[Bug target/57837] New: ARM function pointer tailcall miscompilation regression

["me at williamgrant dot id.au" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57837

            Bug ID: 57837
           Summary: ARM function pointer tailcall miscompilation
                    regression
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: me at williamgrant dot id.au

Created attachment 30469
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30469&action=edit
arm.md fix

The fix for PR target/19599 in svn trunk r198928 causes predicated function
pointer tailcalls for some ARM targets to lose their predicate and execute
unconditionally, resulting in a crash or other misbehaviour. I was able to
reproduce the miscompilation, manifesting as a segfault, using -march=armv4t
-marm -O3, as below:

gcc -march=armv4t -marm -O3 -o bx-test bx-test.c

----
void baz() {};

void (*bar)() = baz;

void foo(int c) {
    if (c == 1) {
        foo(0);
    }
    bar();
}

int main(int argc, char **argv) {
    foo(1);
}
----

Today's trunk gives the following asm:

----
foo:
        cmp     r0, #1
        stmfd   sp!, {r4, lr}
        ldr     r4, .L8
        ldrne   r3, [r4]
        ldmnefd sp!, {r4, lr}
         bx     r3      @ indirect register sibling call
----

Note the bx to an r3 that is uninitialised when r0 == 1; it should actually be
a bxne. The bug in arm.md is fairly clear: the %? is missing from two bx
instructions, so the predicate is omitted. After identifying the bad code I
discovered that the issue was raised in review, but deemed irrelevant
(http://gcc.gnu.org/ml/gcc-patches/2013-05/msg01022.html).

The attached patch fixes the bug for me.