LLVM cannot remove switch even if all branches jump to same code

[AngelicosPhosphoros]

godbolt link

Code:

; ModuleID = 'playground.c03e094e-cgu.0'
source_filename = "playground.c03e094e-cgu.0"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

%"MyEnum<'_>::A" = type { [1 x i64], { ptr, i64 } }
%"MyEnum<'_>::B" = type { [1 x i64], { ptr, i64 } }
%"MyEnum<'_>::C" = type { [1 x i64], { ptr, i64 } }
%"MyEnum<'_>::D" = type { [1 x i64], { ptr, i64 } }

; playground::test_bounds
; Function Attrs: mustprogress nofree norecurse nosync nounwind nonlazybind readonly willreturn uwtable
define noundef i64 @get_lengths(
    ptr noalias nocapture noundef readonly align 8 dereferenceable(24) %en
) unnamed_addr #0 {
start:
  %_2 = load i64, ptr %en, align 8, !range !2, !noundef !3
  switch i64 %_2, label %bb2 [
    i64 0, label %bb3
    i64 1, label %bb4
    i64 2, label %bb5
    i64 3, label %bb1
  ]

bb2:                                              ; preds = %start
  unreachable

bb3:                                              ; preds = %start
  %0 = getelementptr inbounds %"MyEnum<'_>::A", ptr %en, i64 0, i32 1, i32 1
  br label %bb6

bb4:                                              ; preds = %start
  %1 = getelementptr inbounds %"MyEnum<'_>::B", ptr %en, i64 0, i32 1, i32 1
  br label %bb6

bb5:                                              ; preds = %start
  %2 = getelementptr inbounds %"MyEnum<'_>::C", ptr %en, i64 0, i32 1, i32 1
  br label %bb6

bb1:                                              ; preds = %start
  %3 = getelementptr inbounds %"MyEnum<'_>::D", ptr %en, i64 0, i32 1, i32 1
  br label %bb6

bb6:                                              ; preds = %bb3, %bb4, %bb5, %bb1
  %.0.in = phi ptr [ %3, %bb1 ], [ %2, %bb5 ], [ %1, %bb4 ], [ %0, %bb3 ]
  %.0 = load i64, ptr %.0.in, align 8
  ret i64 %.0
}

attributes #0 = {
    mustprogress
    nofree
    norecurse
    nosync
    nounwind
    nonlazybind
    readonly
    willreturn
    uwtable "probe-stack"="__rust_probestack"
    "target-cpu"="x86-64"
}

!llvm.module.flags = !{!0, !1}

!0 = !{i32 7, !"PIC Level", i32 2}
!1 = !{i32 2, !"RtLibUseGOT", i32 1}
!2 = !{i64 0, i64 4}
!3 = !{}

Generated ASM:

get_lengths:                            # @get_lengths
        mov     rax, qword ptr [rdi]
        jmp     qword ptr [8*rax + .LJTI0_0]
.LBB0_1:                                # %bb6
        mov     rax, qword ptr [rdi + 16]
        ret
.LJTI0_0:
        .quad   .LBB0_1
        .quad   .LBB0_1
        .quad   .LBB0_1
        .quad   .LBB0_1

If you look at the asm, it is clear that jump and jump-table can be removed because all branches jump to same place.

[nikic]

This will be fixed by https://discourse.llvm.org/t/rfc-replacing-getelementptr-with-ptradd/68699, or an intermediate step towards it (e.g. canonicalizing all constant GEPs to i8 in InstCombine would be sufficient).