[Bug rtl-optimization/26537] New: Basic block reordering inserts redundant instruction

[dann at godzilla dot ics dot uci dot edu]

This code: 
extern char *nl_langinfo (int) __attribute__ ((__nothrow__));

char *
xtermEnvEncoding(void)
{
  static char *result;

  if (result == 0)
    result = nl_langinfo(50);
  return result;
}
gets compile by gcc-4.1.0 -march=i686 -mtune=i686 to:

xtermEnvEncoding:
[snip]
.L6:
        movl    $50, (%esp)
        call    nl_langinfo
        movl    %eax, result.1281
        movl    result.1281, %eax   <---- note this
        leave
        ret

Note the redundant mov instruction. 4.0 does not generate that extra
instruction.

The extra instruction seems to be generated by the bbro pass. Here is the RTL
dump for the .44.rnreg pass: nothing unusual

(call_insn:HI 17 16 19 1 (set (reg:SI 0 ax)
        (call (mem:QI (symbol_ref:SI ("nl_langinfo") [flags 0x41]
<function_decl 0xb7f2f100
 nl_langinfo>) [0 S1 A8])
            (const_int 4 [0x4]))) 531 {*call_value_0} (nil)
    (expr_list:REG_EH_REGION (const_int 0 [0x0])
        (nil))
    (nil))

(insn:HI 19 17 20 1 (set (mem/f/c/i:SI (symbol_ref:SI ("result.1281") [flags
0x2] <var_decl
 0xb7ebb108 result>) [2 result+0 S4 A32])
        (reg:SI 0 ax [orig:58 D.1283 ] [58])) 34 {*movsi_1}
(insn_list:REG_DEP_TRUE 18 (nil
))
    (expr_list:REG_DEAD (reg:SI 0 ax [orig:58 D.1283 ] [58])
        (nil)))


but the next dump, .45.bbro shows that an extra move instruction has been
inserted. 

(call_insn:HI 17 16 19 2 (set (reg:SI 0 ax)
        (call (mem:QI (symbol_ref:SI ("nl_langinfo") [flags 0x41]
<function_decl 0xb7f2f100
 nl_langinfo>) [0 S1 A8])
            (const_int 4 [0x4]))) 531 {*call_value_0} (nil)
    (expr_list:REG_EH_REGION (const_int 0 [0x0])
        (nil))
    (nil))

(insn:HI 19 17 54 2 (set (mem/f/c/i:SI (symbol_ref:SI ("result.1281") [flags
0x2] <var_decl
 0xb7ebb108 result>) [2 result+0 S4 A32])
        (reg:SI 0 ax [orig:58 D.1283 ] [58])) 34 {*movsi_1}
(insn_list:REG_DEP_TRUE 18 (nil
))
    (expr_list:REG_DEAD (reg:SI 0 ax [orig:58 D.1283 ] [58])
        (nil)))

(insn 54 19 55 2 (set (reg/f:SI 0 ax [orig:61 result ] [61])
        (mem/f/c/i:SI (symbol_ref:SI ("result.1281") [flags 0x2] <var_decl
0xb7ebb108 resul
t>) [2 result+0 S4 A32])) 34 {*movsi_1} (nil)
    (nil))

This problem is one of the causes for PR23153.


-- 
           Summary: Basic block reordering inserts redundant instruction
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dann at godzilla dot ics dot uci dot edu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26537