[Mesa-dev] [Bug 94972] blend failures on llvmpipe with llvm 3.7 due to vector selects

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=94972

            Bug ID: 94972
           Summary: blend failures on llvmpipe with llvm 3.7 due to vector
                    selects
           Product: Mesa
           Version: git
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Mesa core
          Assignee: mesa-dev at lists.freedesktop.org
          Reporter: sroland at vmware.com
        QA Contact: mesa-dev at lists.freedesktop.org

Using vector selects (cdf7c6b83dad7eb6a7600af61403315b02dcf13f) caused some
regressions when using llvm 3.7 (large number of blend tests in deqp, also
mentioned in bug 94957, piglit gl-1.0-blend-func, and dolphin was mentioned on
IRC).

The problem is actually entirely llvm's fault after some digging (initially
wasn't sure if we weren't relying on some undefined behavior, hence tracked
this down), which very obviously miscompiles rather simple vector selects (I
suspect this only affects per-byte selects, possibly only when constant values
are involved).
It only seems to affect 3.7, I've tried 3.3 through 3.8 and everything else
worked.

Mostly filing a bug so I've got something to refer to when working around it in
mesa.

define <16 x i8> @novsel(<16 x i8> %val1, <16 x i8> %val2) {
entry:
  %val1a = and <16 x i8> %val1, <i8 0, i8 0, i8 0, i8 -1, i8 0, i8 0, i8 0, i8
-1, i8 0, i8 0, i8 0, i8 -1, i8 0, i8 0, i8 0, i8 -1>
  %val2rgb = and <16 x i8> %val2, <i8 -1, i8 -1, i8 -1, i8 0, i8 -1, i8 -1, i8
-1, i8 0, i8 -1, i8 -1, i8 -1, i8 0, i8 -1, i8 -1, i8 -1, i8 0>
  %res = or <16 x i8> %val1a, %val2rgb
  ret <16 x i8> %res
}

define <16 x i8> @vsel(<16 x i8> %val1, <16 x i8> %val2) {
entry:
  %res = select <16 x i1> <i1 false, i1 false, i1 false, i1 true, i1 false, i1
false, i1 false, i1 true, i1 false, i1 false, i1 false, i1 true, i1 false, i1
false, i1 false, i1 true>, <16 x i8> %val1, <16 x i8> %val2
   ret <16 x i8> %res
}

The former gets compiled to:
        vmovdqa .LCPI0_0(%rip), %xmm2   # xmm2 =
[255,255,255,0,255,255,255,0,255,255,255,0,255,255,255,0]
        vpblendvb       %xmm2, %xmm1, %xmm0, %xmm0
        retq

But the latter to:
        vmovdqa .LCPI1_0(%rip), %xmm2   # xmm2 =
[0,0,0,255,0,0,0,255,255,255,255,255,255,255,255,255]
        vpblendvb       %xmm2, %xmm0, %xmm1, %xmm0
        retq

So only the first 8 of the 16 values are correct (and yes the .LCPI1_0 byte
values look the same as indicated in the comment).

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