[llvm-dev] Can the entry of a function be messed up in a slightly edited IR .ll file?

[Peng Yu via llvm-dev]

Hi,

I manually added the following code (labeled with ADDED, lines without
ADDED are to show the context) to a .ll file compiled from a .bc file.
(This .bc file can run correctly.) No other lines are changed in the
.ll file.

declare void @__trace_init() ; ADDED
declare void @__trace_block_enter(i8*, i8*) ; ADDED

@mystr.1 = private unnamed_addr constant [5 x i8] c"main\00", align 1 ; ADDED
@mystr.2 = private unnamed_addr constant [3 x i8] c"bb\00", align ; ADDED
; Function Attrs: noinline nounwind optnone uwtable
define dso_local i32 @main(i32 %arg, i8** %arg1, i8** %arg2) #0 !dbg !10092 {
bb:
  call void @__trace_init() ; ADDED
  call void @__trace_block_enter(i8* getelementptr inbounds ([5 x i8],
[5 x i8]* @mystr.1, i32 0, i32 0), i8* getelementptr inbounds ([3 x
i8], [3 x i8]* @mystr.2, i32 0, i32 0)) ; ADDED
  %tmp = alloca i32, align 4

__trace_init() and __trace_block_enter() are defined in trace.c as
shown below. Then I compile the .ll file and trace.c to two new .bc
files, and llvm-link them to generate a single final .bc file. Then I
run the final .bc file using lli with the environment variable
TRACE_OUTFILE set. Depending on the command line options to the final
.bc file, sometimes, the TRACE_OUTFILE contains more lines than the
number of times "__trace_block_enter" printed to the screen.

This is really strange to me. I can only think of one possibility,
__trace_block_enter() may be entered multiple times not at the entry
points. Could this ever happen? Are there any other hypotheses why
this could happen and how to test them? Thanks.

$ cat trace.c
/* vim: set noexpandtab tabstop=2: */
#include <stdio.h>
#include <stdlib.h>

static FILE *fp;

void __trace_init() {
    puts("__trace_init");
    const char *s=getenv("TRACE_OUTFILE");
    if(s) {
        fp = fopen(s, "w");
        if(!fp) {
            perror("Failed to open TRACE_OUTFILE");
            exit(1);
        }
    } else {
        fputs(
                "\x1b[31;1mThe environment variable TRACE_OUTFILE must
be set.\x1b[m\n"
                , stderr
                );
        exit(1);
    }
}

void __trace_block_enter(char *caller, char *basic_block) {
    puts("__trace_block_enter");
    fprintf(fp, "%s\t%s\n", caller, basic_block);
}

void __trace_func_enter(char *caller, char *basic_block, char *callee) {
    fprintf(fp, "%s\t%s\t%s\n", caller, basic_block, callee);
}

-- 
Regards,
Peng