Fix Thumb text sections' flags under COFF/Windows (1/2)

[aguinet]

Summary

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. Forcing the ARM mode by default will makes clang/llvm complains that this isn't supported (I didn't dug further).

[compnerd]

Im not sure that this change is really worth it. We do not support Windows ARM at all. The ARM NT is a pure thumb mode execution. I don't think you can even get MSVC to generate code for ARM mode. That said, with the minor tweak to the overly verbose comment, I don't think that this harms anything as we normalize the target triple to thumb anyways and we have a number of other assumptions based on the fact that the triple is set to thumb.

lib/MC/MCObjectFileInfo.cpp

608

I think that the comment is a bit verbose. Plus, the comment is better applied to the flag calculation below. I would recommend something like:

Set the `IMAGE_SCN_MEM_16BIT` flag when compiling for thumb mode.  This is
used to indicate to the linker that the text segment contains thumb instructions
and to set the ISA selection bit for calls accordingly.

[rengolin]

I agree with Saleem. LGTM.

[rengolin]

r273880

lib/MC/MCObjectFileInfo.cpp

                        COFF::IMAGE_SCN_MEM_READ | COFF::IMAGE_SCN_MEM_WRITE,
       SectionKind::getData());
 
-  bool IsWoA = T.getArch() == Triple::arm || T.getArch() == Triple::thumb;
+  // If we are under Windows and compiling for thumb, we need to add an extra
+  // flag to the .text section to inform the linker thumb code is present! (see
+  // below).
+  // The flag is IMAGE_SCN_MEM_16BIT, which is documented by Microsoft as
+  // "reserved for future use" (see
+  // https://msdn.microsoft.com/en-us/library/windows/hardware/gg463119.aspx),
+  // but the dumpbin.exe tool from Visual Studio confirms that this set the
+  // corresponding "thumb code" flag on a section.
+  // Without this flag, the linker will interpret the code inside this section
+  // as ARM code, and will fail at lots of things (starting by the
+  // relocations).
+  const bool IsThumb = T.getArch() == Triple::thumb;
 
   CommDirectiveSupportsAlignment = true;
 

       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());