[Bug gold/21453] New: Safe ICF doesn't work with -fno-plt

[hjl.tools at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=21453

            Bug ID: 21453
           Summary: Safe ICF doesn't work with -fno-plt
           Product: binutils
           Version: 2.29 (HEAD)
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gold
          Assignee: ccoutant at gmail dot com
          Reporter: hjl.tools at gmail dot com
                CC: ian at airs dot com, rahulchaudhry at chromium dot org,
                    tmsriram at google dot com
  Target Milestone: ---
            Target: i386, x86_64

When fno-plt is used, GOT relocation will be used to call external functions:

00000000 <main>:
   0:   8d 4c 24 04             lea    0x4(%esp),%ecx
   4:   83 e4 f0                and    $0xfffffff0,%esp
   7:   ff 71 fc                pushl  -0x4(%ecx)
   a:   55                      push   %ebp
   b:   89 e5                   mov    %esp,%ebp
   d:   53                      push   %ebx
   e:   51                      push   %ecx
   f:   83 ec 10                sub    $0x10,%esp
  12:   e8 fc ff ff ff          call   13 <main+0x13>   13: R_386_PC32 
__x86.get_pc_thunk.bx
  17:   81 c3 02 00 00 00       add    $0x2,%ebx        19: R_386_GOTPC
_GLOBAL_OFFSET_TABLE_
  1d:   8b 83 00 00 00 00       mov    0x0(%ebx),%eax   1f: R_386_GOT32X       
foo_glob
  23:   89 45 f4                mov    %eax,-0xc(%ebp)
  26:   ff 93 00 00 00 00       call   *0x0(%ebx)       28: R_386_GOT32X       
bar_glob
  2c:   ff 93 00 00 00 00       call   *0x0(%ebx)       2e: R_386_GOT32X       
foo_normal
  32:   ff 93 00 00 00 00       call   *0x0(%ebx)       34: R_386_GOT32X       
foo_default
  38:   b8 00 00 00 00          mov    $0x0,%eax
  3d:   83 c4 10                add    $0x10,%esp
  40:   59                      pop    %ecx
  41:   5b                      pop    %ebx
  42:   5d                      pop    %ebp
  43:   8d 61 fc                lea    -0x4(%ecx),%esp
  46:   c3                      ret    

Gold needs to check opcode for branches to enable safe ICF:

[hjl@gnu-6 gold-1]$ cat x.c
extern int foo_normal(void);
extern int foo_default(void);
extern int foo_glob(void);
extern int bar_glob(void);

int main()
{
  int (*p)() = foo_glob;
  (void)p;
  bar_glob ();
  foo_normal();
  foo_default();
  return 0;
}

[hjl@gnu-6 gold-1]$ cat y.c
int
foo_normal(void)
{
  return 1;
}

int 
foo_default(void)
{
  return 1;
}

int foo_glob(void)
{
  return 2;
}

int bar_glob(void)
{
  return 2;
}
[hjl@gnu-6 gold-1]$ make x
/export/build/gnu/gcc-7/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc-7/build-x86_64-linux/gcc/ -m32 -c -fPIE
-ffunction-sections -fno-plt -o x.o x.c
/export/build/gnu/gcc-7/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc-7/build-x86_64-linux/gcc/ -m32 -c -fPIE
-ffunction-sections -fno-plt -o y.o y.c
./ld -pie --icf=safe -Map x.map -m elf_i386 -o x x.o y.o
[hjl@gnu-6 gold-1]$ cat x.map

Discarded input sections

 .group         0x00000000        0x8 x.o
 .note.GNU-stack
                0x00000000        0x0 x.o
 .group         0x00000000        0x8 y.o
 .note.GNU-stack
                0x00000000        0x0 y.o

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