[cfe-dev] Is it a va_arg bug in clang?

[Richard Smith via cfe-dev]

On Thu, Jan 7, 2016 at 11:15 PM, Shi, Steven via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> For the variadic function error with AMD64 abi and windows calling
> convention on 64bits x86, I find it has been tracked in Bug 20847 (
> https://llvm.org/bugs/show_bug.cgi?id=20847) (
> http://reviews.llvm.org/D1622#inline-9345). Do we still plan to fix it?
>
>
>
> You know, I meet exactly same va_arg mistake with llvm3.7 when I enable
> the Uefi firmware (http://www.uefi.org/) build with clang. The ms_abi is
> the Uefi firmware binary module standard interface. I really hope this bug
> fix can been checked in as soon as possible. If we cannot fix it in short
> time, could we offer a temporary fix patch based on llvm3.7?
>

This is already fixed in clang trunk. Your code is rejected as follows:

<stdin>:15:17: error: 'va_start' used in Win64 ABI function
                va_start (Marker, Format);
                ^

You need to use __builtin_ms_va_list, __builtin_ms_va_start,
__builtin_ms_va_end to use ms_abi varargs semantics. (In particular, note
that __builtin_ms_va_list is a different type from va_list, so this can't
"just work" in the way you expect it to.)


> *Steven Shi*
>
> *Intel\SSG\STO\UEFI Firmware*
>
>
>
> Tel: +86 021-61166522
>
> iNet: 821-6522
>
>
>
> *From:* Shi, Steven
> *Sent:* Thursday, January 07, 2016 11:07 PM
> *To:* 'cfe-dev at lists.llvm.org'
> *Subject:* Is it a va_arg bug in clang?
>
>
>
> Hello,
>
> I’m trying to use the clang3.7 to compile  64bits x86 code with Microsoft
> ABI in Ubuntu 64bit, but I find the va_arg get the wrong variable argument
> from the va_list. Below is my test code.
>
>
>
> #include <stdio.h>
>
> #include <stdarg.h>
>
>
>
> void
>
> __attribute__((ms_abi))
>
> Foo_va_list (
>
>     int                       VaNum,
>
>     const char  *Format,
>
>   ...
>
>   )
>
> {
>
>                 va_list  Marker;
>
>                 long long    Value;
>
>
>
>                 va_start (Marker, Format);
>
>                 for (int i = 0; i < VaNum; i++ ) {
>
>                                 Value = va_arg (Marker, int);
>
>                                 printf("Value = 0x%llx\n", Value);
>
>                 }
>
>                 va_end (Marker);
>
> }
>
>
>
>
>
> int main()
>
> {
>
>   Foo_va_list (16, "0123456789abcdef= %x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x
> \n", 0,1,2,3,4,5,6,7,8,9,0xa,0xb,0xc,0xd,0xe,0xf);
>
>   return 0;
>
> }
>
>
>
> The expected output are:
>
> Value = 0x0
>
> Value = 0x1
>
> Value = 0x2
>
> Value = 0x3
>
> Value = 0x4
>
> Value = 0x5
>
> Value = 0x6
>
> Value = 0x7
>
> Value = 0x8
>
> Value = 0x9
>
> Value = 0xa
>
> Value = 0xb
>
> Value = 0xc
>
> Value = 0xd
>
> Value = 0xe
>
> Value = 0xf
>
>
>
> But the real output are:
>
> Value = 0x0
>
> Value = 0x1
>
> Value = 0x2
>
> Value = 0x3
>
> Value = 0x4
>
> Value = 0x5
>
> Value = 0x0
>
> Value = 0x1
>
> Value = 0x2
>
> Value = 0x3
>
> Value = 0x4
>
> Value = 0x5
>
> Value = 0x6
>
> Value = 0x7
>
> Value = 0x8
>
> Value = 0x9
>
>
>
> After I remove the Microsoft ABI definition __attribute__((ms_abi)) above
>  Foo_va_list () in the code, the output will become correct. I find GCC
> also have similar wrong behavior with __attribute__((ms_abi)). I don’t know
> how to make the __attribute__((ms_abi)) work correctly with va_arg.  I
> appreciate if any suggestion.
>
>
>
>
>
>
>
>
>
>
>
>
>
> *Steven Shi*
>
> *Intel\SSG\STO\UEFI Firmware*
>
>
>
> Tel: +86 021-61166522
>
> iNet: 821-6522
>
>
>
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
>
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