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digitalmars.D.bugs - [Issue 9200] New: Wrong SIMD cod generated

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=9200

           Summary: Wrong SIMD cod generated
           Product: D
           Version: D2
          Platform: x86_64
        OS/Version: Linux
            Status: NEW
          Severity: major
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: jerro.public gmail.com



When compiled with no flags, the following program gives wrong results:

import std.stdio;
import core.simd;

double2 * v(double* a)
{
    return cast(double2*)a;
}

void main()
{
    double2 a;
    auto p = cast(double*) &a;
    p[0] = 1;
    p[1] = 2;

    double2 b = v(p)[0];
    v(p)[0] = b;

    writeln(p[0 .. 2]); // prints [1, 0]
}

Disassembly of the relevant part of the code:


call   426344 <_D3tmp1vFPdZPNhG2d>
movapd xmm0,XMMWORD PTR [rax]
movapd XMMWORD PTR [rbp-0x10],xmm0
movapd xmm1,XMMWORD PTR [rbp-0x10]
movsd  QWORD PTR [rbp-0x40],xmm1        ; should be movapd
mov    rdi,QWORD PTR [rbp-0x20]
call   426344 <_D3tmp1vFPdZPNhG2d>
movsd  xmm1,QWORD PTR [rbp-0x40]        ; should be movapd
movapd XMMWORD PTR [rax],xmm1

This happens with both DMD 2.060 and the latest version of 2.061 from github.
It doesn't happen if I use either -O flag or -inline. It doesn't happen with
LDC or GDC.

I have only tested this on linux.

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next sibling parent d-bugmail puremagic.com writes:
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I managed to reduce it a bit further:

import std.stdio;
import core.simd;

double2 * v(double2* a)
{
    return a;
}

void main()
{
    double2 a = [1, 2];

    *v(&a) = a;

    writeln(a.array);
}

And the disassembly:


movsd  QWORD PTR [rbp-0x20],xmm1
lea    rdi,[rbp-0x10]
call   4263f4 <_D3tmp1vFPNhG2dZPNhG2d>
movsd  xmm1,QWORD PTR [rbp-0x20]
movapd XMMWORD PTR [rax],xmm1

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

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



21:10:51 PST ---
This is happening in cod3.c REGSAVE::save() and REGSAVE::restore().
Unfortunately, just changing the opcodes doesn't work because MOVAPD requires
16 bit alignment of the operands. Fixing that exposes further problems.

Essentially, it'll have to wait a bit.

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 This is happening in cod3.c REGSAVE::save() and REGSAVE::restore().
 Unfortunately, just changing the opcodes doesn't work because MOVAPD requires
 16 bit alignment of the operands. Fixing that exposes further problems.
 
 Essentially, it'll have to wait a bit.


I know nothing about the DMD back end, so this may be an obviously bad idea,
but if alignment is the main problem, wouldn't using MOVUPD work in the
meantime?

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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19:00:01 PST ---
MOVUPD is terribly slow.

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prev sibling next sibling parent d-bugmail puremagic.com writes:
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yebblies <yebblies gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
                 CC|                            |yebblies gmail.com
           Severity|major                       |critical





 MOVUPD is terribly slow.


Terribly slow is still much better than wrong-code.

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Jan 14 2013
prev sibling parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=9200


Walter Bright <bugzilla digitalmars.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |SIMD
             Status|NEW                         |RESOLVED
           Platform|x86_64                      |All
         Resolution|                            |FIXED
         OS/Version|Linux                       |All



12:44:51 PST ---
Fixed here:

https://github.com/D-Programming-Language/dmd/commit/c33809cc201b4697b384209eb3a7a623e8e871e9#src/backend/cod3.c

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