Otherwise
> unsharing is safe.
Okay.
Will unnecessary unshareing produce redundant IRs?
All my questions for unshare_expr relate to a LTO bug that I currently stuck
with
when using .ACCESS_WITH_SIZE in bound sanitizer (only with -flto, without
-flto, no issue):
[opc@qinzhao-aarch64-ol8 gcc]$ sh t
during IPA pass: modref
t.c:20:1: internal compiler error: tree code ‘ssa_name’ is not supported in LTO
streams
0x14c3993 lto_write_tree
../../latest-gcc-write/gcc/lto-streamer-out.cc:561
0x14c3aeb lto_output_tree_1
And the value of the tree node that triggered the ICE is:
(gdb) call debug_tree(expr)
<ssa_name 0xfffff5761e60 type <error_mark 0xfffff56c0e58>
nothrow
def_stmt
version:13 in-free-list>
Is there any good way to debug LTO bug?
Thanks a lot for the help.
Qing
>
> Richard
>
>> Thanks a lot for your help.
>>
>> Qing
