[LLVMbugs] [Bug 21137] New: Missing memory folding with new vector shuffle lowering.

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Thu Oct 2 13:06:56 PDT 2014


            Bug ID: 21137
           Summary: Missing memory folding with new vector shuffle
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: qcolombet at apple.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 13121
  --> http://llvm.org/bugs/attachment.cgi?id=13121&action=edit
IR to reproduce the problem

When using the new vector shuffle lowering, some folding opportunities are

To reproduce:
llc -x86-experimental-vector-shuffle-lowering=true missing_folding.ll -o new.s
llc -x86-experimental-vector-shuffle-lowering=false missing_folding.ll -o old.s

diff -U 10 old.s new.s
-    pshufd    $27, (%rdi), %xmm0      ## xmm0 = mem[3,2,1,0]
+    movaps    (%rdi), %xmm0
+    shufps    $27, %xmm0, %xmm0       ## xmm0 = xmm0[3,2,1,0]

The shuffle instruction used in the new lowering is on a different than the old
one (different domain). Thus, it is likely we need to teach how to fold the
operand for that one too.

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