Target-Triple changes in assembly; Who is wrong?

Clang Frontend
#1

Hello Clang-People,

I try to understand the system behind the target triple of clang - but I don’t get it. I used clang to compile the same code with two different targets:
–target=x86_64-pc-windows-msvc and –target=x86_64-pc-windows-elf.

I let the dragon compile my code and wished for assembly output. The dragon did fine but the output confused me.

When I looked into the functions I discovered strange difference. For example:
msvc target generated:
movq %rdx, %rax

but in elf the same line is:
movq %rcx, %rax

Why is there (and in other places) such difference? The target is still windows, but the elf code is breaking the windows __thiscall calling convention. I expected that both output are compatible to windows.

Is the dragon wrong, or is it me?

Kind regards
Björn

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#2

Without the context of the code being compiled, it’s impossible to say if LLVM is right or not. It’s worth mentioning though that there is no __thiscall convention on x64. That’s a 32-bit-only thing.

If I had to guess, I’d say this is some struct return ABI detail that’s going wrong.

#3

Which kind of context would help you to judge who is wrong?
Do you want the two assembly files? The C++ source code?

#4

(When you receive 2 Mails I’m sorry. There were problems with my mail program)

Which kind of context would help you to judge who is wrong?
Do you want the two assembly files? The C++ source code?