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digitalmars.D.bugs - [Issue 8091] New: Optimizer generates wrong code when reducing comparisons.

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8091

           Summary: Optimizer generates wrong code when reducing
                    comparisons.
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: wrong-code
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: k.hanazuki gmail.com


--- Comment #0 from Kasumi Hanazuki <k.hanazuki gmail.com> 2012-05-12 13:24:17
PDT ---
DMD optimizer generates a wrong code for the program below.
The second assertion unexpectedly fails if you specify -O flag to DMD,
while it passes as expected without optimization.

 $ dmd -m32 -run test.d      # fine
 $ dmd -m32 -O -run test.d   # AssertError test(8): Assertion failure

Tested against DMDv2.060 (git HEAD) on 32-bit Linux.
Found by hos_lyric
<https://twitter.com/#!/hos_lyric_/status/201270597265793024>.

----

int solve(int n) {
    int a = (n % 3 == 0) ? 1 : (n % 3 == 1) ? 1 : 0;
    return (a != 0) ? a : 0;
}

void main() {
    assert(solve(0) ==  1);
    assert(solve(1) ==  1); // line 8, fails with -O
    assert(solve(2) ==  0);
}

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--- Comment #1 from Kasumi Hanazuki <k.hanazuki gmail.com> 2012-05-12 23:58:16
PDT ---
more simplification:

----

int solve(int n) {
    int a = (n == 0) ? 1 : (n == 1) ? 1 : 0;
    return (a != 0) ? a : 0;
}

void main() {
    assert(solve(0) ==  1);
    assert(solve(1) ==  1); // fails with -O
    assert(solve(2) ==  0);
}

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug yahoo.com.au
           Severity|normal                      |critical


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--- Comment #2 from Don <clugdbug yahoo.com.au> 2012-05-16 12:17:27 PDT ---
Marginally reduced (this is what the early optimizer passes turn this into):

int solve(int n) {
    int a = n ? (n>=1u) : 1;
    return a ? a : 0;
}

void main() {
    assert(solve(1) ==  1);
}

It looks as after common subexpression elimination of a, the branch address
from the >= is out by 1. The generated code is:

        push    EAX
        cmp    dword ptr -4[EBP],0
        je    L1C
        mov    ECX,1
        cmp    dword ptr -4[EBP],1
        jae    L1F
        mov    ECX,0
        jmp short    L1F <--- wrong address
L1C:        xor    ECX,ECX
        inc    ECX
L1F:        je    L25        <---- shouldn't jump here
        mov    EAX,ECX    <---- should jump here instead
        jmp short    L27
L25:        xor    EAX,EAX
L27:        mov    ESP,EBP

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--- Comment #3 from github-bugzilla puremagic.com 2012-05-17 19:59:04 PDT ---
Commit pushed to dmd-1.x at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/8a689e26e1ba6565d76b05d045674967e83aef60
fix Issue 8091 - Optimizer generates wrong code when reducing comparisons

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--- Comment #4 from github-bugzilla puremagic.com 2012-05-17 20:01:21 PDT ---
Commit pushed to master at https://github.com/D-Programming-Language/dmd

https://github.com/D-Programming-Language/dmd/commit/40f81b41931eea6768b38b44802e35cd4f32f419
fix Issue 8091 - Optimizer generates wrong code when reducing comparisons

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prev sibling parent d-bugmail puremagic.com writes:
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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |bugzilla digitalmars.com
         Resolution|                            |FIXED


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