[llvm-bugs] [Bug 40306] New: [x86] shuffle lowering creates strange shuffle mask

via llvm-bugs llvm-bugs at lists.llvm.org
Mon Jan 14 08:06:16 PST 2019


            Bug ID: 40306
           Summary: [x86] shuffle lowering creates strange shuffle mask
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

This manifested in:
...but it's independent of that patch as shown with this example:

define <8 x i16> @shuf_zeros_undefs(<8 x i16> %x) {
  %r = shufflevector <8 x i16> zeroinitializer, <8 x i16> %x, <8 x i32> <i32 9,
i32 1, i32 2, i32 3, i32 undef, i32 undef, i32 undef, i32 undef>
  ret <8 x i16> %r

Clearly, we have undefs in the high elements of the result, but we get this
unexpected pshufb lowering:

$ llc -o - weird_shufb.ll -mattr=avx
        vpshufb LCPI0_0(%rip), %xmm0, %xmm0 ## xmm0 =


That's not a miscompile, but why are the high elements choosing from the input?

Debug output shows that we created this intermediate state:

              t12: v8i16 = X86ISD::UNPCKL t17, t2
            t19: v8i16 = X86ISD::PSHUFHW t12, Constant:i8<-24>
          t20: v4i32 = bitcast t19
        t22: v4i32 = X86ISD::PSHUFD t20, Constant:i8<-26>
      t23: v8i16 = bitcast t22
    t25: v8i16 = X86ISD::PSHUFLW t23, Constant:i8<75>


So this leads to a question that may not be answerable in the DAG or
statically: what is the ideal x86 lowering for that shuffle? 

The pshufb is obviously the smallest code, but should we favor a solution that
doesn't need to load anything?

That could be a shift+blend immediate with zero:
        vpsrld  $16, %xmm0, %xmm0
        vpxor   %xmm1, %xmm1, %xmm1
        vpblendw        $1, %xmm0, %xmm1, %xmm0 ## xmm0 =

Or shift+zext:
        vpsrld  $16, %xmm0, %xmm0
        vpmovzxdq %xmm0, %xmm0

Or if we're ok with a load, but just want to avoid pshufb, it could be a
        vpsrld  $16, %xmm0, %xmm0
        vpand   LCPI0_0(%rip), %xmm0, %xmm0

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