LLVM Create VarArg Function and access var args - llvm

Question

I have been trying to create a function using the module pass in LLVM. What I am trying to do is create a variable argument function and then add the logic to manipulate the variable arguments.
For example:
/\*can do this\*/
int foo(int a, ...)
{
double var1;
//can't figure out how to add any of this using llvm
va_list ap;
va_start(ap, a);
va_arg(var1,double);
va_end(ap);
}
Creating the function type is easy because I just set the vararg boolean to true. What do I do after that?

Answer 1

I always use clang to check what it needs to convert for c/c++ lang.
Use llvm instruction va_arg and intinsics llvm.va_start, llvm.va_end, llvm.va_copy to use llvm variable argument support.
you also need target-specific value type “va_list” for functions that operates on arguments that use this.
; This struct is different for every platform. For most platforms,
; it is merely an i8*.
%struct.va_list = type { i8* }
; For Unix x86_64 platforms, va_list is the following struct:
; %struct.va_list = type { i32, i32, i8*, i8* }
ref http://llvm.org/docs/LangRef.html#variable-argument-handling-intrinsics
for your listed code,
; ModuleID = 'test.c'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-linux-gnu"
%struct.__va_list_tag = type { i32, i32, i8*, i8* }
; Function Attrs: nounwind uwtable
define i32 #foo(i32 %a, ...) #0 {
%1 = alloca i32, align 4
%2 = alloca i32, align 4
%var1 = alloca double, align 8
%ap = alloca [1 x %struct.__va_list_tag], align 16
store i32 %a, i32* %2, align 4
%3 = getelementptr inbounds [1 x %struct.__va_list_tag]* %ap, i32 0, i32 0
%4 = bitcast %struct.__va_list_tag* %3 to i8*
call void #llvm.va_start(i8* %4)
%5 = getelementptr inbounds [1 x %struct.__va_list_tag]* %ap, i32 0, i32 0
%6 = getelementptr inbounds %struct.__va_list_tag* %5, i32 0, i32 1
%7 = load i32* %6
%8 = icmp ule i32 %7, 160
br i1 %8, label %9, label %15
; <label>:9 ; preds = %0
%10 = getelementptr inbounds %struct.__va_list_tag* %5, i32 0, i32 3
%11 = load i8** %10
%12 = getelementptr i8* %11, i32 %7
%13 = bitcast i8* %12 to double*
%14 = add i32 %7, 16
store i32 %14, i32* %6
br label %20
; <label>:15 ; preds = %0
%16 = getelementptr inbounds %struct.__va_list_tag* %5, i32 0, i32 2
%17 = load i8** %16
%18 = bitcast i8* %17 to double*
%19 = getelementptr i8* %17, i32 8
store i8* %19, i8** %16
br label %20
; <label>:20 ; preds = %15, %9
%21 = phi double* [ %13, %9 ], [ %18, %15 ]
%22 = load double* %21
%23 = getelementptr inbounds [1 x %struct.__va_list_tag]* %ap, i32 0, i32 0
%24 = bitcast %struct.__va_list_tag* %23 to i8*
call void #llvm.va_end(i8* %24)
%25 = load i32* %1
ret i32 %25
}
; Function Attrs: nounwind
declare void #llvm.va_start(i8*) #1
; Function Attrs: nounwind
declare void #llvm.va_end(i8*) #1
; Function Attrs: nounwind uwtable
define i32 #main() #0 {
ret i32 0
}