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

[Teresa Johnson via llvm-dev]

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);
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>         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
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>


-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |
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