[Bug target/81720] New: [arm] Invalid code generation

["oarias at knights dot ucf.edu" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81720

            Bug ID: 81720
           Summary: [arm] Invalid code generation
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: oarias at knights dot ucf.edu
  Target Milestone: ---

Created attachment 41928
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41928&action=edit
Testcase to generate the bug.

Greetings,

Yesterday, I ran into a problem with the compiler issuing invalid instructions
when compiling code with optimization levels of -O2 or higher. I narrowed it
down to the following testcase [attached]:

int main(void) {
        volatile unsigned int read_value = *((volatile unsigned
int*)0x00000000);
        return 0;
}

When compiled with

$ arm-none-eabi-gcc -S -c testcase.c -O1 -mthumb -mno-thumb-interwork
-mcpu=cortex-m4

The following assembly is generated [trimmed, see full attachment]:

main:
        @ Volatile: function does not return.
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        movs    r3, #0
        ldr     r3, [r3]
        .inst   0xdeff
        .size   main, .-main

The .inst 0xdeff should definitely not be there. Any code that happens after
the dereference takes place is never emitted by the compiler. The dereference
in question is actually valid for the target I am working with [bare-metal ARM
system]. Of course, the assembler happily picks this up and creates object
files with the erroneous code.

I am using the following gcc:

$ arm-none-eabi-gcc --version
arm-none-eabi-gcc (Arch Repository) 7.1.0
Copyright (C) 2017 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.

Which has been compiled as:
$ arm-none-eabi-gcc -v
Using built-in specs.
COLLECT_GCC=arm-none-eabi-gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/arm-none-eabi/7.1.0/lto-wrapper
Target: arm-none-eabi
Configured with: /build/arm-none-eabi-gcc/src/gcc-7-20170504/configure
--target=arm-none-eabi --prefix=/usr --with-sysroot=/usr/arm-none-eabi
--with-native-system-header-dir=/include --libexecdir=/usr/lib
--enable-languages=c,c++ --enable-plugins --disable-decimal-float
--disable-libffi --disable-libgomp --disable-libmudflap --disable-libquadmath
--disable-libssp --disable-libstdcxx-pch --disable-nls --disable-shared
--disable-threads --disable-tls --with-gnu-as --with-gnu-ld --with-system-zlib
--with-newlib --with-headers=/usr/arm-none-eabi/include
--with-python-dir=share/gcc-arm-none-eabi --with-gmp --with-mpfr --with-mpc
--with-isl --with-libelf --enable-gnu-indirect-function
--with-host-libstdcxx='-static-libgcc -Wl,-Bstatic,-lstdc++,-Bdynamic -lm'
--with-pkgversion='Arch Repository' --with-bugurl=https://bugs.archlinux.org/
--with-multilib-list=armv6-m,armv7-m,armv7e-m,armv7-r
Thread model: single
gcc version 7.1.0 (Arch Repository) 


Any advice? Thank you.

Cheers,
Orlando.