[Bug other/89222] New: [7.x regression] ARM thumb-2 misoptimisation of func ptr call with -O2 or -Os

[jifl-bugzilla at jifvik dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89222

            Bug ID: 89222
           Summary: [7.x regression] ARM thumb-2 misoptimisation of func
                    ptr call with -O2 or -Os
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jifl-bugzilla at jifvik dot org
  Target Milestone: ---

Created attachment 45616
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45616&action=edit
C file to demonstrate problem

[ I have set the Component as "Other" as I don't know which part of GCC's
optimization is at fault]

On GCC 7.3.0 I am experiencing a problem where GCC is misoptimizing code built
with either -O2 or -Os. The effect of the misoptimization is that it calls a
function pointer without the LS bit set, even though the function is a Thumb
function (it's Cortex-M, so ARM mode isn't even allowed).

The attached pared-down code demonstrates this, although I couldn't manage to
divorce it from my embedded run-time, so unfortunately you will have to inspect
the asm :-/

The key part is that if a certain magic constant is 1, then it fails. This
constant is used to compare against function pointers (it is the value of
SIG_DFL from signal.h and this sort of comparison is standard practise, e.g. in
glibc etc.). Values of SIG_DFL other than 1 work - it is the fact it is 1 that
confuses GCC.

In the asm (I used -Os as it made more tractable output), right at the start of
main(), r5 is loaded with e.g. 0x200003a7. Then this value plus one is put into
$r6. For most of the rest of the function r6 is left as-is, and is saved and
restored around the call to the hal_setjmp function. Shortly after, there is a
"blx r6" and that's when the processor gets the exception.

The C file shows this when built with: arm-eabi-gcc -c -mcpu=cortex-m3 -mthumb
-Os signal1.c
Or -O2 equally.