[Bug middle-end/47466] New: c++ __builtin_expect() regression

[Bug middle-end/47466] New: c++ __builtin_expect() regression

[guillaume at morinfr dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47466

           Summary: c++ __builtin_expect() regression
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: guillaume@morinfr.org


Created attachment 23127
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23127
test case

This is a followup from http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42233

While the example given there now works, a slightly different example leads to
the same problem: the "unlikely" path is favored by the compiler (the likely
case will branch).

$ cat foo.cpp
struct EnumStruct {
    enum Enum { ONE, TWO, THREE };
    char e;
    EnumStruct(Enum _e) : e(_e) {}
    operator Enum() const { return (Enum)e; }
};

extern void unlikelyCall();

#define unlikely(x) __builtin_expect(!!(x), 0)

int test_expect(const EnumStruct& s)  {
    if (unlikely(s != EnumStruct::ONE && s != EnumStruct::TWO)) {
        unlikelyCall();
        return 1;
    }
    return 0;
}
int test_expect2(const EnumStruct& s)  {
    if (unlikely(s != EnumStruct::ONE) &&
        unlikely(s != EnumStruct::TWO)) {
        unlikelyCall();
        return 1;
    }
    return 0;
}
$ g++45 -O3 -S -o foo.S foo.cpp
$ cat foo.S | c++filt 
    .file    "foo.cpp"
    .text
    .p2align 4,,15
.globl test_expect(EnumStruct const&)
    .type    test_expect(EnumStruct const&), @function
test_expect(EnumStruct const&):
.LFB4:
    .cfi_startproc
    subq    $8, %rsp
    .cfi_def_cfa_offset 16
    movsbl    (%rdi), %edx
    xorl    %eax, %eax
    testl    %edx, %edx
    je    .L2
    cmpl    $1, %edx
    je    .L2
    call    unlikelyCall()
    movl    $1, %eax
.L2:
    addq    $8, %rsp
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc
.LFE4:
    .size    test_expect(EnumStruct const&), .-test_expect(EnumStruct const&)
    .p2align 4,,15
.globl test_expect2(EnumStruct const&)
    .type    test_expect2(EnumStruct const&), @function
test_expect2(EnumStruct const&):
.LFB5:
    .cfi_startproc
    subq    $8, %rsp
    .cfi_def_cfa_offset 16
    movsbl    (%rdi), %edx
    testl    %edx, %edx
    jne    .L10
    xorl    %eax, %eax
.L7:
    addq    $8, %rsp
    .cfi_remember_state
    .cfi_def_cfa_offset 8
    ret
.L10:
    .cfi_restore_state
    xorl    %eax, %eax
    subl    $1, %edx
    je    .L7
    call    unlikelyCall()
    movl    $1, %eax
    jmp    .L7
    .cfi_endproc
.LFE5:
    .size    test_expect2(EnumStruct const&), .-test_expect2(EnumStruct const&)
    .ident    "GCC: (GNU) .5.2 20100819 (prerelease)"


Same symptom as bug 42233.  The unlikely path is favored and breaking the
"unlilkely(foo && bar)" into "unlikely(foo) && unlikely(bar)" works around the
problem.

I verified that this example leads to the likely path being favored with gcc
4.1.2 from CentOS, 4.2.1 from FreeBSD.  gcc 4.3 seems to have the same problem

[Bug middle-end/47466] c++ __builtin_expect() regression

[guillaume at morinfr dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47466

--- Comment #1 from Guillaume Morin <guillaume at morinfr dot org> 2011-01-26 18:26:34 UTC ---
A recent 4.6 snapshot shows the same issue

    .file    "foo.cpp"
    .text
    .p2align 4,,15
    .globl    test_expect(EnumStruct const&)
    .type    test_expect(EnumStruct const&), @function
test_expect(EnumStruct const&):
.LFB4:
    .cfi_startproc
    subq    $8, %rsp
    .cfi_def_cfa_offset 16
    movsbl    (%rdi), %edx
    xorl    %eax, %eax
    testl    %edx, %edx
    je    .L2
    cmpl    $1, %edx
    je    .L2
    call    unlikelyCall()
    movl    $1, %eax
.L2:
    addq    $8, %rsp
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc
.LFE4:
    .size    test_expect(EnumStruct const&), .-test_expect(EnumStruct const&)
    .p2align 4,,15
    .globl    test_expect2(EnumStruct const&)
    .type    test_expect2(EnumStruct const&), @function
test_expect2(EnumStruct const&):
.LFB5:
    .cfi_startproc
    subq    $8, %rsp
    .cfi_def_cfa_offset 16
    movsbl    (%rdi), %edx
    xorl    %eax, %eax
    testl    %edx, %edx
    jne    .L10
.L7:
    addq    $8, %rsp
    .cfi_remember_state
    .cfi_def_cfa_offset 8
    ret
.L10:
    .cfi_restore_state
    subl    $1, %edx
    je    .L7
    .p2align 4,,5
    call    unlikelyCall()
    movl    $1, %eax
    jmp    .L7
    .cfi_endproc
.LFE5:
    .size    test_expect2(EnumStruct const&), .-test_expect2(EnumStruct const&)
    .ident    "GCC: (Debian 20110116-1) 4.6.0 20110116 (experimental) [trunk
revision 168860]"
    .section    .note.GNU-stack,"",@progbits

[Bug middle-end/47466] c++ __builtin_expect() regression

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47466

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011-12-15
     Ever Confirmed|0                           |1

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-12-15 22:29:48 UTC ---
Confirmed, the C testcase works though.