[Bug target/43137] New: redundant register move for sign extending

["carrot at google dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Compile following code with options -march=armv7-a -mthumb -Os

int foo();
long long bar22()
{
  int result = foo();
  return result;
}

GCC generates:

bar22:
        push    {r3, lr}
        bl      foo
        asrs    r3, r0, #31    // A
        mov     r1, r3         // B
        pop     {r3, pc}

Instruction A,B can be simplified to "asrs r1, r0, #31".

In rtl expand, result was first extended to a DI register and then the DI
register was moved to return register. In later passs the low half of the
return value and variable result were recognized as in the same register, so
the low part move was removed. After register allocation I get:

(call_insn 5 2 7 2 src/t5.c:5 (parallel [
            (set (reg:SI 0 r0)
                (call (mem:SI (symbol_ref:SI ("foo") [flags 0x41] 
<function_decl 0x7f6e60fe0600 foo>) [0 S4 A32])
                    (const_int 0 [0x0])))
            (use (const_int 0 [0x0]))
            (clobber (reg:SI 14 lr))
        ]) 255 {*call_value_insn} (nil)
    (nil))

(insn 7 5 22 2 src/t5.c:5 (set (reg:DI 2 r2 [orig:136 result ] [136])
        (sign_extend:DI (reg/v:SI 0 r0 [orig:133 result ] [133]))) 691
{*thumb2_extendsidi2} (nil))

(insn 22 7 15 2 src/t5.c:7 (set (reg:SI 1 r1 [+4 ])
        (reg:SI 3 r3 [ result+4 ])) 658 {*thumb2_movsi_insn} (nil))

The high part register move didn't get any chance to be removed until the end
of compilation.


-- 
           Summary: redundant register move for sign extending
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: carrot at google dot com
 GCC build triplet: i686-linux
  GCC host triplet: i686-linux
GCC target triplet: arm-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43137