Jake Goulding

Uninitialized variable warnings and compiler optimizations

A coworker recently got the may be used uninitialized in this function warning from GCC and asked me to take a look at it to see if the warning was spurious or if he was missing something. While trying to boil his code down to a concise example, we found an example where the warning got flipped – we no longer got a warning even though we both expected one.

Here is the example code we ended up with. foo is a function that should set value when the parameter is true. If the parameter is not set, then value is undefined. The main simply calls foo, using the command-line argument count to control what is passed to the function.

(uninit-warning.c) download
#include <stdio.h>
#include <stdlib.h>

foo(int do_it) {
  int value;

  if (do_it) {
    value = 7;

  if (7 == value) {
    printf("It is 7\n");

main(int argc, char *argv[])
  foo(argc < 2);

  return 0;

We compiled it with GCC, expecting to see a warning.

$ gcc --version
gcc (Gentoo 4.4.4-r2 p1.2, pie-0.4.5) 4.4.4
$ gcc -o uninit-gcc uninit.c -O3 -Wall -pedantic

Strange, no warning. What does Clang say?

$ clang --version
clang version 2.8 (branches/release_28)
$ clang -o uninit-clang uninit.c -O3 -Wall -pedantic

Neither compilation produces warnings! We tried compiling with -O0 and -Os, in case some optimization was being applied, but the results were the same. Curiosity piqued, we dropped down to the next most logical level – assembly.

GCC disassembly

0000000000400570 <foo>:
  400570:       be 9c 06 40 00          mov    $0x40069c,%esi
  400575:       bf 01 00 00 00          mov    $0x1,%edi
  40057a:       31 c0                   xor    %eax,%eax
  40057c:       e9 d7 fe ff ff          jmpq   400458 <__printf_chk@plt>
  400581:       66 66 66 66 66 66 2e    nopw   %cs:0x0(%rax,%rax,1)
  400588:       0f 1f 84 00 00 00 00 
  40058f:       00 

If you want to follow along, I used the AMD64 Architecture Programmer’s Manual as an assembly reference.

GCC starts out by loading the address of the string into %esi, then loads 1 into %edi and %eax is then XORed against itself, which sets %eax to 0. According to the X86_64 calling convention, %edi and %esi are the first and second integral parameters to the function about to be called and %eax is used as the return value from the current function. Since we only have one parameter to printf, and foo is a void function, I think that %edi and %eax must have something to do with calling a variadic function. The code unconditionally jumps to printf. The function finishes off with nopw, a fancy way of using a NOP to pad out to a nice even boundary.

Clang disassembly

0000000000400540 <foo>:
  400540:       55                      push   %rbp
  400541:       48 89 e5                mov    %rsp,%rbp
  400544:       bf 5c 06 40 00          mov    $0x40065c,%edi
  400549:       5d                      pop    %rbp
  40054a:       e9 e1 fe ff ff          jmpq   400430 <puts@plt>
  40054f:       90                      nop

Clang produces pretty much the same code, but does some different gyrations. The contents of %rpb are pushed onto the stack, then %rsp is moved to %rpb. The address of the string is loaded into %edi and %rbp is restored – never having been used. Clang has optimized the call to printf to instead use the simpler puts function, so we don’t see the same mucking about with %edi and %eax. However, it is still an unconditional jump!

Clang LLVM output

Clang has an interesting feature that lets you dump the abstract syntax tree just like assembly.

define void @foo(i32 %do_it) nounwind {
  %puts = tail call i32 @puts(i8* getelementptr inbounds ([8 x i8]* @str, i64 0, i64 0))
  ret void

We can see that it ignores the parameter do_it.

Visual Studio 2010

Reddit user DeliciouslyMoist shows that Visual Studio 2010 does have a warning.

cl -c -Ox -W4 test.cpp
test.cpp(18) : warning C4100: 'argv' : unreferenced formal parameter
test.cpp(12) : warning C4701: potentially uninitialized local variable 'value' used

Along with the generated assembly. Note this is Windows assembly, so the Linux calling conventions above don’t apply.

?foo@@YAXH@Z (void __cdecl foo(int)):
  00000000: 83 7C 24 04 00     cmp         dword ptr [esp+4],0
  00000005: 75 07              jne         0000000E
  00000007: 83 7C 24 04 07     cmp         dword ptr [esp+4],7
  0000000C: 75 0D              jne         0000001B
  0000000E: C7 44 24 04 00 00  mov         dword ptr [esp+4],offset $SG4825
        00 00
  00000016: E9 00 00 00 00     jmp         _printf

Clang static analyzer

As Reddit user masklinn points out, the clang static analyzer does output a warning, even though the clang compiler does not.

$ clang --analyze uninit.c 
uninit.c:12:9: warning: The right operand of '==' is a garbage value
  if (7 == value) {
      ^  ~~~~~
1 warning generated.


The behavior and generated code of the compilers is interesting, but I think they are allowed to do whatever they want when optimizing undefined code, so I can’t fault them for that.

What both my coworker and I were surprised was the lack of warning. In general, we have come to rely on GCC to be overly zealous about providing warnings. I’d much rather have an extraneous warning that I can silence instead of no warning at all.

Edit 2010-01-28 – added Visual Studio and clang analyzer output from reddit.