[Bug target/70743] New: Extend -fno-plt to function pointer

["hjl.tools at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70743

            Bug ID: 70743
           Summary: Extend -fno-plt to function pointer
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hjl.tools at gmail dot com
                CC: ubizjak at gmail dot com
  Target Milestone: ---
            Target: x86

Even when -fno-plt is used, GCC still generates PLT for function
pointer:

[hjl@gnu-6 plt-2]$ cat x.i
extern void foo (void);

void *
_start (void)
{
  foo ();
  return foo;
}
[hjl@gnu-6 plt-2]$ cat foo.c
void
foo (void)
{
}
[hjl@gnu-6 plt-2]$ make
/export/build/gnu/gcc/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc/build-x86_64-linux/gcc/ -O2 -fno-plt -c -o x.o x.i
/export/build/gnu/gcc/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc/build-x86_64-linux/gcc/ -O2 -fno-plt -fPIC    -c -o
foo.o foo.c
ld -shared  -o foo.so foo.o
ld  -o x x.o foo.so
readelf -r x

Relocation section '.rela.dyn' at offset 0x268 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name +
Addend
000000600470  000200000006 R_X86_64_GLOB_DAT 00000000004002b0 foo + 0

Relocation section '.rela.plt' at offset 0x280 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name +
Addend
000000600490  000200000007 R_X86_64_JUMP_SLO 00000000004002b0 foo + 0
objdump -dw x

x:     file format elf64-x86-64


Disassembly of section .plt:

00000000004002a0 <foo@plt-0x10>:
  4002a0:       ff 35 da 01 20 00       pushq  0x2001da(%rip)        # 600480
<_GLOBAL_OFFSET_TABLE_+0x8>
  4002a6:       ff 25 dc 01 20 00       jmpq   *0x2001dc(%rip)        # 600488
<_GLOBAL_OFFSET_TABLE_+0x10>
  4002ac:       0f 1f 40 00             nopl   0x0(%rax)

00000000004002b0 <foo@plt>:
  4002b0:       ff 25 da 01 20 00       jmpq   *0x2001da(%rip)        # 600490
<_GLOBAL_OFFSET_TABLE_+0x18>
  4002b6:       68 00 00 00 00          pushq  $0x0
  4002bb:       e9 e0 ff ff ff          jmpq   4002a0 <foo@plt-0x10>

Disassembly of section .text:

00000000004002c0 <_start>:
  4002c0:       48 83 ec 08             sub    $0x8,%rsp
  4002c4:       ff 15 a6 01 20 00       callq  *0x2001a6(%rip)        # 600470
<_DYNAMIC+0x140>
  4002ca:       b8 b0 02 40 00          mov    $0x4002b0,%eax
  4002cf:       48 83 c4 08             add    $0x8,%rsp
  4002d3:       c3                      retq   
[hjl@gnu-6 plt-2]$ 

GCC can generate

        movq    foo@GOTPCREL(%rip), %rax

for function pointer.  It will remove PLT and only needs one run-time
relocation:
readelf -r x

Relocation section '.rela.dyn' at offset 0x268 contains 1 entries:
  Offset          Info           Type           Sym. Value    Sym. Name +
Addend
0000006003c8  000200000006 R_X86_64_GLOB_DAT 0000000000000000 foo + 0
objdump -dw x

x:     file format elf64-x86-64


Disassembly of section .text:

0000000000400280 <_start>:
  400280:       48 83 ec 08             sub    $0x8,%rsp
  400284:       ff 15 3e 01 20 00       callq  *0x20013e(%rip)        # 6003c8
<_DYNAMIC+0x100>
  40028a:       48 8b 05 37 01 20 00    mov    0x200137(%rip),%rax        #
6003c8 <_DYNAMIC+0x100>
  400291:       48 83 c4 08             add    $0x8,%rsp
  400295:       c3                      retq   
[hjl@gnu-6 plt-2]$