[Bug target/67226] New: Incorrect code generated for tail call, where parameters are structs passed by value, -O2 is used, and target is ARM

[jack.d.whitham at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67226

            Bug ID: 67226
           Summary: Incorrect code generated for tail call, where
                    parameters are structs passed by value, -O2 is used,
                    and target is ARM
           Product: gcc
           Version: 5.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jack.d.whitham at gmail dot com
  Target Milestone: ---

Created attachment 36188
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36188&action=edit
Test case to reproduce bug, run "show_bug.sh" on ARMv7 system with GCC 5.2.1

I compiled this code with "gcc (Debian 5.2.1-15) 5.2.1 20150808" on an ARMv7
target (RPi2).

typedef struct assembly_operand_t
{   int   type, value, symtype, symflags, marker;
} assembly_operand;

void t2(assembly_operand to, assembly_operand from)
{   if (to.value == from.value) return;
    if (to.value == 0) assemblez_1(32, from);
    else               assemblez_store(to, from);
}

My code calls the function t2 with to.value == 0. If I compile t2 with -O0 or
-O1, assemblez_1 is called as expected with the correct arguments. If I compile
t2 with -O2, the "from" structure is copied incorrectly. The "type" field has
the incorrect value.

I have seen this bug on GCC 4.6.x and 4.7.x as well as 5.2.1 and it seems to
behave the same regardless of whether the target ISA is 32-bit ARM or Thumb.
However it does not occur on x86/x86_64. For ARM targets, it appears that -O2
causes the call to assemblez_1 to be generated as a tail call whereas it is an
ordinary call with -O1/-O0.

In the attached test case the same code is compiled with both -O0 and -O2. Both
copies are then executed. The test case shows that the "from.type" field is
corrupted in the -O2 case. See the "show_bug.sh" script for the command-line
options used. 

More information:

This GCC is from https://packages.debian.org/sid/gcc.
# gcc --version
gcc (Debian 5.2.1-15) 5.2.1 20150808
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.