https://sourceware.org/bugzilla/show_bug.cgi?id=29900
Bug ID: 29900
Summary: Partial Linking removes alignment from linker
directives
Product: binutils
Version: 2.39
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: ld
Assignee: unassigned at sourceware dot org
Reporter: denys.li...@ipg-automotive.com
Target Milestone: ---
Hello everyone,
the following bug report is related to Windows and can be reproduced with the
mingw-w64 toolchain, that is available at:
https://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win64/Personal%20Builds/mingw-builds/8.1.0/threads-win32/seh/x86_64-8.1.0-release-win32-seh-rt_v6-rev0.7z/download
A Microsoft Visual Studio installation might be required as well.
All commands are considered to be done at cmd.exe.
First, extract mingw-w64 and append its bin folder (D:\mingw64\bin in my case)
to the PATH environment variable.
> set PATH=%PATH%"D:\mingw64\bin";
The potential bug can be reproduced with the following source code:
// aligned.c
int var;
int aligned_var __attribute__((aligned(32)));
Compile aligned.c, but dont run the linker
> gcc.exe -c -o aligned.o aligned.c
Do a partial linking step:
> ld.exe -r -o linked.o aligned.o
As far as I can tell it is not possible to check the symbol alignment with one
of the binutils tools. Therefore I use dumpbin.exe, which is part of MS Visual
Studio.
Extend the PATH environment variable in order to use dumpbin.exe. The actual
path depends on the installed Visual Studio version and might be different on
your system.
> set PATH=%PATH%"C:\Program Files (x86)\Microsoft Visual
> Studio\Community\VC\Tools\MSVC\14.23.28105\bin\Hostx64\x64";
Now check the alignment of the symbols in the object file.
> dumpbin.exe /directives aligned.o
Microsoft (R) COFF/PE Dumper Version 14.23.28107.0
Copyright (C) Microsoft Corporation. All rights reserved.
Dump of file aligned.o
File Type: COFF OBJECT
Linker Directives
-----------------
-aligncomm:var,2
-aligncomm:aligned_var,5
Summary
0 .bss
0 .data
30 .drectve
20 .rdata$zzz
0 .text
The dumpbin utility shows the alignment of the symbols var (2^2) and
aligned_var (2^5) like expected. Lets do the same check for the (partial)
linked object file.
> dumpbin.exe /directives linked.o
Microsoft (R) COFF/PE Dumper Version 14.23.28107.0
Copyright (C) Microsoft Corporation. All rights reserved.
Dump of file linked.o
File Type: COFF OBJECT
Summary
0 .bss
0 .data
20 .rdata$zzz
0 .text
As we can see the alignment information got lost at the partial linking step. I
think this is a bug, isn't it?
The behavior is the same for the latest Binutils (2.39 at this time) as well.
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