How to insert reference to external symbol in RTL properly?

[Arslan Khan <arslankhan52@gmail.com>]

Hi,
I am a beginner to GCC and am trying to implement safe stack using a
GCC Plugin for embedded systems. I am using a cross compiler, with GCC
version:

arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors
6-2018-q4-major) 6.3.1 20170620 (release) [ARM/embedded-6-branch
revision 249437]

The shadow stack is placed on a different RAM chip and is protected
using software fault isolation (SFI). For SFI i have used a GIMPLE
pass, but to save the Link Register in the protected memory i had to
use an RTL pass, because FWIU GIMPLE has no notion of registers. Anyhow i
am able to generate insn(s) for saving stack pointer to a symbol
"shadowStack", created by RTL pass (currently
there is no indexing, wanted to start simple). Here's the insn
generated for the prologue:

(insn 21 2 20 2 (set (reg/f:SI 110)
        (symbol_ref:SI ("shadowStack") [flags 0x2])) ./test.c:35 -1
     (nil))
(insn 20 21 5 2 (set (mem/c:SI (reg/f:SI 110) [0  S4 A32])
        (reg:SI 14 lr)) ./test.c:35 -1
     (nil))

I have tried to copy what cc1 actually generated for an external
reference. The one thing that is different is that there is no
information about the variable declaration ins symbol_ref as
shadowStack is not defined in source and is purely introduced by the
RTL pass.  My plan is to provide shadowStack using a library (which in
the end version would be an array). I did something similar for GIMPLE
pass to call functions in an external library and it worked for me.
However when i run it i get the following error message:

./test.c: In function 'iowrite_example':
./test.c:40:1: error: insn does not satisfy its constraints:
 }
 ^
(insn 21 2 20 (set (reg/f:SI 110)
        (symbol_ref:SI ("shadowStack") [flags 0x2])) ./test.c:35 172
{*arm_movsi_insn}
     (nil))
*** WARNING *** there are active plugins, do not report this as a bug
unless you can reproduce it without enabling any plugins.
Event                                        | Plugins
PLUGIN_ATTRIBUTES               | plugin
PLUGIN_START_UNIT               | plugin
PLUGIN_ALL_IPA_PASSES_START      | plugin
./test.c:40:1: internal compiler error: in extract_constrain_insn, at
recog.c:2190
0xc8ff4c _fatal_insn(char const*, rtx_def const*, char const*, int, char const*)
    /home/build/toolchain/gcc-arm-none-eabi-6-2017-q2-update/src/gcc/gcc/rtl-error.c:108
0xc8ffac _fatal_insn_not_found(rtx_def const*, char const*, int, char const*)
    /home/build/toolchain/gcc-arm-none-eabi-6-2017-q2-update/src/gcc/gcc/rtl-error.c:119
0xc3b8e0 extract_constrain_insn(rtx_insn*)
    /home/build/toolchain/gcc-arm-none-eabi-6-2017-q2-update/src/gcc/gcc/recog.c:2190
0x104dad2 note_invalid_constants
    /home/build/toolchain/gcc-arm-none-eabi-6-2017-q2-update/src/gcc/gcc/config/arm/arm.c:17555
0x104fab3 arm_reorg
    /home/build/toolchain/gcc-arm-none-eabi-6-2017-q2-update/src/gcc/gcc/config/arm/arm.c:18413
0xc8a5e6 execute
    /home/build/toolchain/gcc-arm-none-eabi-6-2017-q2-update/src/gcc/gcc/reorg.c:3952

FWIU these instruction generated are accepted by some RTL templates
but the constraints on operands are not satisfied. I tried to debug
the constrain_operands function, and is turns out we are trying to
match constraints for arm_movsi_insn pattern. So the constraints were:

constraints[0]    const char *    0x1a76cf3 "=rk,r,r,r,rk,m"
constraints[1]    const char *    0x1a76d02 "rk,I,K,j,mi,rk"

My guess was that it should've matched second last alternative but it
didn't, but i am pretty sure there is nothing wrong with
constrain_operands, but something is wrong with the way i generated
the reference. Is there any place i can look for on how to do this
properly? if required i can share the code here as well for the pass
(basically i just used gen_rtx_SYMBOL_REF for creating the reference,
and to be sure there's only one SYMBOL_REF
(https://gcc.gnu.org/ml/gcc/2003-06/msg02331.html) i save the ref in a
global variable). Any pointers on what could be wrong in the way i
generated the reference? Secondly is there any effort going on
incorporating safe stack with GCC? Lastly please tell me if this
question belongs here or gcc-help list? i tried gcc-help but it looks
like most question there are for GCC users not developers.

Thanks,
Arslan