Fwd: [llvm-dev] [patch] fix Thumb text sections' flags under COFF/Windows

[Adrien Guinet via llvm-commits]

Hello!

I had no answer from llvm-dev for this patch below, I'm reposting it
here 'cause I think I got the wrong mailing list? (according to
http://llvm.org/docs/DeveloperPolicy.html#making-and-submitting-a-patch
where I forgot the keyword *commit* mailing-list).

Let me know if I missed something or need more information!

Regards,

On 06/19/2016 06:36 PM, Adrien Guinet via llvm-dev wrote:
> Hello everyone,
> 
> Here is attached a patch that fixes the flags of text sections when
> compiling thumb code under Windows using clang. The patch has been made
> against the svn commit r273084.
> 
> The main issue is that the "thumb" flag wasn't set for some of these
> sections, making MSVC's link.exe fails to correctly relocate code
> against the symbols inside these sections. link.exe could fail for
> instance with the "fixup is not aligned for target 'XX'" error. If
> linking doesn't fail, the relocation process goes wrong in the end and
> invalid code is generated by the linker.
> 
> To reproduce the error, one can compile this file as a static library:
> 
>> #include <stdio.h>
>>
>> void test(const char* s)
>> {
>> 	const void* ptr = &snprintf;
>> 	printf("%s: %p\n", s, ptr);
>> }
> 
> and then create a dynamic library that links against this static library
> (which calls the 'test' function). A visual studio project can be
> provided for people interested.
> 
> As a side note, I fixed something I think is a potential bug here:
> 
>> Index: lib/MC/MCObjectFileInfo.cpp
>> ===================================================================
>> --- lib/MC/MCObjectFileInfo.cpp (revision 273084)
>> +++ lib/MC/MCObjectFileInfo.cpp (working copy)
>> @@ -595,7 +595,7 @@
>>                         COFF::IMAGE_SCN_MEM_READ | COFF::IMAGE_SCN_MEM_WRITE,
>>        SectionKind::getData());
>>  
>> -  bool IsWoA = T.getArch() == Triple::arm || T.getArch() == Triple::thumb;
>> +  const bool IsThumb = T.getArch() == Triple::thumb;
>>  
>>    CommDirectiveSupportsAlignment = true;
>>  
>> @@ -606,7 +606,7 @@
>>        SectionKind::getBSS());
>>    TextSection = Ctx->getCOFFSection(
>>        ".text",
>> -      (IsWoA ? COFF::IMAGE_SCN_MEM_16BIT : (COFF::SectionCharacteristics)0) |
>> +      (IsThumb ? COFF::IMAGE_SCN_MEM_16BIT : (COFF::SectionCharacteristics)0) |
>>            COFF::IMAGE_SCN_CNT_CODE | COFF::IMAGE_SCN_MEM_EXECUTE |
>>            COFF::IMAGE_SCN_MEM_READ,
>>        SectionKind::getText());
> 
> Indeed, the "SCN_MEM_16BIT" flag (which indicate that the text sections
> contain thumb code [1]) is set even if we are compiling non-thumb code.
> I couldn't check this as thumb mode seems to be forced in
> clang/lib/Driver/ToolChain.cpp, function ToolChain::ComputeLLVMTriple :
> 
>>     // FIXME: this is invalid for WindowsCE
>>     if (getTriple().isOSWindows())
>>       ThumbDefault = true;
>> ...
>>     if ((InputType != types::TY_PP_Asm && Args.hasFlag(options::OPT_mthumb,
>>          options::OPT_mno_thumb, ThumbDefault)) || IsMProfile) {
>>       if (IsBigEndian)
>>         ArchName = "thumbeb";
>>       else
>>         ArchName = "thumb";
>>     }
> 
> Commenting this and forcing the ARM mode by default will makes
> clang/llvm complains that this isn't supported (I didn't dug further).
> 
> If more information are necessary, I'd be glad to provide them!
> Thanks!
> 
> Regards,
> 
> [1] : in the official COFF documentation (https://msdn.microsoft.com/en-us/library/windows/hardware/gg463119.aspx), this flag is set as
> "reserved for future use". However, using the dumpbin.exe tool provided
> with Visual Studio (with the /headers flag), it seems that this flag is
> the one that set the section as containing thumb code.

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