[Bug target/63679] New: [4.9 Regression][AArch64] Failure to constant fold.

[belagod at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63679

            Bug ID: 63679
           Summary: [4.9 Regression][AArch64] Failure to constant fold.
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: belagod at gcc dot gnu.org

When this piece of code is compiled with -O3 -mgeneral-regs-only

int __attribute__ ((noinline))
foo ()
{
  const int a[8] = { 0, 1, 2, 3, 4, 5, 6, 7 };
  int i, sum;

  sum = 0;
  for (i = 0; i < sizeof (a) / sizeof (*a); i++)
    sum += a[i];

  return sum;
}

4.9 gcc generates:

foo:
    sub    sp, sp, #32
    mov    w0, 28
    add    sp, sp, 32
    ret
    .size    foo, .-foo
    .ident    "GCC: (unknown) 4.9.2 20140930 (prerelease)"

5.0 generates:

foo:
    adrp    x0, .LANCHOR0
    sub    sp, sp, #32
    add    x0, x0, :lo12:.LANCHOR0
    ldr    x7, [x0]
    ldr    x6, [x0, 16]
    ldr    x1, [x0, 8]
    sbfx    x5, x7, 32, 32
    ldr    x0, [x0, 24]
    add    w2, w6, w7
    str    x0, [sp, 24]
    mov    x4, x1
    str    x1, [sp, 8]
    sbfx    x1, x6, 32, 32
    ldr    x3, [sp, 24]
    add    w1, w1, w5
    add    w1, w1, w2
    str    x7, [sp]
    add    w0, w3, w4
    sbfx    x4, x4, 32, 32
    sbfx    x3, x3, 32, 32
    add    w0, w0, w1
    add    w3, w4, w3
    str    x6, [sp, 16]
    add    w0, w3, w0
    add    sp, sp, 32
    ret
    .size    foo, .-foo
    .section    .rodata
    .align    3
.LANCHOR0 = . + 0
.LC0:
    .word    0
    .word    1
    .word    2
    .word    3
    .word    4
    .word    5
    .word    6
    .word    7
    .ident    "GCC: (unknown) 5.0.0 20141023 (experimental)"

Constant-folding seems to have got a bit messed up. I've observed this only on
aarch64-none-elf-gcc. 5.0 x86_64 seems to work fine. 

foo:
.LFB0:
    .cfi_startproc
    movl    $28, %eax
    ret
    .cfi_endproc
.LFE0:
    .size    foo, .-foo
    .section    .text.unlikely
.LCOLDE0:
    .text
.LHOTE0:
    .ident    "GCC: (GNU) 5.0.0 20141023 (experimental)"
    .section    .note.GNU-stack,"",@progbits


Looks like a aarch64-specific backend issue.

$ aarch64-none-elf-gcc -v
Target: aarch64-none-elf
Configured with: /work/dev/arm/src/gcc/configure --target=aarch64-none-elf
--prefix=/work/dev/arm/bin//install --with-gmp=/work/dev/arm/bin//host-tools
--with-mpfr=/work/dev/arm/bin//host-tools
--with-mpc=/work/dev/arm/bin//host-tools
--with-cloog=/work/dev/arm/bin//host-tools
--with-isl=/work/dev/arm/bin//host-tools --with-pkgversion=unknown
--disable-shared --disable-nls --disable-threads --disable-tls
--enable-checking=yes --enable-languages=c,c++ --with-newlib
Thread model: single
gcc version 5.0.0 20141023 (experimental) (unknown)