[LLVMdev] [BUG] Varargs example in LangRef segfaults

[Reid Kleckner]

The problem is that the type of the va_list is platform-specific. No IR
example will work on all platforms, unless you overallocate. The LangRef
should probably be updated to use the SysV x86_64 va_list as that is the
most common and mention in an IR comment that the type depends on the
platform.


On Tue, Aug 26, 2014 at 12:37 PM, Ramkumar Ramachandra <artagnon at gmail.com>
wrote:

> Hi,
>
> So the Variable Argument Handling Intrinsics section of the LangRef
> (http://llvm.org/docs/LangRef.html#variable-argument-handling-intrinsics)
> lists an example that segfaults. Try the following on x86_64:
>
> -- 8< --
> define i32 @test(i32 %X, ...) {
>   ; Initialize variable argument processing
>   %ap = alloca i8*
>   %ap2 = bitcast i8** %ap to i8*
>   call void @llvm.va_start(i8* %ap2)
>
>   ; Read a single integer argument
>   %tmp = va_arg i8** %ap, i32
>
>   ; Demonstrate usage of llvm.va_copy and llvm.va_end
>   %aq = alloca i8*
>   %aq2 = bitcast i8** %aq to i8*
>   call void @llvm.va_copy(i8* %aq2, i8* %ap2)
>   call void @llvm.va_end(i8* %aq2)
>
>   ; Stop processing of arguments.
>   call void @llvm.va_end(i8* %ap2)
>   ret i32 %tmp
> }
>
> define i32 @main() {
>   %call = call i32 (i32, ...)* @test(i32 1, i32 3)
>   ret i32 %call
> }
>
> declare void @llvm.va_start(i8*)
> declare void @llvm.va_copy(i8*, i8*)
> declare void @llvm.va_end(i8*)
> -- 8< --
>
> It happens because va_arg is apparently not implemented properly on
> X86 (I saw tests/Codegen/X86/vaargs.ll). What should be done about the
> situation?
> 1. Update the LangRef.
> 2. Fix va_arg for x86.
>
> If (2) is the way to go, I'll take a stab at it.
>
> Thanks.
>
> Ram
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