CODEWITHSUNDEEP
llvmllvm-clangllvm-ir
teslik
teslik

Reputation: 21

LLVM Create VarArg Function and access var args

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?

Upvotes: 1

Views: 2839

Answers (1)

Chirag Patel
Chirag Patel

Reputation: 1161

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
    }

Upvotes: 4

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