[llvm-dev] Expected behavior of lld during LTO for global symbols (Attr Internal/Common)

[Rui Ueyama via llvm-dev]

Looks like this is indeed related to r360841.

In C, there are distinctions between declarations, definitions and
tentative definitions. Global variables declared with "extern" are
declarations. Global variables that don't have "extern" and have
initializers are definitions. If global variables have neither "extern" nor
initializers, they are called tentative definitions.

Common symbols represent tentative definitions.

Tentative definition get special treatment in the linker. Usually if you
define the same symbol in two object files, a linker report an error.
However, common symbols are allowed to duplicate. Two or more common
symbols are merged and then placed to the .bss section, so that they will
be zero-initialized at runtime.

So, a global variable defined as `struct Node* head` is actually a common
symbol.

I'm not sure why LTO cannot internalize common symbols though. Teresa, is
this expected?

On Mon, Jun 10, 2019 at 11:06 PM Teresa Johnson <tejohnson at google.com>
wrote:

> My guess is that it is due to lld change r360841 on that date (Introduce
> CommonSymbol). +Rui for comments.
>
> On Mon, Jun 10, 2019 at 4:45 AM Mani, Suresh via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>>
>>
>>
>>
>> Hi ,
>>
>>
>>
>> I have an issue during LTO phase of llvm compiler which is as follows,
>>
>>
>>
>>
>>
>> File t3.c
>>
>> ---------
>>
>>
>>
>>
>>
>> #include <stdio.h>
>>
>> #include <stdlib.h>
>>
>>
>>
>> // A linked list node
>>
>> struct Node {
>>
>>     int data;
>>
>>     struct Node* next;
>>
>>     struct Node* prev;
>>
>> };
>>
>>
>>
>> *struct Node* head;*
>>
>>
>>
>> /* Given a reference (pointer to pointer) to the head of a list
>>
>> and an int, inserts a new node on the front of the list. */
>>
>> void push(struct Node** head_ref, int new_data)
>>
>> {
>>
>>     struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));
>>
>>
>>
>>     new_node->data = new_data;
>>
>>
>>
>>     new_node->next = (*head_ref);
>>
>>     new_node->prev = NULL;
>>
>>
>>
>>     if ((*head_ref) != NULL)
>>
>>         (*head_ref)->prev = new_node;
>>
>>
>>
>>     (*head_ref) = new_node;
>>
>> }
>>
>>
>>
>>
>>
>> // This function prints contents of linked list starting from the given
>> node
>>
>> void printList(struct Node* node)
>>
>> {
>>
>>     struct Node* last;
>>
>>     printf("\nTraversal in forward direction \n");
>>
>>     while (node != NULL) {
>>
>>         printf(" %d ", node->data);
>>
>>         last = node;
>>
>>         node = node->next;
>>
>>     }
>>
>>
>>
>>     printf("\nTraversal in reverse direction \n");
>>
>>     while (last != NULL) {
>>
>>         printf(" %d ", last->data);
>>
>>         last = last->prev;
>>
>>     }
>>
>> }
>>
>>
>>
>>
>>
>> /* Driver program to test above functions*/
>>
>> int main()
>>
>> {
>>
>>
>>
>>     head = NULL;
>>
>>     push(&head, 7);
>>
>>     push(&head, 1);
>>
>>     push(&head, 4);
>>
>>
>>
>>     printList(head);
>>
>>
>>
>>     return 0;
>>
>> }
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> Compiler invocation:
>>
>> --------------------
>>
>>
>>
>> clang -flto -fuse-ld=lld -O3 t3.c -o a.out
>>
>>
>>
>>
>>
>> Expected behavior during LTO:
>>
>> ------------------------------
>>
>>
>>
>> The compiler optimization during LTO needs to figure out that variable
>> "head" is not referred by any precompiled object or library.
>>
>> Until May-16-2019 variable "head" had internal attribute as follows,
>>
>>
>>
>> @head = internal global %struct.Node* null, align 8
>>
>>
>>
>> And the compiler was rightly able to recognize that "head" is not
>> referred by any external precompiled object or library.
>>
>>
>>
>> But after May-16-2019  the attribute of head was changed as follows,
>>
>>
>>
>> @head = common dso_local global %struct.Node* null, align 8
>>
>>
>>
>>
>>
>> Not sure if this is correct behavior?
>>
>>
>>
>> If this is a correct behavior then can you please let me know how could
>> the compiler figure out that variable "head" is not referred by any
>> external precompiled object or library?
>>
>>
>>
>>
>>
>> Thanks
>>
>> M Suresh
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>
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