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From: "vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/55315] New: comparing address to constant is folded in cse
Date: Tue, 13 Nov 2012 23:00:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55315

             Bug #: 55315
           Summary: comparing address to constant is folded in cse
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vries@gcc.gnu.org


Consider test.c:
...
int data[4096];

int
f (void)
{
  return ((unsigned int) &data[0]) == 0xdeadbea0U;
}
...

Although the address is not available at compile time, the compiler (mips
target) concludes it's not equal to the constant:
...
$ gcc test.c -O2 -o- -S
...
f:
        j       $31
        move    $2,$0
...

The comparison:
  ((unsigned int) &data[0]) == 0xdeadbea0U
is transformed into this by expand:
  ((unsigned int) &data[0]) + (~0xdeadbea0U + 1) == 0

Then cse uses this part of nonzero_address_p:
...
    case PLUS:
      if (CONST_INT_P (XEXP (x, 1)))
        return nonzero_address_p (XEXP (x, 0));
...
to determine that ((unsigned int) &data[0]) + (~0xdeadbea0U + 1) is non-null,
while there is no evidence that the PLUS is an address.

This is similar to PR29519, and the test-case of this PR is mentioned in
comment 5.

configure line:
...
Target: mipsisa32r2-sde-elf
Configured with: src/gcc-mainline/configure --build=i686-pc-linux-gnu
--host=i686-pc-linux-gnu --target=mipsisa32r2-sde-elf --enable-threads
--disable-libmudflap --disable-libssp --disable-libstdcxx-pch --with-gnu-as
--with-gnu-ld --enable-languages=c,c++ --disable-shared --enable-lto
--with-newlib --disable-nls --disable-shared --disable-threads --disable-libssp
--disable-libgomp --without-headers --with-newlib --disable-decimal-float
--disable-libffi --disable-libquadmath --disable-libitm --disable-libatomic
--enable-languages=c --with-build-sysroot=install/mipsisa32r2-sde-elf
--with-gmp=obj/pkg-mainline-0-mipsisa32r2-sde-elf/fsf-mainline-0-mipsisa32r2-sde-elf.extras/host-libs-i686-pc-linux-gnu/usr
--with-mpfr=obj/pkg-mainline-0-mipsisa32r2-sde-elf/fsf-mainline-0-mipsisa32r2-sde-elf.extras/host-libs-i686-pc-linux-gnu/usr
--with-mpc=obj/pkg-mainline-0-mipsisa32r2-sde-elf/fsf-mainline-0-mipsisa32r2-sde-elf.extras/host-libs-i686-pc-linux-gnu/usr
--with-host-libstdcxx='-static-libgcc -Wl,-Bstatic,-lstdc++,-Bdynamic -lm'
--with-isl=obj/pkg-mainline-0-mipsisa32r2-sde-elf/fsf-mainline-0-mipsisa32r2-sde-elf.extras/host-libs-i686-pc-linux-gnu/usr
--with-cloog=obj/pkg-mainline-0-mipsisa32r2-sde-elf/fsf-mainline-0-mipsisa32r2-sde-elf.extras/host-libs-i686-pc-linux-gnu/usr
--with-libelf=obj/pkg-mainline-0-mipsisa32r2-sde-elf/fsf-mainline-0-mipsisa32r2-sde-elf.extras/host-libs-i686-pc-linux-gnu/usr
--disable-libgomp --disable-libitm --enable-poison-system-directories
--with-build-time-tools=install/mipsisa32r2-sde-elf/bin
Thread model: single
gcc version 4.8.0 20121113 (experimental) (GCC) 
...