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digitalmars.D.bugs - [Issue 3558] New: Optimizer bug results in false if condition being taken

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           Summary: Optimizer bug results in false if condition being
                    taken
           Product: D
           Version: unspecified
          Platform: x86
        OS/Version: Linux
            Status: NEW
          Severity: critical
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: janzert janzert.com


--- Comment #0 from Janzert <janzert janzert.com> 2009-11-29 05:28:04 PST ---
I've been chasing a bug for a few days and finally have it narrowed down to the
following example. Basically a condition that should evaluate to false is taken
anyway.

The printf line below should never be executed, but it is if compiled under
linux with "dmd -release -O -inline badbranch.d".

I first started chasing this while using 1.043 but have since upgraded to 1.052
and still see it. Not surprisingly I also see it with phobos or tango. It has
also been reported to behave the same with 2.034.

badbranch.d:

extern(C) int printf(char*, ...);

struct Container
{
    ulong[2] bits = [0UL, 1];
}

int called(ulong value)
{
    value = value * 3;
    return value;
}

int test(Container* c, int[] shift)
{
    int count = 0;
    if (c.bits[0])
        count = 1;
    count |= called(c.bits[1]) << shift[0];
    // This is always false, but is taken anyway.
    if (c.bits[0])
        printf("Impossible output %lld\n", c.bits[0]);

    return count;
}

int main(char[][] args)
{
    int[] shift = [0];
    Container c;
    return test(&c, shift);
}

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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug yahoo.com.au


--- Comment #1 from Don <clugdbug yahoo.com.au> 2009-11-30 17:15:12 PST ---
Reduced test case only requires -O. It's a bit peculiar.

int main()
{
    long[1] c = [0]; // must be long

    int [1] d = [1];
    int k = 0;    
    if (!d[0]) 
       k = 1;       
    k = d[0] + k + k;

    if (c[0]) assert(c[0]);

    return k;
}

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Walter Bright <bugzilla digitalmars.com> changed:

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


--- Comment #2 from Walter Bright <bugzilla digitalmars.com> 2009-11-30
17:36:08 PST ---
Fixed changeset 272.

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--- Comment #3 from Walter Bright <bugzilla digitalmars.com> 2009-11-30
17:39:13 PST ---
Oops, I meant 3521 was fixed!

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--- Comment #4 from Don <clugdbug yahoo.com.au> 2010-02-10 01:25:37 PST ---
Test case for DMC 8.42n. Hits the breakpoint when compiled with -O.
=======================================
const static int d[1] = {1};
const static long long c[1] = {0};

void crash()
{
  _asm  int 3;
}
int main()
{
    int k = 1;    
    if (d[0]) 
       k = 0;           
    k = 2*d[0] + k;
    if (c[0]) crash();
    return k;
}
=======================================

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Don <clugdbug yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch


--- Comment #5 from Don <clugdbug yahoo.com.au> 2010-02-11 14:24:13 PST ---
This was hard to track down! I had to delve into most of the back-end to work
out what's going on here. As usual with these nightmare bugs, the fix is
simple:

PATCH: cod1.c, loaddata(), line 3365.

    else if (sz == 8)
    {   code *c1;
        int i;

        c = allocreg(&regm,&reg,TYoffset);    /* get a register */
        i = sz - REGSIZE;
        ce = loadea(e,&cs,0x8B,reg,i,0,0);    /* MOV reg,data+6 */
        if (tyfloating(tym))        // TYdouble or TYdouble_alias
        gen2(ce,0xD1,modregrm(3,4,reg));    // SHL reg,1
        c = cat(c,ce);

        while ((i -= REGSIZE) >= 0)
        {
        c1 = loadea(e,&cs,0x0B,reg,i,regm,0);    // OR reg,data+i
+        if (i == 0)
+            c1->Iflags |= CFpsw;   // We need the flags
        c = cat(c,c1);
        }
    }
    else if (sz == LNGDBLSIZE)            // TYldouble

ROOT CAUSE: 
A condition of the form 'if (x)', where x is long or ulong, is coded as:  asm {
mov reg, low_dword; OR reg, high_dword; } But, the code needs to mark the flags
as needing to be preserved. (CFpsw, I have no idea what that stands for!).
Then, later on, in cgsched.c, the Pentium scheduler thinks it can move this
instruction behind other flag-modifying instructions, and it wants to do that
in this case in order to do instruction pairing for the U and V pipes.

In the original test case, there's an ADD which gets moved after the OR. Then,
the branch instruction which follows this gets the flags from the ADD instead
of the OR, and so it branches incorrectly. The bug is only manifested in weird
cases where pairing opportunities arise, so it is extremely fragile. It's not
really an optimiser bug, but it only happens when there are pairing advantages
when instructions are moved around, which only happens when many registers are
being used and modified, and this in turn only happens when variables are in
registers.

BTW: This bug probably also affects the 16-bit C compiler in the 4-byte int
case; the flags aren't marked as required for any of the comparisons.

That's a long explanation, but it took me ages to track this down.
I think I need a beer.

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--- Comment #6 from Janzert <janzert janzert.com> 2010-02-12 02:35:40 PST ---
Wow, that's amazing that you were able and persistent enough to track that
down. Thanks for finding the cause. Hopefully it can be applied and make it
into a release quickly.

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Walter Bright <bugzilla digitalmars.com> changed:

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


--- Comment #7 from Walter Bright <bugzilla digitalmars.com> 2010-03-08
22:21:27 PST ---
Fixed dmd 1.057 and 2.041

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