I would like to be able to instrument va_arg, but when I generate a bc
file for a test case using:
llvm-gcc -O3 -emit-llvm vararg.c -c -o vararg.bc
I do not see va_arg. Instead, it seems the args are accessed through
%struct.__va_list_tag, which makes things a bit trickier to
instrument. Is there a way to force llvm-gcc to use va_arg?
Perhaps there is some documentation about va_list_tag or llvm's use of
stdarg that would help me understand what is going on?
Here is the C code...
int sum(int n, ...) {
int sum = 0;
va_list list;
va_start(list, n);
int i;
for (i=0; i<n; ++i) {
sum += va_arg(list, int);
}
va_end(list);
return sum;
}
And the snippet from vararg.ll:
; ModuleID = 'vararg.bc'
target datalayout
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
target triple = "x86_64-unknown-linux-gnu"
%struct.__va_list_tag = type { i32, i32, i8*, i8* }
define i32 @sum(i32 %n, ...) nounwind {
entry:
%list = alloca [1 x %struct.__va_list_tag], align 8
; <[1 x %struct.__va_list_tag]*> [#uses=4]
%list12 = bitcast [1 x %struct.__va_list_tag]* %list to i8*
; <i8*> [#uses=2]
call void @llvm.va_start(i8* %list12)
%0 = icmp sgt i32 %n, 0 ; <i1> [#uses=1]
br i1 %0, label %bb.nph, label %bb7
bb.nph: ; preds = %entry
%1 = getelementptr [1 x %struct.__va_list_tag]* %list, i64 0,
i64 0, i32 0 ; <i32*> [#uses=2]
%2 = getelementptr [1 x %struct.__va_list_tag]* %list, i64 0,
i64 0, i32 3 ; <i8**> [#uses=1]
%3 = getelementptr [1 x %struct.__va_list_tag]* %list, i64 0,
i64 0, i32 2 ; <i8**> [#uses=2]
br label %bb
bb: ; preds = %bb5, %bb.nph
%i.011 = phi i32 [ 0, %bb.nph ], [ %indvar.next, %bb5 ]
; <i32> [#uses=1]
%sum.012 = phi i32 [ 0, %bb.nph ], [ %14, %bb5 ]
; <i32> [#uses=1]
%4 = load i32* %1, align 8 ; <i32> [#uses=3]
%5 = icmp ult i32 %4, 48 ; <i1> [#uses=1]
br i1 %5, label %bb3, label %bb4
bb3: ; preds = %bb
%6 = load i8** %2, align 8 ; <i8*> [#uses=1]
%7 = inttoptr i32 %4 to i8* ; <i8*> [#uses=1]
%8 = ptrtoint i8* %7 to i64 ; <i64> [#uses=1]
%ctg2 = getelementptr i8* %6, i64 %8 ; <i8*> [#uses=1]
%9 = add i32 %4, 8 ; <i32> [#uses=1]
store i32 %9, i32* %1, align 8
br label %bb5
bb4: ; preds = %bb
%10 = load i8** %3, align 8 ; <i8*> [#uses=2]
%11 = getelementptr i8* %10, i64 8 ; <i8*> [#uses=1]
store i8* %11, i8** %3, align 8
br label %bb5
bb5: ; preds = %bb4, %bb3
%addr.0.0 = phi i8* [ %10, %bb4 ], [ %ctg2, %bb3 ]
; <i8*> [#uses=1]
%12 = bitcast i8* %addr.0.0 to i32* ; <i32*> [#uses=1]
%13 = load i32* %12, align 4 ; <i32> [#uses=1]
%14 = add i32 %13, %sum.012 ; <i32> [#uses=2]
%indvar.next = add i32 %i.011, 1 ; <i32> [#uses=2]
%exitcond = icmp eq i32 %indvar.next, %n ; <i1>
[#uses=1]
br i1 %exitcond, label %bb7, label %bb
bb7: ; preds = %bb5, %entry
%sum.0.lcssa = phi i32 [ 0, %entry ], [ %14, %bb5 ]
; <i32> [#uses=1]
call void @llvm.va_end(i8* %list12)
ret i32 %sum.0.lcssa
}