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From "Martin Sebor (JIRA)" <j...@apache.org>
Subject [jira] Commented: (STDCXX-335) std::min() suboptimal
Date Wed, 07 Feb 2007 21:17:05 GMT

    [ https://issues.apache.org/jira/browse/STDCXX-335?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12471106
] 

Martin Sebor commented on STDCXX-335:
-------------------------------------

I see something similar with the latest trunk, gcc 4.1.0 on SPARC:

_Z8test_minii:
        cmp     %o1, %o0
        st      %o0, [%sp+68]
        st      %o1, [%sp+72]
        bge     .LL11
         add    %sp, 72, %o0
        jmp     %o7+8
         ld     [%o0], %o0
.LL11:
        add     %sp, 68, %o0
        jmp     %o7+8
         ld     [%o0], %o0

_Z6my_minii:
        cmp     %o0, %o1
        bg,a    .LL5
         mov    %o1, %o0
.LL5:
        jmp     %o7+8
         nop


Sun C++ 5.8 also produces worse code:

                        __1cItest_min6Fii_i_:
/* 000000          3 */         st      %o0,[%sp+68]
/* 0x0004          4 */         cmp     %o1,%o0
/* 0x0008            */         bge,pn  %icc,.L77000003
/* 0x000c          3 */         st      %o1,[%sp+72]
/* 0x0010          4 */         ba      .L77000004
/* 0x0014            */         add     %sp,72,%o5
                        .L77000003:
/* 0x0018          4 */         add     %sp,68,%o5
                        .L77000004:
/* 0x001c          4 */         retl    ! Result =  %o0
/* 0x0020            */         ld      [%o5],%o0 ! volatile

                        __1cGmy_min6Fii_i_:
/* 000000          9 */         cmp     %o0,%o1
/* 0x0004            */         bl,a,pt %icc,.L77000011
/* 0x0008          8 */         or      %g0,%o0,%o1
                        .L77000011:
/* 0x000c          9 */         retl    ! Result =  %o0
/* 0x0010            */         or      %g0,%o1,%o0


> std::min() suboptimal
> ---------------------
>
>                 Key: STDCXX-335
>                 URL: https://issues.apache.org/jira/browse/STDCXX-335
>             Project: C++ Standard Library
>          Issue Type: Bug
>          Components: 25. Algorithms
>    Affects Versions: 4.1.3
>         Environment: gcc 3.2.3, x86 Linux
>            Reporter: Mark Brown
>
> I don't know if it's the compiler that's generating worse code for the stdcxx version
of min or if the function is not implemented as efficiently as it could be but the disassembly
for the two functions below shows that my_min() is one instruction shorter than test_min():
> #include <algorithm>
> int test_min (int x, int y) { return std::min (x, y); }
> int my_min (int x, int y) { return x < y ? x : y; }
> _Z8test_minii:
>         pushl   %ebp
>         movl    %esp, %ebp
>         movl    12(%ebp), %eax
>         cmpl    %eax, 8(%ebp)
>         jg      .L2
>         leal    8(%ebp), %eax   <<< extra load?
> .L4:
>         movl    (%eax), %eax
>         leave
>         ret
> _Z6my_minii:
>         pushl   %ebp
>         movl    %esp, %ebp
>         movl    12(%ebp), %eax
>         cmpl    8(%ebp), %eax
>         jle     .L6
>         movl    8(%ebp), %eax
> .L6:
>         leave
>         ret

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