[Bug target/100936] New: %p and %P modifiers should not emit segment overrides

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100936

            Bug ID: 100936
           Summary: %p and %P modifiers should not emit segment overrides
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ubizjak at gmail dot com
  Target Milestone: ---

Following testcase:

--cut here--
__seg_gs int var = 123;

static int
*foo (void)
{
  int *addr;

  asm ("lea %p1, %0" : "=r"(addr) : "m"(var));

  return addr;
}

static int
bar (int *addr)
{
  int val;

  asm ("mov %%gs:%1, %0" : "=r"(val) : "m"(*addr));

  return val;
}

int
baz (void)
{
  int *addr = foo();
  int val = bar (addr);

  return val;
}
--cut here--

emits assembly warning when compiled on x86 target:

gcc -O2 -c lea.c
lea.c: Assembler messages:
lea.c:8: Warning: segment override on `lea' is ineffectual

$ objdump -d lea.o

lea.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <baz>:
   0:   65 48 8d 04 25 00 00    lea    %gs:0x0,%rax
   7:   00 00 
   9:   65 8b 00                mov    %gs:(%rax),%eax
   c:   c3                      retq   

The problem is with %p operand modifier, which should emit raw symbol name:

   P -- if PIC, print an @PLT suffix.  For -fno-plt, load function
        address from GOT.
   p -- print raw symbol name.

but it also emits its segment override. As shown in the above example, it is
not possible to use LEA to load its address into a register.

Similar problem is with %P modifier, trying to CALL or JMP to overriden
symbol,e.g:

        call %gs:zzz
        jmp %gs:zzz

call.s:1: Warning: skipping prefixes on `call'
call.s:2: Warning: skipping prefixes on `jmp'