llvm dev - Jun 2010 - [LLVMdev] Passing structures by value on Windows

Hello, LLVM team!

I faced a problem with passing structures by value on Windows (both
x86 and x64). I tried to write simple { i32 i32 } Vector2D addition
using IR builder (signature is Vector2D add(Vector2D a, Vector2D b)).
x86 version worked fine, but on x64 function returned incorrect value.

Some research showed, that { i16 i16 } addition also fails on x86, so
I guess the problem is in passing structures as values.

On x64 VC++ passes two { i32 i32 } structs in RCX and RDX respectively
and reads result from RAX, but it seems LLVM reads parameters from
ECX, EDX (first vector) and R8D, R9D (second vector).

Currently, I can't figure out how to dump IR, but there is a link with
disassembly shown by Visual Studio for generated functions:
comparing { i32 i32 } add on 32-bit and 64-bit (first one works):
http://pastebin.com/ijjCNWKJ

Best regards,
Milovanov Victor.

Hello
> On x64 VC++ passes two { i32 i32 } structs in RCX and RDX respectively
> and reads result from RAX, but it seems LLVM reads parameters from
> ECX, EDX (first vector) and R8D, R9D (second vector).
It's a frontend task to lower the code to follow the vendor ABI. In
this case you have to bitcast your struct to i64 before passing as
argument.

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

> Is this documented somethere?
This is the standard assumption. The calling convention can be complex
enough and operate with the language-dependent definitions (e.g. "C"
structs, etc.), thus frontend should lower such constructs as needed.

For reference, normal x86-64 ABI has pretty complex rules wrt passing
aggregates by value. You might want to look into llvm-gcc as an
example.

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

---------- Forwarded message ----------
From: lost <lostfreeman at gmail.com>
Date: 2010/6/5
Subject: Re: [LLVMdev] Passing structures by value on Windows
To: Anton Korobeynikov <anton at korobeynikov.info>


Ok, but how to bitcast structure?
Documentation<http://llvm.org/docs/LangRef.html#i_bitcast>says that
bitcast does not applicable to aggregate values.


2010/6/3 Anton Korobeynikov <anton at
korobeynikov.info>:
>> Is this documented somethere?
> This is the standard assumption. The calling convention can be complex
> enough and operate with the language-dependent definitions (e.g.
"C"
> structs, etc.), thus frontend should lower such constructs as needed.
>
> For reference, normal x86-64 ABI has pretty complex rules wrt passing
> aggregates by value. You might want to look into llvm-gcc as an
> example.
>
> --
> With best regards, Anton Korobeynikov
> Faculty of Mathematics and Mechanics, Saint Petersburg State University
>
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On Sat, Jun 5, 2010 at 2:16 PM, lost <lostfreeman at gmail.com>
wrote:
>
>
> ---------- Forwarded message ----------
> From: lost <lostfreeman at gmail.com>
> Date: 2010/6/5
> Subject: Re: [LLVMdev] Passing structures by value on Windows
> To: Anton Korobeynikov <anton at korobeynikov.info>
>
>
> Ok, but how to bitcast structure? Documentation says that bitcast does not
> applicable to aggregate values.
Just make an alloca, store the members of the struct to it, cast it to
an i64* (or whatever is appropriate), and load from it.

-Eli