[LLVMbugs] [Bug 8411] New: ARM back end is not very clever with shufflevector

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=8411

           Summary: ARM back end is not very clever with shufflevector
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: ARM
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: Edmund.Grimley-Evans at arm.com
                CC: llvmbugs at cs.uiuc.edu


Created an attachment (id=5635)
 --> (http://llvm.org/bugs/attachment.cgi?id=5635)
C source code for two functions that load a Q reg then store half of it

In gethalf.c (attached) there are two functions that load a Q reg then
store half of it.

The first function does it the way <arm_neon.h> implements
vget_low_s16: it goes via a vector of two float64s. The intermediate
LLVM assembler code uses extractelement and three bitcasts, and the
final assembler code is nice and simple:

f1:
        vld1.16 {d16, d17}, [r1]
        vst1.16 {d16}, [r0]
        bx      lr

The second function uses __builtin_shufflevector. The intermediate LLVM
assembler code is very simple: just a shufflevector. However, the final
assembler code is very bad: the shuffle is implemented by going via the
stack (and there are some redundant address calculations);

        vld1.16 {d16, d17}, [r1]
        mov     r1, sp
        vstmia  r1, {d16, d17}
        vldr.64 d16, [sp]
        vst1.16 {d16}, [r0]

It would be nice if use of shufflevector would give good output code.
If you are dealing with code that has a lot of shuffles this is
probably the canonical way these operations will be represented.

The command lines were:

clang -cc1 -triple thumbv7-eabi -target-cpu cortex-a8 gethalf.c -O3 -emit-llvm
-o gethalf.ll

clang -cc1 -triple thumbv7-eabi -target-cpu cortex-a8 gethalf.c -O3 -S -o
gethalf.s

If you want to try it without Clang:

llc -mcpu cortex-a8 gethalf.ll -o gethalf.s

I'm at revision 116700.

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