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From: "tony.poppleton at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/47477] New: [4.6 regression] Sub-optimal mov at end of method
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47477

           Summary: [4.6 regression] Sub-optimal mov at end of method
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tony.poppleton@gmail.com
              Host: Linux x86-64


Whilst investigating PR35926, I noticed a slight inefficiency in code generated
by 4.6.0 (20110115) versus that of 4.5.1.

Duplicating the C code here from that PR for easy reference:

typedef struct toto_s *toto_t;
toto_t add (toto_t a, toto_t b) {
  int64_t tmp = (int64_t)(intptr_t)a + ((int64_t)(intptr_t)b&~1L);
  return (toto_t)(intptr_t) tmp;
}

The ASM generated by 4.6.0 with flags -O3 is:

        .file   "PR35926.c"
        .text
        .p2align 4,,15
        .globl  add
        .type   add, @function
add:
.LFB0:
        .cfi_startproc
        pushl   %ebx
        .cfi_def_cfa_offset 8
        .cfi_offset 3, -8
        movl    12(%esp), %eax
        movl    8(%esp), %ecx
        popl    %ebx
        .cfi_def_cfa_offset 4
        .cfi_restore 3
        andl    $-2, %eax
        addl    %eax, %ecx       <==== order of regs inverted
        movl    %ecx, %eax       <==== resulting in unnecessary movl
        ret
        .cfi_endproc
.LFE0:
        .size   add, .-add
        .ident  "GCC: (GNU) 4.6.0 20110115 (experimental)"
        .section        .note.GNU-stack,"",@progbits

In 4.5.1, the last bit is one instruction shorter, with just:
        addl    %ecx, %eax
        ret

A bug search revealed a similar sounding PR44249, however that is a regression
in 4.5 too apparently, yet this only affects 4.6.