[clang] [llvm] [DebugInfo] Use DW_op_bit_piece for structured bindings of bitfields (PR #85665)

[Adrian Prantl via cfe-commits]

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@@ -8,8 +8,8 @@ struct S0 {
 // CHECK-LABEL: define dso_local void @_Z3fS0v
 // CHECK:                        alloca %struct.S0, align 4
 // CHECK-NEXT:    [[TMP0:%.*]] = alloca %struct.S0, align 4
-// CHECK:         call void @llvm.dbg.declare(metadata ptr [[TMP0]], metadata [[S0_A:![0-9]+]], metadata !DIExpression())
-// CHECK-NEXT:    call void @llvm.dbg.declare(metadata ptr [[TMP0]], metadata [[S0_B:![0-9]+]], metadata !DIExpression(DW_OP_plus_uconst, 2))
+// CHECK:         call void @llvm.dbg.declare(metadata ptr [[TMP0]], metadata [[S0_A:![0-9]+]], metadata !DIExpression(DW_OP_bit_piece, 16, 0))
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adrian-prantl wrote:

https://dwarfstd.org/issues/191025.1.html
https://dwarfstd.org/issues/161206.2.html

I think you are right about the semantics of `DW_OP_bit_piece`.
I also think I misunderstood the problem the first time around.

Here a and b are not fragments of a large struct, but we want to express the opposite. They point into the same alloca. Here's incorrect ToT output:


https://github.com/llvm/llvm-project/pull/85665