[Bug binutils/20337] New: Objdump incorrectly disassembles zero-length functions

[aivchenk at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20337

            Bug ID: 20337
           Summary: Objdump incorrectly disassembles zero-length functions
           Product: binutils
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: binutils
          Assignee: unassigned at sourceware dot org
          Reporter: aivchenk at gmail dot com
  Target Milestone: ---

When compiling the following code:

int f() {}
int g() {}
int h() {}
int main() {
    return 0;
}

with Clang/LLVM ($clang a.cpp -O2; I used 3.7) functions f, g, h, main have the
same starting address:

0000000000400500 g     F .text  0000000000000000              f()
0000000000400500 g     F .text  0000000000000000              g()
0000000000400500 g     F .text  0000000000000000              h()
0000000000400500 g     F .text  0000000000000003              main

While it is controversial
(http://stackoverflow.com/questions/26533740/do-distinct-functions-have-distinct-addresses)
whether clang produces a sane result here, the objdump behavior for this file
is clearly confusing and incorrect:

Here is the output of 'objdump -Cd a.out':

...
0000000000400500 <f()>:
  400500:       31 c0                   xor    %eax,%eax
  400502:       c3                      retq   
  400503:       66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
  40050a:       00 00 00 
  40050d:       0f 1f 00                nopl   (%rax)
...

The output from objdump should label this function as 'main' since the size of
the function 'f' is 0 as stated in the symbol table, and the size of 'main' is
3 (which is apparently used by objdump to determine the function boundaries).

There are no functions 'g()', 'h()' and 'main()' in the objdump output, but
they probably should be present since mentioned in the symbol table. Or
probably it is okay that they are missing since there is no assembly code for
them (once 'f()' in the dump is renamed to 'main()', and 'f()' becomes missing
in the dump).

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