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digitalmars.D.bugs - [Issue 5581] New: [64-bit] Wrong code with bitwise operations on bools

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           Summary: [64-bit] Wrong code with bitwise operations on bools
           Product: D
           Version: D2
          Platform: x86_64
        OS/Version: Windows
            Status: NEW
          Keywords: wrong-code
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: dsimcha yahoo.com


--- Comment #0 from David Simcha <dsimcha yahoo.com> 2011-02-14 20:10:25 PST ---
The following code produces obviously implausible results on 64-bit builds. 
result ends up being bigger than 0b111, which is the biggest number that
correct code could generate here.  (The correct value, which 32-bit builds
produce, is 3.)  This only happens in this test case when -O and -release are
disabled, though I think it still happens with -O and -release enabled in the
program I reduced this bug from.

import std.stdio;

void screwy(string[] data) {
    immutable size_t mid = data.length / 2;
    immutable uint result = ((cast(uint) (data[0] < data[mid])) << 2) |
                            ((cast(uint) (data[0] < data[$ - 1])) << 1) |
                            (cast(uint) (data[mid] < data[$ - 1]));

    stderr.writefln("%b", result);
}

void main() {
    auto stuff = ["a", "a", "b"];
    screwy(stuff);
}

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--- Comment #1 from David Simcha <dsimcha yahoo.com> 2011-02-15 11:29:13 PST ---
I think this is actually a long-standing, hard to reproduce, non-deterministic
codegen bug that strikes on 32-bit as well, though due to the non-deterministic
nature of it, under different circumstances.  Here's some code that segfaults
on 32 with stock DMD 2.051, only with -O -inline -release enabled.  This is
slightly less reduced than the 64 version because anytime I try to reduce it
more, the bug goes away.

import std.stdio;

bool comp(long a, long b) { return a < b; }

size_t medianOf3(T)(T[] data) {
    // Removing this print statement changes the result.
    stderr.writeln(data[0], '\t', data[$ / 2], '\t', data[$ - 1]);
    immutable size_t mid = data.length / 2;
    immutable uint result = ((cast(uint) (comp(data[0], data[mid]))) << 2) |
                            ((cast(uint) (comp(data[0], data[$ - 1]))) << 1) |
                            (cast(uint) (comp(data[mid], data[$ - 1])));

    assert(result != 2 && result != 5 && result < 8); // Cases 2, 5 can't
happen.
    switch(result) {
        case 1:  // 001
        case 6:  // 110
            return data.length - 1;
        case 3:  // 011
        case 4:  // 100
            return 0;
        case 0:  // 000
        case 7:  // 111
            return mid;
        default:
            assert(0);
    }
    assert(0);
}


void main() {
    long[] stuff = [4602111750194969114, 4591797823464657051,
4579622497201165564];
    medianOf3(stuff);
}

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David Simcha <dsimcha yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         OS/Version|Windows                     |Linux


--- Comment #2 from David Simcha <dsimcha yahoo.com> 2011-02-15 11:31:50 PST ---
Forgot to mention:  The 32-bit version still only segfaults on Linux.

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David Simcha <dsimcha yahoo.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Platform|x86_64                      |All


--- Comment #3 from David Simcha <dsimcha yahoo.com> 2011-02-15 15:59:13 PST ---
I did a little bisecting.  This issue appeared in 2.050.  It's possible that
it's been around since before that but requires different magic numbers on
earlier releases, though I doubt it because I would have probably noticed the
bug if it was around for a long time.  I do my day to day work on Windows but
use Linux for the occasional computationally intensive project.  I haven't done
much serious (i.e. Linux-based) computation since 2.050 was released, but I was
doing a lot of it earlier on.  The function I isolated this test case from was
around in its current form back then and was definitely getting called a lot in
those projects.

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--- Comment #4 from David Simcha <dsimcha yahoo.com> 2011-02-15 16:16:59 PST ---
Created an attachment (id=913)
Diff of 2.049 vs. 2.050 ASM output.

One more hint:  Here's a diff of the ASM output for the function body of
medianOf3 between 2.049 (test49.asm) and 2.050 (test50.asm).  The most notable
difference is that in 2.050 the comparison function gets inlined.  In 2.049 it
doesn't.

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--- Comment #5 from David Simcha <dsimcha yahoo.com> 2011-02-15 17:25:48 PST ---
Unfortunately this appears to be a Heisenbug.  When I put print statements in
to figure out what's going on and call the switch statement function manually,
it starts working.  However, the one hint I did get from mucking around in
switch_.d is that replacing the memcmp() call with a manual for loop and
recompiling druntime solves the problem.  Not sure if this is due to the
Heisenbug nature of this thing or if it indicates that the memcmp() call is the
culprit.

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--- Comment #6 from David Simcha <dsimcha yahoo.com> 2011-02-15 18:10:30 PST ---
Ignore the last comment.  It was directed to the wrong bug report.

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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bugzilla digitalmars.com


--- Comment #7 from Walter Bright <bugzilla digitalmars.com> 2011-02-15
21:48:42 PST ---
https://github.com/D-Programming-Language/dmd/commit/175bb7540db3ad16655772fe47e4dcbeab7fa380

https://github.com/D-Programming-Language/dmd/commit/cfc773a5bf99b6e636b03644787b71f4acc1c622

This addresses the 64 bit code gen problems, not the 32 bit issue.

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Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED


--- Comment #8 from Walter Bright <bugzilla digitalmars.com> 2011-02-15
22:39:38 PST ---
Fix for 32 bit code (completely different from 64 bit bug):

https://github.com/D-Programming-Language/dmd/commit/52879f36cb28b05813161002d559fecc11a0cbe6

https://github.com/D-Programming-Language/dmd/commit/525de3d38d8202a810352e6811328afa19166591

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