[llvm-bugs] [Bug 42794] New: [X86] Unnecessary stack manipulation remaining from memcpy/extract_subvector

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42794

            Bug ID: 42794
           Summary: [X86] Unnecessary stack manipulation remaining from
                    memcpy/extract_subvector
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: bisqwit at iki.fi, craig.topper at gmail.com,
                    llvm-bugs at lists.llvm.org, llvm-dev at redking.me.uk,
                    spatel+llvm at rotateright.com

Split off from [Bug #42674]

Current codegen for 16/32/64 byte sums: https://godbolt.org/z/76g4k2

The 32-byte reduction below should have been able to remove memcpy's stack
spill/reload and use the original load directly.

64-byte reduction manages to do this (see Godbolt) but leaves the rsp stack
manipulations.

    #include <string.h>
    unsigned char calculate_checksum(const void* ptr)
    {
        unsigned char bytes[32], result = 0;
        memcpy(bytes, ptr, 32); // Endianess does not matter.
        for(unsigned n=0; n<32; ++n) result += bytes[n];
        return result;
    }


        push    rbp
        mov     rbp, rsp
        and     rsp, -32
        sub     rsp, 64
        vmovups ymm0, ymmword ptr [rdi]
        vmovaps ymmword ptr [rsp], ymm0
        vmovdqa xmm0, xmmword ptr [rsp]
        vpaddb  xmm0, xmm0, xmmword ptr [rsp + 16]
        vpshufd xmm1, xmm0, 78          # xmm1 = xmm0[2,3,0,1]
        vpaddb  xmm0, xmm0, xmm1
        vpxor   xmm1, xmm1, xmm1
        vpsadbw xmm0, xmm0, xmm1
        vpextrb eax, xmm0, 0
        mov     rsp, rbp
        pop     rbp
        vzeroupper
        ret

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