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Date: Sat, 15 Nov 2008 00:07:00 -0000
Subject: [Bug c/38126]  New: suboptimal code for (a && b || !a && !b)
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From: "sebor at roguewave dot com" <gcc-bugzilla@gcc.gnu.org>
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I would expect gcc to generate comparable code for both functions below, or
perhaps even better code for foo() than for bar() since the code in foo() is
likely to be more common than the equivalent code in bar(). However, the code
produced for foo() is suboptimal in comparison to the code for bar(). In my
timings on x86 with gcc 4.3.0 at -O2, foo() appears to run about 5% slower than
bar().

$ cat t.c && gcc -S -O2 t.c && cat t.s
int foo (int *a, int *b) { return a && b || !a && !b; }
int bar (int *a, int *b) { return !!a == !!b; }
        .file   "t.c"
        .text
        .p2align 4,,15
.globl foo
        .type   foo, @function
foo:
.LFB2:
        testq   %rdi, %rdi
        je      .L2
        testq   %rsi, %rsi
        movl    $1, %eax
        je      .L2
        rep
        ret
        .p2align 4,,10
        .p2align 3
.L2:
        testq   %rdi, %rdi
        sete    %al
        testq   %rsi, %rsi
        sete    %dl
        andl    %edx, %eax
        movzbl  %al, %eax
        ret
.LFE2:
        .size   foo, .-foo
        .p2align 4,,15
.globl bar
        .type   bar, @function
bar:
.LFB3:
        testq   %rdi, %rdi
        sete    %al
        testq   %rsi, %rsi
        setne   %dl
        xorl    %edx, %eax
        movzbl  %al, %eax
        ret
.LFE3:
        .size   bar, .-bar
        .section        .eh_frame,"a",@progbits
.Lframe1:
        .long   .LECIE1-.LSCIE1
.LSCIE1:
        .long   0x0
        .byte   0x1
        .string "zR"
        .uleb128 0x1
        .sleb128 -8
        .byte   0x10
        .uleb128 0x1
        .byte   0x3
        .byte   0xc
        .uleb128 0x7
        .uleb128 0x8
        .byte   0x90
        .uleb128 0x1
        .align 8
.LECIE1:
.LSFDE1:
        .long   .LEFDE1-.LASFDE1
.LASFDE1:
        .long   .LASFDE1-.Lframe1
        .long   .LFB2
        .long   .LFE2-.LFB2
        .uleb128 0x0
        .align 8
.LEFDE1:
.LSFDE3:
        .long   .LEFDE3-.LASFDE3
.LASFDE3:
        .long   .LASFDE3-.Lframe1
        .long   .LFB3
        .long   .LFE3-.LFB3
        .uleb128 0x0
        .align 8
.LEFDE3:
        .ident  "GCC: (GNU) 4.3.0 20080428 (Red Hat 4.3.0-8)"
        .section        .note.GNU-stack,"",@progbits


-- 
           Summary: suboptimal code for (a && b || !a && !b)
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sebor at roguewave dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38126