Wrong code generated with -fstack-protector and -msingle-pic-base

[Ilya Oleinik]

Compilation of the following piece of code with gcc-9 2019-q4 when -fstack-protector -fpie
-msingle-pic-base are set, uses wrong register for GOT access:

void test() {
  return;
}

Result:

.LFB0:
 @ args = 0, pretend = 0, frame = 8
 @ frame_needed = 0, uses_anonymous_args = 0
 push {lr}
 ldr r3, .L6
 sub sp, sp, #12
 ldr r3, [r2, r3] // r2 is used instead of r9
 ldr r3, [r3]
 str r3, [sp, #4]
 mov r3,#0
 ldr r3, .L6
 ldr r3, [r2, r3]
 ldr r2, [r3]
 ldr r3, [sp, #4]
 eors r2, r3, r2
 bne .L5
 add sp, sp, #12
 @ sp needed
 ldr pc, [sp], #4
.L5:
 bl __stack_chk_fail(PLT)
.L7:
 .align 2
.L6:
 .word __stack_chk_guard(GOT)

Without -msingle-pic-base code is correct:

.LFB0:
 @ args = 0, pretend = 0, frame = 8
 @ frame_needed = 0, uses_anonymous_args = 0
 push {lr}
 ldr r2, .L6 // r2 is initialized with GOT base
 ldr r3, .L6+4
.LPIC0:
 add r2, pc
 ldr r3, [r2, r3]
 sub sp, sp, #12
 ldr r3, [r3]
 str r3, [sp, #4]
 mov r3,#0
 ldr r2, .L6+8
 ldr r3, .L6+4
.LPIC1:
 add r2, pc
 ldr r3, [r2, r3]
 ldr r2, [r3]
 ldr r3, [sp, #4]
 eors r2, r3, r2
 bne .L5
 add sp, sp, #12
 @ sp needed
 ldr pc, [sp], #4
.L5:
 bl __stack_chk_fail(PLT)
.L7:
 .align 2
.L6:
 .word _GLOBAL_OFFSET_TABLE_-(.LPIC0+4)
 .word __stack_chk_guard(GOT)
 .word _GLOBAL_OFFSET_TABLE_-(.LPIC1+4)

Problem is not present on gcc-8 or 7:

.LFB0:
        @ args = 0, pretend = 0, frame = 8
        @ frame_needed = 0, uses_anonymous_args = 0
        push {lr}
        ldr r3, .L6
        ldr r3, [r9, r3] // r9 is used as base (correct)
        sub sp, sp, #12
        ldr r2, [r3]
        str r2, [sp, #4]
        ldr r2, [sp, #4]
        ldr r3, [r3]
        cmp r2, r3
        bne .L5
        add sp, sp, #12
        @ sp needed
        ldr pc, [sp], #4
.L5:
        bl __stack_chk_fail(PLT)