[llvm-bugs] [Bug 39944] New: missed optimizations in memcpy like code

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39944

            Bug ID: 39944
           Summary: missed optimizations in memcpy like code
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: trass3r at gmail.com
                CC: llvm-bugs at lists.llvm.org

https://godbolt.org/z/g7OHz3

void foo(char* __restrict dst, const char* __restrict buf) {
    for (int i=0; i<8; ++i)
        *dst++ = *buf++;
}

$ clang -Os -march=haswell
define dso_local void @_Z3fooPcPKc(i8* noalias nocapture, i8* noalias nocapture
readonly) local_unnamed_addr #0 {
  br label %3

  %4 = phi i64 [ 0, %2 ], [ %10, %3 ]
  %5 = getelementptr i8, i8* %1, i64 %4
  %6 = getelementptr i8, i8* %0, i64 %4
  %7 = bitcast i8* %5 to i64*
  %8 = load i64, i64* %7, align 1, !tbaa !2
  %9 = bitcast i8* %6 to i64*
  store i64 %8, i64* %9, align 1, !tbaa !2
  %10 = add i64 %4, 8
  %11 = icmp eq i64 %4, 0
  br i1 %11, label %12, label %3, !llvm.loop !5

foo(char*, char const*):                            # @foo(char*, char const*)
        xor     eax, eax
        mov     ecx, 8
.LBB0_1:
        mov     rdx, qword ptr [rsi + rax]
        mov     qword ptr [rdi + rax], rdx
        add     rax, 8
        cmp     rax, rcx
        jne     .LBB0_1


And with -O2 or -O3 this is unrolled into byte-sized reads!

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