[llvm-bugs] [Bug 47700] New: Missed bitfield optimization

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47700

            Bug ID: 47700
           Summary: Missed bitfield optimization
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

union u
{
    unsigned long ulong_val;
    struct {
        unsigned long a:4;
        unsigned long b:60;
    };
};

union u pack(union u in)
{
    union u ret;
    ret.ulong_val  |= in.b;
    ret.ulong_val <<= 4;
    ret.ulong_val  |= in.a;
    return ret;
}

pack(u):                              # @pack(u)
        movq    %rdi, %rax
        orq     $-16, %rax
        ret
GCC:
pack(u):
        movq    %rdi, %rax
        ret
----------------------------------------
define i64 @src(i64 %0) {
%1:
  %2 = alloca i64 8, align 8
  %3 = alloca i64 8, align 8
  %4 = gep inbounds * %3, 8 x i32 0, 1 x i64 0
  store i64 %0, * %4, align 8
  %5 = bitcast * %3 to *
  %6 = bitcast * %5 to *
  %7 = load i64, * %6, align 8
  %8 = lshr i64 %7, 4
  %9 = bitcast * %2 to *
  %10 = load i64, * %9, align 8
  %11 = or i64 %10, %8
  store i64 %11, * %9, align 8
  %12 = bitcast * %2 to *
  %13 = load i64, * %12, align 8
  %14 = shl i64 %13, 4
  store i64 %14, * %12, align 8
  %15 = bitcast * %3 to *
  %16 = bitcast * %15 to *
  %17 = load i64, * %16, align 8
  %18 = and i64 %17, 15
  %19 = bitcast * %2 to *
  %20 = load i64, * %19, align 8
  %21 = or i64 %20, %18
  store i64 %21, * %19, align 8
  %22 = gep inbounds * %2, 8 x i32 0, 1 x i64 0
  %23 = load i64, * %22, align 8
  ret i64 %23
}
=>
define i64 @tgt(i64 %0) {
%1:
  ret i64 %0
}
Transformation seems to be correct!

https://alive2.llvm.org/ce/z/PJbo7v


Godbolt: https://godbolt.org/z/zb8zdx

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