[Bug ld/30957] New: aarch64: unnecessary bti veneer

[nsz at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=30957

            Bug ID: 30957
           Summary: aarch64: unnecessary bti veneer
           Product: binutils
           Version: 2.41
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: nsz at gcc dot gnu.org
  Target Milestone: ---

bti veneer is only needed if the target instruction is not bti (or
paciasp,...), but bfd ld sometimes emits the veneer unnecessarily. (gnu
property is for BTI marking)

(introduced by the fix for bug 30076 and causes large number of bti veneers in
large executables that uncovered other veneer issues see bug 30930 )

$ cat a.s
        .section        .a.text,"ax",@progbits
        .align  2
        .global a_func
        .type   a_func, %function
a_func:
        hint    34 // bti c
        b       b_func

        .section        .note.gnu.property,"a"
        .align  3
        .word   4
        .word   16
        .word   5
        .string "GNU"
        .word   3221225472
        .word   4
        .word   1
        .align  3
$ cat b.s
        .section        .b.text,"ax",@progbits
        .align  2
        .global b_func
        .type   b_func, %function
        nop // offset the function so it is not at section start
b_func:
        hint    34 // bti c
        ret
        .global _start
        .type   _start, %function
_start:
        b       a_func

        .section        .note.gnu.property,"a"
        .align  3
        .word   4
        .word   16
        .word   5
        .string "GNU"
        .word   3221225472
        .word   4
        .word   1
        .align  3
$ as -o a.o a.s
$ as -o b.o b.s
$ ld --section-start=.a.text=0x00040000 --section-start=.b.text=0x08041000 a.o
b.o
$ objdump -d a.out
a.out:     file format elf64-littleaarch64


Disassembly of section .a.text:

0000000000040000 <a_func>:
   40000:       d503245f        bti     c
   40004:       14000005        b       40018 <__b_func_veneer>
   40008:       1400000a        b       40030 <__b_func_veneer+0x18>
   4000c:       d503201f        nop

0000000000040010 <__a_func_bti_veneer>:
   40010:       d503245f        bti     c
   40014:       17fffffb        b       40000 <a_func>

0000000000040018 <__b_func_veneer>:
   40018:       b0040010        adrp    x16, 8041000 <__bss_end__+0x7c30e88>
   4001c:       9100c210        add     x16, x16, #0x30
   40020:       d61f0200        br      x16
        ...

Disassembly of section .b.text:

0000000008041000 <b_func-0x4>:
 8041000:       d503201f        nop

0000000008041004 <b_func>:
 8041004:       d503245f        bti     c
 8041008:       d65f03c0        ret

000000000804100c <_start>:
 804100c:       14000003        b       8041018 <__a_func_veneer>
 8041010:       1400000a        b       8041038 <__b_func_bti_veneer+0x8>
 8041014:       d503201f        nop

0000000008041018 <__a_func_veneer>:
 8041018:       f0fbfff0        adrp    x16, 40000 <a_func>
 804101c:       91004210        add     x16, x16, #0x10
 8041020:       d61f0200        br      x16
        ...

0000000008041030 <__b_func_bti_veneer>:
 8041030:       d503245f        bti     c
 8041034:       17fffff4        b       8041004 <b_func>

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