[Bug target/83970] New: -mindirect-branch=thunk -fno-plt generates CET-incompatible thunk

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83970

            Bug ID: 83970
           Summary: -mindirect-branch=thunk -fno-plt generates
                    CET-incompatible thunk
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hjl.tools at gmail dot com
  Target Milestone: ---
            Target: x86

[hjl@gnu-bdx-1 indirect-got-1]$ cat x.i
void func (void);

void
bar (void)
{
  func ();
}
[hjl@gnu-bdx-1 indirect-got-1]$
/export/build/gnu/gcc-test/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc-test/build-x86_64-linux/gcc/ -O2
-mindirect-branch=thunk -fno-plt -S x.i
[hjl@gnu-bdx-1 indirect-got-1]$ cat x.s
        .file   "x.i"
        .text
        .p2align 4,,15
        .globl  bar
        .type   bar, @function
bar:
.LFB0:
        .cfi_startproc
        pushq   func@GOTPCREL(%rip)
        jmp     __x86_indirect_thunk
        .cfi_endproc
.LFE0:
        .size   bar, .-bar
        .section       
.text.__x86_indirect_thunk,"axG",@progbits,__x86_indirect_thunk,comdat
        .globl  __x86_indirect_thunk
        .hidden __x86_indirect_thunk
        .type   __x86_indirect_thunk, @function
__x86_indirect_thunk:
        .set    __x86_return_thunk,__x86_indirect_thunk
        .globl  __x86_return_thunk
        .hidden __x86_return_thunk
.LFB1:
        .cfi_startproc
        call    .LIND1
.LIND0:
        pause
        lfence
        jmp     .LIND0
.LIND1:
        lea     8(%rsp), %rsp
        ret
        .cfi_endproc
.LFE1:
        .ident  "GCC: (GNU) 8.0.1 20180115 (experimental)"
        .section        .note.GNU-stack,"",@progbits
[hjl@gnu-bdx-1 indirect-got-1]$ 

For i386, since all registers may be used for function call, there
isn't much we can do.  But there are a couple scratch registers
available for function call.   We can generate

bar:
.LFB0:
        .cfi_startproc
        movq    func@GOTPCREL(%rip), %r11
        jmp     __x86_indirect_thunk_r11
        .cfi_endproc
.LFE0:
        .size   bar, .-bar

which is easier to change __x86_indirect_thunk_r11 to be compatible
with CET.