[lucy-issues] [jira] [Updated] (LUCY-234) Brittle object struct ABI

["Marvin Humphrey (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/LUCY-234?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Marvin Humphrey updated LUCY-234:
---------------------------------

    Attachment: member_offset_variable.c
                member_offset_constant.c

The files member_offset_constant.c and member_offset_variable.c illustrate one
approach: accessing member vars via variable offsets at runtime.

When those files are compiled to assembler with optimization enabled
({{cc -Wall -Wextra -c -S -O2}}), here are the only differences:

{noformat}
$ diff -u member_offset_constant.s member_offset_variable.s
--- member_offset_constant.s	2012-09-07 11:16:10.000000000 -0700
+++ member_offset_variable.s	2012-09-07 11:16:05.000000000 -0700
@@ -12,7 +12,8 @@
 	movq	%rsp, %rbp
 Ltmp4:
 	.cfi_def_cfa_register %rbp
-	movl	%esi, 8(%rdi)
+	movq	_Foo_num_OFFSET(%rip), %rax
+	movl	%esi, (%rdi,%rax)
 	popq	%rbp
 	ret
 	.cfi_endproc
@@ -30,7 +31,8 @@
 	movq	%rsp, %rbp
 Ltmp9:
 	.cfi_def_cfa_register %rbp
-	movl	8(%rdi), %eax
+	movq	_Foo_num_OFFSET(%rip), %rax
+	movl	(%rdi,%rax), %eax
 	popq	%rbp
 	ret
 	.cfi_endproc
$
{noformat}
                
> Brittle object struct ABI
> -------------------------
>
>                 Key: LUCY-234
>                 URL: https://issues.apache.org/jira/browse/LUCY-234
>             Project: Lucy
>          Issue Type: Bug
>          Components: Clownfish
>            Reporter: Marvin Humphrey
>         Attachments: member_offset_constant.c, member_offset_variable.c
>
>
> If an upstream parcel adds, removes or rearranges member variables,
> downstream extensions that build on the affected classes will break.

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